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Research Techniques Made Simple: Mouse Bacterial Skin Infection Models for Immunity Research

Open ArchivePublished:May 11, 2020DOI:https://doi.org/10.1016/j.jid.2020.04.012
      Bacterial skin infections are a major societal health burden and are increasingly difficult to treat owing to the emergence of antibiotic-resistant strains such as community-acquired methicillin-resistant Staphylococcus aureus. Understanding the immunologic mechanisms that provide durable protection against skin infections has the potential to guide the development of immunotherapies and vaccines to engage the host immune response to combat these antibiotic-resistant strains. To this end, mouse skin infection models allow researchers to examine host immunity by investigating the timing, inoculum, route of infection and the causative bacterial species in different wild-type mouse backgrounds as well as in knockout, transgenic, and other types of genetically engineered mouse strains. To recapitulate the various types of human skin infections, many different mouse models have been developed. For example, four models frequently used in dermatological research are based on the route of infection, including (i) subcutaneous infection models, (ii) intradermal infection models, (iii) wound infection models, and (iv) epicutaneous infection models. In this article, we will describe these skin infection models in detail along with their advantages and limitations. In addition, we will discuss how humanized mouse models such as the human skin xenograft on immunocompromised mice might be used in bacterial skin infection research.

      Abbreviation:

      AD (atopic dermatitis)

      Introduction

      The skin provides the first line of defense by providing a physical barrier with a low pH and temperature, an abundance of antimicrobial peptides, and the normal healthy skin microbiome that protects against microbial invasion. However, when the protective skin barrier is damaged or breached or develops a microbial dysbiosis, skin infections can arise. Staphylococcus aureus is the leading cause of skin and soft tissue infections in the US (
      • Dantes R.
      • Mu Y.
      • Belflower R.
      • Aragon D.
      • Dumyati G.
      • Harrison L.H.
      • et al.
      National burden of invasive methicillin-resistant Staphylococcus aureus infections, United States, 2011.
      ,
      • Suaya J.A.
      • Mera R.M.
      • Cassidy A.
      • O’Hara P.
      • Amrine-Madsen H.
      • Burstin S.
      • et al.
      Incidence and cost of hospitalizations associated with Staphylococcus aureus skin and soft tissue infections in the United States from 2001 through 2009.
      ). With the emergence of antibiotic-resistant bacterial clinical isolates such as community-acquired methicillin-resistant S. aureus, it is critical to understand the host immune responses that promote bacterial clearance to develop nonantibiotic immune-based therapies to prevent and/or treat skin infections. To investigate these immunologic processes, mouse models that mimic various human skin infections have been developed and have been instrumental in identifying novel immunotherapeutic targets. This review will discuss different mouse skin infection models along with a human skin xenograft model and the advantages and limitations of each model. Although we will focus on S. aureus and other bacterial pathogens to describe each mouse skin infection model, these models do not exclude or may not be representative of fungal, parasitic, or viral skin infections.

      Summary Points

      Advantages

      • Mouse skin infection models are powerful tools to elucidate immune mechanisms of protection and identify therapeutic targets against skin infections.
      • Human skin xenografts on immunocompromised mice provide the potential to validate findings from mouse infection models in human skin.

      Limitations

      • Immune responses can differ against the same infectious agent depending on the skin infection model used and should be verified in each model separately.
      • There are inherent immunologic and physiological differences between mouse and human skin.

      Mouse models of skin infection

      Mouse skin infection models can be categorized into four groups on the basis of the depth of infection: (i) subcutaneous infection in which the bacteria are inoculated below the dermis, (ii) intradermal infection in which the bacteria are inoculated into the dermis, (iii) wound infection in which the bacteria are inoculated into full-thickness incisional or excisional wounds, and (iv) epicutaneous infection in which the surface of the skin is exposed to the bacterial inoculum (Figure 1). These four models will be described in the context of S. aureus skin infections. Finally, we will discuss the potential for translational studies with human skin xenografts. Understanding the strengths and weaknesses of each model will help provide key insights into which the system is most appropriate to study specific immunologic responses as summarized in Table 1.
      Figure thumbnail gr1
      Figure 1Graphical and photographic representations of bacterial skin infection models. (a) Graphical representation of mouse skin infection models as defined by the depth of infection in the skin. (b) Representative clinical photographs of each of the following skin infection models (left panel: control; right panel: experimental): (i) epicutaneous infection where bacteria was inoculated on the surface of intact skin by applying a gauze soaked with bacteria or swabbing (
      • Dai T.
      • Kharkwal G.B.
      • Tanaka M.
      • Huang Y.Y.
      • Bil de Arce V.J.
      • Hamblin M.R.
      Animal models of external traumatic wound infections.
      ,
      • Malhotra N.
      • Yoon J.
      • Leyva-Castillo J.M.
      • Galand C.
      • Archer N.
      • Miller L.S.
      • et al.
      IL-22 derived from γδ T cells restricts Staphylococcus aureus infection of mechanically injured skin.
      ,
      • Williams M.R.
      • Costa S.K.
      • Zaramela L.S.
      • Khalil S.
      • Todd D.A.
      • Winter H.L.
      • et al.
      Quorum sensing between bacterial species on the skin protects against epidermal injury in atopic dermatitis.
      ), (ii) wound infection where Staphylococcus aureus was inoculated on a full-thickness skin incisional or splint-sutured excisional wound (
      • Archer N.K.
      • Wang Y.
      • Ortines R.V.
      • Liu H.
      • Nolan S.J.
      • Liu Q.
      • et al.
      Preclinical models and methodologies for monitoring Staphylococcus aureus infections using noninvasive optical imaging.
      ,
      • Morimoto K.
      • Ozawa T.
      • Awazu K.
      • Ito N.
      • Honda N.
      • Matsumoto S.
      • et al.
      Photodynamic therapy using systemic administration of 5-aminolevulinic acid and a 410-nm wavelength light-emitting diode for methicillin-resistant Staphylococcus aureus-infected ulcers in mice.
      ), (iii) intradermal infection model where S. aureus was inoculated into the dermis of the dorsal skin and developed dermonecrosis (
      • Liu H.
      • Archer N.K.
      • Dillen C.A.
      • Wang Y.
      • Ashbaugh A.G.
      • Ortines R.V.
      • et al.
      Staphylococcus aureus epicutaneous exposure drives skin inflammation via IL-36-Mediated T cell responses.
      ), and (iv) subcutaneous infection where S. aureus was inoculated into the subcutaneous tissue, which led to dermonecrosis and muscle necrosis (
      • Tseng C.W.
      • Sanchez-Martinez M.
      • Arruda A.
      • Liu G.Y.
      Subcutaneous infection of methicillin resistant Staphylococcus aureus (MRSA).
      ). Permission to reproduce the images in b were obtained from the respective original manuscripts, cited above.
      Table 1Summary of Mouse Skin Infection Models for Immunity Research
      TypeHuman RelevanceModel DescriptionLimitationsReferences
      Subcutaneous model
      • Cellulitis
      • Necrotizing fasciitis
      • Myositis
      • Inoculation of bacteria into subcutaneous tissue.
      • Used to study infection in the context of dermal, subcutaneous, and muscular tissues.
      • Some measurements require invasive sampling.
      (
      • Berube B.J.
      • Sampedro G.R.
      • Otto M.
      • Bubeck Wardenburg J.
      The PSMα locus regulates production of Staphylococcus aureus alpha-toxin during infection.
      ;
      • Jeong S.
      • Kim H.Y.
      • Kim A.R.
      • Yun C.H.
      • Han S.H.
      Propionate ameliorates Staphylococcus aureus skin infection by attenuating bacterial growth.
      ;
      • Nippe N.
      • Varga G.
      • Holzinger D.
      • Löffler B.
      • Medina E.
      • Becker K.
      • et al.
      Subcutaneous infection with S. aureus in mice reveals association of resistance with influx of neutrophils and Th2 response.
      ;
      • Tseng C.W.
      • Sanchez-Martinez M.
      • Arruda A.
      • Liu G.Y.
      Subcutaneous infection of methicillin resistant Staphylococcus aureus (MRSA).
      )
      Intradermal model
      • Folliculitis
      • Furuncle
      • Cellulitis
      • Erysipelas
      • Injection of bacteria into the dermis.
      • Used to elucidate the epidermal and dermal contribution to protection against infection.
      • Immunologic differences between humans and mice (e.g., γδ T cells subsets, composition of circulating immune cells).
      (
      • Asai A.
      • Tsuda Y.
      • Kobayashi M.
      • Hanafusa T.
      • Herndon D.N.
      • Suzuki F.
      Pathogenic role of macrophages in intradermal infection of methicillin-resistant Staphylococcus aureus in thermally injured mice.
      ;
      • Brown E.L.
      • Dumitrescu O.
      • Thomas D.
      • Badiou C.
      • Koers E.M.
      • Choudhury P.
      • et al.
      The Panton-Valentine leukocidin vaccine protects mice against lung and skin infections caused by Staphylococcus aureus USA300.
      ;
      • Dillen C.A.
      • Pinsker B.L.
      • Marusina A.I.
      • Merleev A.A.
      • Farber O.N.
      • Liu H.
      • et al.
      Clonally expanded γδ T cells protect against Staphylococcus aureus skin reinfection.
      )
      Wound models
       Incisional wound
      • Diabetic ulcerative skin infections
      • Surgery site infections
      • External wound infections
      • Polymicrobial infections
      • Infection of full-thickness skin incisions.
      • Examination of wound healing and immunity in response to bacterial infection.
      • Differences in wound healing mechanisms between mouse and humans.
      (
      • Guo Y.
      • Ramos R.I.
      • Cho J.S.
      • Donegan N.P.
      • Cheung A.L.
      • Miller L.S.
      In vivo bioluminescence imaging to evaluate systemic and topical antibiotics against community-acquired methicillin-resistant Staphylococcus aureus-infected skin wounds in mice.
      ;
      • Ortines R.V.
      • Liu H.
      • Cheng L.I.
      • Cohen T.S.
      • Lawlor H.
      • Gami A.
      • et al.
      Neutralizing alpha-toxin accelerates healing of Staphylococcus aureus-infected wounds in nondiabetic and diabetic mice.
      ;
      • Zolfaghari P.S.
      • Packer S.
      • Singer M.
      • Nair S.P.
      • Bennett J.
      • Street C.
      • et al.
      In vivo killing of Staphylococcus aureus using a light-activated antimicrobial agent.
      )
       Excisional wound
      • Diabetic ulcerative skin infections
      • Surgery site infections
      • External wound infections
      • Polymicrobial infections
      • Infection of full-thickness skin excisional wounds.
      • With film or suturing, can better replicate re-epithelization wound closure of human skin.
      • Differences in wound healing mechanisms between mouse and humans.
      (
      • Fila G.
      • Kasimova K.
      • Arenas Y.
      • Nakonieczna J.
      • Grinholc M.
      • Bielawski K.P.
      • et al.
      Murine model imitating chronic wound infections for evaluation of antimicrobial photodynamic therapy efficacy.
      ;
      • Shi C.M.
      • Nakao H.
      • Yamazaki M.
      • Tsuboi R.
      • Ogawa H.
      Mixture of sugar and povidone-iodine stimulates healing of MRSA-infected skin ulcers on db/db mice.
      )
      Epicutaneous models
       Gauze infection
      • AD skin inflammation
      • Covered topical infection on intact or tape-stripped skin.
      • Used to investigate disease pathogenesis of AD in humans.
      • Unknown contribution of depilatory cream to skin inflammation.
      • Contact through gauze does not replicate the direct colonization of bacteria on the patient skin.
      (
      • Liu H.
      • Archer N.K.
      • Dillen C.A.
      • Wang Y.
      • Ashbaugh A.G.
      • Ortines R.V.
      • et al.
      Staphylococcus aureus epicutaneous exposure drives skin inflammation via IL-36-Mediated T cell responses.
      ;
      • Nakatsuji T.
      • Chen T.H.
      • Two A.M.
      • Chun K.A.
      • Narala S.
      • Geha R.S.
      • et al.
      Staphylococcus aureus exploits epidermal barrier defects in atopic dermatitis to trigger cytokine expression.
      ;
      • Williams M.R.
      • Costa S.K.
      • Zaramela L.S.
      • Khalil S.
      • Todd D.A.
      • Winter H.L.
      • et al.
      Quorum sensing between bacterial species on the skin protects against epidermal injury in atopic dermatitis.
      )
       Swab infection
      • Noninvasive bacterial colonization
      • AD skin inflammation
      • Impetigo
      • Uncovered topical infection on intact or tape-stripped skin.
      • Used to investigate how the skin microbiome influences the local immune system.
      • Exposure to nonphysiological inoculum.
      (
      • Kugelberg E.
      • Norström T.
      • Petersen T.K.
      • Duvold T.
      • Andersson D.I.
      • Hughes D.
      Establishment of a superficial skin infection model in mice by using Staphylococcus aureus and Streptococcus pyogenes.
      ;
      • Linehan J.L.
      • Harrison O.J.
      • Han S.J.
      • Byrd A.L.
      • Vujkovic-Cvijin I.
      • Villarino A.V.
      • et al.
      Non-classical immunity controls microbiota impact on skin immunity and tissue repair.
      ;
      • Malhotra N.
      • Yoon J.
      • Leyva-Castillo J.M.
      • Galand C.
      • Archer N.
      • Miller L.S.
      • et al.
      IL-22 derived from γδ T cells restricts Staphylococcus aureus infection of mechanically injured skin.
      ;
      • Pastagia M.
      • Euler C.
      • Chahales P.
      • Fuentes-Duculan J.
      • Krueger J.G.
      • Fischetti V.A.
      A novel chimeric lysin shows superiority to mupirocin for skin decolonization of methicillin-resistant and -sensitive Staphylococcus aureus strains.
      )
      Human Skin xenograft
      • Closely resembles normal human skin
      • Transplantation of human skin onto immunocompromised mice.
      • Potential to apply murine skin infection models in human skin to validate translational relevance of findings.
      • Limited to immunocompromised mice.
      • Skin infection models are largely untested.
      (
      • Schulz A.
      • Jiang L.
      • de Vor L.
      • Ehrström M.
      • Wermeling F.
      • Eidsmo L.
      • et al.
      Neutrophil recruitment to noninvasive MRSA at the stratum corneum of human skin mediates transient colonization.
      )
      Abbreviation: AD, atopic dermatitis.

      Subcutaneous infection models

      The subcutaneous infection model mimics more invasive infections such as subcutaneous abscesses and cellulitis (
      • McCaig L.F.
      • McDonald L.C.
      • Mandal S.
      • Jernigan D.B.
      Staphylococcus aureus-associated skin and soft tissue infections in ambulatory care.
      ,
      • Miller L.G.
      • Perdreau-Remington F.
      • Rieg G.
      • Mehdi S.
      • Perlroth J.
      • Bayer A.S.
      • et al.
      Necrotizing fasciitis caused by community-associated methicillin-resistant Staphylococcus aureus in Los Angeles.
      ). Upon subcutaneous inoculation of S. aureus into the backs of mice, a deep abscess that is comprised of neutrophils forms around that bacteria. This abscess typically forms below the panniculus carnosus muscle in the deep dermis that primarily involves the subcutaneous fat above the deeper muscle layers (
      • Liese J.
      • Rooijakkers S.H.
      • van Strijp J.A.
      • Novick R.P.
      • Dustin M.L.
      Intravital two-photon microscopy of host-pathogen interactions in a mouse model of Staphylococcus aureus skin abscess formation.
      ,
      • Tseng C.W.
      • Sanchez-Martinez M.
      • Arruda A.
      • Liu G.Y.
      Subcutaneous infection of methicillin resistant Staphylococcus aureus (MRSA).
      ). Thus, this model has been widely used to elucidate immune mechanisms against deep soft tissue infections with various bacterial species such as S. aureus and Streptococcus pyogenes (
      • Medina E.
      Murine model of cutaneous infection with Streptococcus pyogenes.
      ,
      • Tseng C.W.
      • Kyme P.
      • Low J.
      • Rocha M.A.
      • Alsabeh R.
      • Miller L.G.
      • et al.
      Staphylococcus aureus Panton-Valentine leukocidin contributes to inflammation and muscle tissue injury.
      ). For instance, by subcutaneously inoculating different wild-type mouse strains with S. aureus, it was seen that resistance to the bacterial infection was associated with an increasing number of infiltrating neutrophils at the site of infection (
      • Nippe N.
      • Varga G.
      • Holzinger D.
      • Löffler B.
      • Medina E.
      • Becker K.
      • et al.
      Subcutaneous infection with S. aureus in mice reveals association of resistance with influx of neutrophils and Th2 response.
      ). The subcutaneous infection model has also been used to elucidate the role of antimicrobial peptides during S. aureus skin infections. The activities of hBD3 and LL-37 (cathelicidin) were shown to be essential for controlling subcutaneous skin infections by promoting the killing of S. aureus by either maintaining the antistaphylococcal environment or permeabilizing the bacterial membrane, respectively (
      • Cheung G.Y.C.
      • Fisher E.L.
      • McCausland J.W.
      • Choi J.
      • Collins J.W.M.
      • Dickey S.W.
      • et al.
      Antimicrobial peptide resistance mechanism contributes to Staphylococcus aureus infection.
      ). In addition, the subcutaneous model was used to discover an unexpected role for adipocyte-derived LL-37 in the control of S. aureus infection (
      • Zhang L.J.
      • Guerrero-Juarez C.F.
      • Hata T.
      • Bapat S.P.
      • Ramos R.
      • Plikus M.V.
      • et al.
      Innate immunity. Dermal adipocytes protect against invasive Staphylococcus aureus skin infection.
      ). This model can also be used to investigate durable immune responses that protect the host from recurrent infections. For example, reinfected mice showed innate immune memory (e.g., trained memory) of macrophages against a recurrent S. aureus subcutaneous infection (
      • Chan L.C.
      • Rossetti M.
      • Miller L.S.
      • Filler S.G.
      • Johnson C.W.
      • Lee H.K.
      • et al.
      Protective immunity in recurrent Staphylococcus aureus infection reflects localized immune signatures and macrophage-conferred memory.
      ). However, obtaining the muscle lesion size, which is a common readout for the subcutaneous infection model, involves a more invasive procedure that requires sacrificing the mice (
      • Tseng C.W.
      • Sanchez-Martinez M.
      • Arruda A.
      • Liu G.Y.
      Subcutaneous infection of methicillin resistant Staphylococcus aureus (MRSA).
      ).

      Intradermal skin infection models

      The intradermal skin infection model also recapitulates the hallmarks of human S. aureus skin infections, including dermonecrotic lesions and neutrophilic skin abscesses, which correspond to the progression and severity of the infection (
      • Asai A.
      • Tsuda Y.
      • Kobayashi M.
      • Hanafusa T.
      • Herndon D.N.
      • Suzuki F.
      Pathogenic role of macrophages in intradermal infection of methicillin-resistant Staphylococcus aureus in thermally injured mice.
      ,
      • Mölne L.
      • Verdrengh M.
      • Tarkowski A.
      Role of neutrophil leukocytes in cutaneous infection caused by Staphylococcus aureus.
      ). In addition, bioluminescent bacterial strains and in vivo optical imaging systems can be used in conjunction to noninvasively and longitudinally monitor the dynamics of the bacterial infection (
      • Miller L.S.
      • O’Connell R.M.
      • Gutierrez M.A.
      • Pietras E.M.
      • Shahangian A.
      • Gross C.E.
      • et al.
      MyD88 mediates neutrophil recruitment initiated by IL-1R but not TLR2 activation in immunity against Staphylococcus aureus.
      ). Genetically engineered mouse strains are also useful to study the components of the host response required for protection against skin infections. For example, the critical role of the inflammasome and IL-1β–IL-1R signaling in promoting neutrophil recruitment and host defense against S. aureus skin infections was uncovered using mice deficient in ASC, IL-1β, or IL-1R as well as IL-1β-DsRed reporter mice (
      • Cho J.S.
      • Guo Y.
      • Ramos R.I.
      • Hebroni F.
      • Plaisier S.B.
      • Xuan C.
      • et al.
      Neutrophil-derived IL-1β is sufficient for abscess formation in immunity against Staphylococcus aureus in mice.
      ,
      • Miller L.S.
      • Pietras E.M.
      • Uricchio L.H.
      • Hirano K.
      • Rao S.
      • Lin H.
      • et al.
      Inflammasome-mediated production of IL-1beta is required for neutrophil recruitment against Staphylococcus aureus in vivo.
      ,
      • Miller L.S.
      • O’Connell R.M.
      • Gutierrez M.A.
      • Pietras E.M.
      • Shahangian A.
      • Gross C.E.
      • et al.
      MyD88 mediates neutrophil recruitment initiated by IL-1R but not TLR2 activation in immunity against Staphylococcus aureus.
      ). In addition, mice lacking γδ T cells exhibited significant host defense defects owing to impaired IL-17 production (
      • Cho J.S.
      • Pietras E.M.
      • Garcia N.C.
      • Ramos R.I.
      • Farzam D.M.
      • Monroe H.R.
      • et al.
      IL-17 is essential for host defense against cutaneous Staphylococcus aureus infection in mice.
      ). Transgenic reporter mouse strains such as the IL-17A-tdTomato/IL-17F-GFP dual-color reporter mice can provide insights into the expression kinetics and relevant expressing cell types of host-derived cytokines that are important for protection against intradermal S. aureus infections (
      • Marchitto M.C.
      • Dillen C.A.
      • Liu H.
      • Miller R.J.
      • Archer N.K.
      • Ortines R.V.
      • et al.
      Clonal Vγ6+Vδ4+ T cells promote IL-17-mediated immunity against Staphylococcus aureus skin infection.
      ). The intradermal model can also be modified to investigate the mechanisms of immunologic memory by reinfecting mice at a different skin site from the original intradermal infection (
      • Gaidamakova E.K.
      • Myles I.A.
      • McDaniel D.P.
      • Fowler C.J.
      • Valdez P.A.
      • Naik S.
      • et al.
      Preserving immunogenicity of lethally irradiated viral and bacterial vaccine epitopes using a radio- protective Mn2+-peptide complex from Deinococcus.
      ,
      • Montgomery C.P.
      • Daniels M.
      • Zhao F.
      • Alegre M.L.
      • Chong A.S.
      • Daum R.S.
      Protective immunity against recurrent Staphylococcus aureus skin infection requires antibody and interleukin-17A.
      ,
      • Sampedro G.R.
      • DeDent A.C.
      • Becker R.E.
      • Berube B.J.
      • Gebhardt M.J.
      • Cao H.
      • et al.
      Targeting Staphylococcus aureus α-toxin as a novel approach to reduce severity of recurrent skin and soft-tissue infections.
      ). Remarkably, using an S. aureus intradermal skin reinfection model, a clonal population of γδ T cells was found to expand in the draining lymph nodes and traffic to the site of infection to confer protection against a secondary S. aureus intradermal infection (
      • Dillen C.A.
      • Pinsker B.L.
      • Marusina A.I.
      • Merleev A.A.
      • Farber O.N.
      • Liu H.
      • et al.
      Clonally expanded γδ T cells protect against Staphylococcus aureus skin reinfection.
      ). Despite the widespread use of S. aureus intradermal models of infection, inherent biological differences between mice and humans need to be considered, such as the activity of specific S. aureus toxins that are highly active against human but not mouse cells, especially superantigens such as toxic shock syndrome toxin-1 (
      • Salgado-Pabón W.
      • Schlievert P.M.
      Models matter: the search for an effective Staphylococcus aureus vaccine.
      ). To overcome this limitation, humanized mice that express the human receptors (e.g., HLA-DR4 transgenic mice) targeted by these S. aureus toxins have been developed, in which toxic shock syndrome toxin-1 has superantigen activity (
      • Xu S.X.
      • Gilmore K.J.
      • Szabo P.A.
      • Zeppa J.J.
      • Baroja M.L.
      • Haeryfar S.M.
      • et al.
      Superantigens subvert the neutrophil response to promote abscess formation and enhance Staphylococcus aureus survival in vivo.
      ). Mouse immune cells are also less sensitive to the cytolytic activity of Panton‒Valentine leukocidin and α-hemolysin (
      • Hongo I.
      • Baba T.
      • Oishi K.
      • Morimoto Y.
      • Ito T.
      • Hiramatsu K.
      Phenol-soluble modulin alpha 3 enhances the human neutrophil lysis mediated by Panton-Valentine leukocidin.
      ,
      • Spaan A.N.
      • Henry T.
      • van Rooijen W.J.M.
      • Perret M.
      • Badiou C.
      • Aerts P.C.
      • et al.
      The staphylococcal toxin Panton-Valentine Leukocidin targets human C5a receptors.
      ,
      • Tseng C.W.
      • Biancotti J.C.
      • Berg B.L.
      • Gate D.
      • Kolar S.L.
      • Müller S.
      • et al.
      Increased susceptibility of humanized NSG mice to Panton-Valentine leukocidin and Staphylococcus aureus skin infection.
      ). However, whereas α-hemolysin mainly has cytolytic activity against leukocytes and development of large purulent abscesses in humans, it induces keratinocyte cell death in mice that manifests as large dermonecrotic lesions (
      • Kennedy A.D.
      • Bubeck Wardenburg J.
      • Gardner D.J.
      • Long D.
      • Whitney A.R.
      • Braughton K.R.
      • et al.
      Targeting of alpha-hemolysin by active or passive immunization decreases severity of USA300 skin infection in a mouse model.
      ). Additional interrogation of the pathophysiology and immunologic responses can be done by histological, flow cytometric, RNA, or protein analyses to verify the relevance and validity of phenotypic observations (
      • Marchitto M.C.
      • Dillen C.A.
      • Liu H.
      • Miller R.J.
      • Archer N.K.
      • Ortines R.V.
      • et al.
      Clonal Vγ6+Vδ4+ T cells promote IL-17-mediated immunity against Staphylococcus aureus skin infection.
      ). Mice have an abundant population of γδ T cells called dendritic epidermal T cells, which are not present in human epidermis (albeit 1–10% of resident T cells in the dermis of human skin are γδ T cells) (
      • Nielsen M.M.
      • Witherden D.A.
      • Havran W.L.
      γδ T cells in homeostasis and host defence of epithelial barrier tissues.
      ). Mice also have more subsets of γδ T cells that reside at different layers of the skin with both conserved and distinct physiological functions as those in humans (
      • Girardi M.
      Immunosurveillance and immunoregulation by gammadelta T cells.
      ,
      • Suwanpradid J.
      • Holcomb Z.E.
      • MacLeod A.S.
      Emerging skin T-cell functions in response to environmental insults.
      ). In addition to the differences in skin resident immune cells, there are other general immunologic differences between the two species that could lead to discrepancies in infection outcomes between mice and humans (
      • McGovern N.
      • Schlitzer A.
      • Gunawan M.
      • Jardine L.
      • Shin A.
      • Poyner E.
      • et al.
      Human dermal CD14⁺ cells are a transient population of monocyte-derived macrophages.
      ). In humans, neutrophils make up the majority of circulating leukocytes, whereas lymphocytes exist in higher percentages in mice (
      • Mestas J.
      • Hughes C.C.
      Of mice and not men: differences between mouse and human immunology.
      ). Furthermore, differences in hair follicles also contribute to differential protective mechanisms in mouse and human skin by influencing the accessibility, mobility, and communication of epithelial cells that initiate an innate immune response against foreign pathogens (
      • Al-Nuaimi Y.
      • Baier G.
      • Watson R.E.
      • Chuong C.M.
      • Paus R.
      The cycling hair follicle as an ideal systems biology research model.
      ,
      • Bekeredjian-Ding I.
      • Stein C.
      • Uebele J.
      The innate immune response against Staphylococcus aureus.
      ,
      • Oh J.W.
      • Kloepper J.
      • Langan E.A.
      • Kim Y.
      • Yeo J.
      • Kim M.J.
      • et al.
      A guide to studying human hair follicle cycling in vivo.
      ). Therefore, it is important to consider a broad spectrum of differences between mice and humans when performing skin infection models.

      Wound infection models

      S. aureus is the most common pathogen isolated from infected skin wounds, with patients with diabetes being particularly susceptible to the development of chronic, nonhealing wounds (
      • Dunyach-Remy C.
      • Ngba Essebe C.
      • Sotto A.
      • Lavigne J.P.
      Staphylococcus aureus toxins and diabetic foot ulcers: role in pathogenesis and interest in diagnosis.
      ,
      • Giurato L.
      • Meloni M.
      • Izzo V.
      • Uccioli L.
      Osteomyelitis in diabetic foot: a comprehensive overview.
      ,
      • Tong S.Y.
      • Davis J.S.
      • Eichenberger E.
      • Holland T.L.
      • Fowler V.G.
      Staphylococcus aureus infections: epidemiology, pathophysiology, clinical manifestations, and management.
      ). Pseudomonas aeruginosa is another invasive bacterial species commonly found in wounds that causes severe tissue damage (
      • Mutluoglu M.
      • Uzun G.
      Pseudomonas infection in a postoperative foot wound.
      ,
      • Sivanmaliappan T.S.
      • Sevanan M.
      Antimicrobial susceptibility patterns of Pseudomonas aeruginosa from diabetes patients with foot ulcers.
      ). Mouse wound infection models replicate multiple features of infected human wounds, such as purulent drainage, necrotic debris, and delayed wound healing. The mouse wound infection model is performed by inoculating bacteria into full-thickness incisional cuts or excisional wounds (
      • Dai T.
      • Kharkwal G.B.
      • Tanaka M.
      • Huang Y.Y.
      • Bil de Arce V.J.
      • Hamblin M.R.
      Animal models of external traumatic wound infections.
      ). For example, incisional wounds can be inoculated with a bioluminescent S. aureus strain in lysozyme M-EGFP reporter mice to longitudinally monitor both the bacterial burden and neutrophil recruitment dynamics during the course of infection and wound healing (Figure 2a–d) (
      • Anderson L.S.
      • Reynolds M.B.
      • Rivara K.R.
      • Miller L.S.
      • Simon S.I.
      A mouse model to assess innate immune response to Staphylococcus aureus infection.
      ,
      • Kim M.H.
      • Liu W.
      • Borjesson D.L.
      • Curry F.R.
      • Miller L.S.
      • Cheung A.L.
      • et al.
      Dynamics of neutrophil infiltration during cutaneous wound healing and infection using fluorescence imaging.
      ). Furthermore, histological analysis of the infected wound skin can be used to analyze neutrophil abscess area, bacterial bandwidth, and the presence of specific cells (Figure 2e) (
      • Cho J.S.
      • Zussman J.
      • Donegan N.P.
      • Ramos R.I.
      • Garcia N.C.
      • Uslan D.Z.
      • et al.
      Noninvasive in vivo imaging to evaluate immune responses and antimicrobial therapy against Staphylococcus aureus and USA300 MRSA skin infections.
      ). The benefits of these different wound infection models include the ability to replicate polymicrobial infections that typically occur in human wounds (
      • Dalton T.
      • Dowd S.E.
      • Wolcott R.D.
      • Sun Y.
      • Watters C.
      • Griswold J.A.
      • et al.
      An in vivo polymicrobial biofilm wound infection model to study interspecies interactions.
      ,
      • Pastar I.
      • Nusbaum A.G.
      • Gil J.
      • Patel S.B.
      • Chen J.
      • Valdes J.
      • et al.
      Interactions of methicillin resistant Staphylococcus aureus USA300 and Pseudomonas aeruginosa in polymicrobial wound infection.
      ). By infecting the wounds of mice with diabetes with polymicrobial isolates from human diabetic foot ulcers,
      • Kalan L.R.
      • Meisel J.S.
      • Loesche M.A.
      • Horwinski J.
      • Soaita I.
      • Chen X.
      • et al.
      Strain- and species-level variation in the microbiome of diabetic wounds is associated with clinical outcomes and therapeutic efficacy.
      were able to correlate strain-specific S. aureus phenotypes in mice with patient outcomes. With the availability of a new strain of bioluminescent S. aureus expressing click beetle red luciferase and a P. aeruginosa lux strain, it is now possible to longitudinally and noninvasively monitor the dynamics of each bacterial strain simultaneously in the context of wound infection (
      • Miller R.J.
      • Crosby H.A.
      • Schilcher K.
      • Wang Y.
      • Ortines R.V.
      • Mazhar M.
      • et al.
      Development of a Staphylococcus aureus reporter strain with click beetle red luciferase for enhanced in vivo imaging of experimental bacteremia and mixed infections.
      ). In addition, different strains of genetically engineered mice with diabetes exist that exhibit impaired host defense against S. aureus wound infections similar to human who have diabetes (
      • Guo Y.
      • Ramos R.I.
      • Cho J.S.
      • Donegan N.P.
      • Cheung A.L.
      • Miller L.S.
      In vivo bioluminescence imaging to evaluate systemic and topical antibiotics against community-acquired methicillin-resistant Staphylococcus aureus-infected skin wounds in mice.
      ,
      • Ortines R.V.
      • Liu H.
      • Cheng L.I.
      • Cohen T.S.
      • Lawlor H.
      • Gami A.
      • et al.
      Neutralizing alpha-toxin accelerates healing of Staphylococcus aureus-infected wounds in nondiabetic and diabetic mice.
      ). It is important to consider the route and depth of infection in the skin as these can affect the immunologic processes involved. For instance, both IL-1α and IL-1β were found to be involved in neutrophil recruitment and immunity against an S. aureus wound infection, whereas IL-1β played a more predominant role against an intradermal S. aureus infection (
      • Cho J.S.
      • Zussman J.
      • Donegan N.P.
      • Ramos R.I.
      • Garcia N.C.
      • Uslan D.Z.
      • et al.
      Noninvasive in vivo imaging to evaluate immune responses and antimicrobial therapy against Staphylococcus aureus and USA300 MRSA skin infections.
      ,
      • Yan C.
      • Gao N.
      • Sun H.
      • Yin J.
      • Lee P.
      • Zhou L.
      • et al.
      Targeting imbalance between IL-1β and IL-1 receptor antagonist ameliorates delayed epithelium wound healing in diabetic mouse corneas.
      ). Different cellular compositions between mouse and human skin may lead to challenges in translating findings in mouse wound infection models. Unlike human skin, mouse skin is highly populated with dendritic epidermal T cells, which are responsible for sensing skin injury to promote wound healing and strengthen skin barrier function (
      • MacLeod A.S.
      • Hemmers S.
      • Garijo O.
      • Chabod M.
      • Mowen K.
      • Witherden D.A.
      • et al.
      Dendritic epidermal T cells regulate skin antimicrobial barrier function.
      ). Another limitation of this model is that wound contraction is much more pronounced in mouse skin than in human skin. Some groups have tried to overcome this limitation by covering the wound bed with a transparent breathable film (that also keeps the wound open longer) or suturing a splint to prevent wound contracture (
      • Griffin D.R.
      • Weaver W.M.
      • Scumpia P.O.
      • Di Carlo D.
      • Segura T.
      Accelerated wound healing by injectable microporous gel scaffolds assembled from annealed building blocks.
      ).
      Figure thumbnail gr2
      Figure 2Staphylococcus aureus skin infection in vivo imaging and histology. Three 8 mm in length, parallel scalpel wounds on the backs of (a–d) LysM-EGFP mice or (e) C57BL/6 mice inoculated with 2 × 106 CFUs per 10 μl of Staphylococcus aureus or no bacteria (none). (a) Representative photographs of in vivo S. aureus bioluminescence. (b) In vivo S. aureus burden as measured by in vivo bioluminescence imaging (mean total flux [photons per second] ± SEM) (logarithmic scale). (c) Representative photographs of in vivo EGFP-neutrophil fluorescence. (d) In vivo fluorescence imaging of EGFP-neutrophil infiltration (mean total flux [photons per second] ± SEM). (e) Representative photomicrographs of sections from skin punch biopsies at 1 day after wounding plus S. aureus infection labeled with H&E stain, anti–Gr-1 mAb (neutrophil marker), and Gram stain. Bars = 150 μm. This figure was derived with permission to reproduce the images and data from
      • Cho J.S.
      • Zussman J.
      • Donegan N.P.
      • Ramos R.I.
      • Garcia N.C.
      • Uslan D.Z.
      • et al.
      Noninvasive in vivo imaging to evaluate immune responses and antimicrobial therapy against Staphylococcus aureus and USA300 MRSA skin infections.
      . CFU, colony-forming unit.

      Epicutaneous infection models

      S. aureus commonly colonizes the lesional skin of human patients with atopic dermatitis (AD), and the level of colonization correlates with disease severity (
      • Byrd A.L.
      • Deming C.
      • Cassidy S.K.B.
      • Harrison O.J.
      • Ng W.I.
      • Conlan S.
      • et al.
      Staphylococcus aureus and Staphylococcus epidermidis strain diversity underlying pediatric atopic dermatitis.
      ,
      • Kong H.H.
      • Oh J.
      • Deming C.
      • Conlan S.
      • Grice E.A.
      • Beatson M.A.
      • et al.
      Temporal shifts in the skin microbiome associated with disease flares and treatment in children with atopic dermatitis.
      ). Patients with hyper-IgE syndrome, which is often caused by a dominant negative mutation in STAT3 gene, have been characterized by atopic manifestations and higher susceptibility to S. aureus and/or Candida cutaneous infections as a result of impaired T helper type 17 development (
      • Horváth R.
      • Rožková D.
      • Lašťovička J.
      • Poloučková A.
      • Sedláček P.
      • Sedivá A.
      • et al.
      Expansion of T helper type 17 lymphocytes in patients with chronic granulomatous disease.
      ,
      • Milner J.D.
      • Brenchley J.M.
      • Laurence A.
      • Freeman A.F.
      • Hill B.J.
      • Elias K.M.
      • et al.
      Impaired T(H)17 cell differentiation in subjects with autosomal dominant hyper-IgE syndrome.
      ). These clinical observations have caused intense interest in understanding the role of S. aureus in the immune pathogenesis of AD skin inflammation. To model this, an S. aureus-soaked gauze pad is applied to the shaved and depilated dorsal skin of mice. The erythematous skin inflammation that mimics human with AD conditions in mice can be measured by disease scoring, epidermal thickening, and elevated serum IgE, whereas the increased skin barrier defect can be measured through transepidermal water loss (
      • Alexander H.
      • Brown S.
      • Danby S.
      • Flohr C.
      Research techniques made simple: transepidermal water loss measurement as a research tool.
      ,
      • Nakamura Y.
      • Oscherwitz J.
      • Cease K.B.
      • Chan S.M.
      • Muñoz-Planillo R.
      • Hasegawa M.
      • et al.
      Staphylococcus δ-toxin induces allergic skin disease by activating mast cells.
      ). The use of mouse genetic cre‒lox systems provides another important tool for researchers to identify the cells involved in immune responses by targeting gene deletion in a specific cell type. For example, a cre‒lox mouse with T-cell‒specific deletion of MyD88 was used to uncover a novel role for IL-36–mediated IL-17 T-cell responses in epicutaneous S. aureus‒driven skin inflammation (
      • Liu H.
      • Archer N.K.
      • Dillen C.A.
      • Wang Y.
      • Ashbaugh A.G.
      • Ortines R.V.
      • et al.
      Staphylococcus aureus epicutaneous exposure drives skin inflammation via IL-36-Mediated T cell responses.
      ). Tape stripping of the skin can be performed before epicutaneous skin infection to recapitulate the barrier defect seen in AD skin. In this model, S. aureus‒derived proteases and phenol-soluble modulin alpha, which are under the regulation of the bacteria quorum sensing system, promote skin inflammation by inducing epidermal proteolysis and skin barrier damage (
      • Williams M.R.
      • Costa S.K.
      • Zaramela L.S.
      • Khalil S.
      • Todd D.A.
      • Winter H.L.
      • et al.
      Quorum sensing between bacterial species on the skin protects against epidermal injury in atopic dermatitis.
      ). Similarly, S. aureus was shown to exploit the barrier defect in filaggrin-deficient mice to promote T helper type 2 and T helper type 22 cytokines that are associated with exacerbation of AD skin inflammation (
      • Nakatsuji T.
      • Chen T.H.
      • Two A.M.
      • Chun K.A.
      • Narala S.
      • Geha R.S.
      • et al.
      Staphylococcus aureus exploits epidermal barrier defects in atopic dermatitis to trigger cytokine expression.
      ). The exploitation of skin barrier defects is not limited to S. aureus but also to vaccinia virus, which is the cause of a life-threatening condition called eczema vaccinatum in patients with AD. Furthermore, cutaneous exposure to vaccinia virus in filaggrin-deficient mice through scarification, which recapitulates the route of exposure during smallpox vaccination in humans, showed IL-17A‒mediated dissemination of the virus in the skin (
      • Oyoshi M.K.
      • Beaupré J.
      • Venturelli N.
      • Lewis C.N.
      • Iwakura Y.
      • Geha R.S.
      Filaggrin deficiency promotes the dissemination of cutaneously inoculated vaccinia virus.
      ). Therefore, the epicutaneous infection model is useful in investigating the host- and pathogen-derived factors that contribute to AD-like skin inflammation and AD-associated complications. Nonetheless, some of these models have used depilatory creams that result in baseline skin inflammation. Moreover, the models that wrap a pathogen-soaked gauze pad around the mouse to artificially expose the mouse skin to the pathogen of interest do not truly recapitulate the normal S. aureus colonization of uncovered skin in patients with AD.
      To investigate the crosstalk between the skin microbiome and host immune cells, an alternate epicutaneous infection model has been developed where a bacteria-soaked cotton swab is rubbed onto the shaved backs of mice (
      • Belkaid Y.
      • Segre J.A.
      Dialogue between skin microbiota and immunity.
      ,
      • Kugelberg E.
      • Norström T.
      • Petersen T.K.
      • Duvold T.
      • Andersson D.I.
      • Hughes D.
      Establishment of a superficial skin infection model in mice by using Staphylococcus aureus and Streptococcus pyogenes.
      ). This model was instrumental in understanding how skin discriminates between commensal and pathogenic skin microbes. In particular, the commensal Staphylococcus epidermidis promoted T regulatory cell expansion and skin immune tolerance in a crucial window in neonatal life (
      • Scharschmidt T.C.
      • Vasquez K.S.
      • Truong H.A.
      • Gearty S.V.
      • Pauli M.L.
      • Nosbaum A.
      • et al.
      A wave of regulatory T cells into neonatal skin mediates tolerance to commensal microbes.
      ). However, S. aureus manipulated IL-1β release to inhibit T regulatory cell expansion and induce skin inflammation (
      • Leech J.M.
      • Dhariwala M.O.
      • Lowe M.M.
      • Chu K.
      • Merana G.R.
      • Cornuot C.
      • et al.
      Toxin-triggered interleukin-1 receptor signaling enables early-life discrimination of pathogenic versus commensal skin bacteria.
      ). Furthermore, the model has been used to understand how the commensal bacterial strain S. epidermis promotes protection against pathogens as well as accelerate wound healing (
      • Linehan J.L.
      • Harrison O.J.
      • Han S.J.
      • Byrd A.L.
      • Vujkovic-Cvijin I.
      • Villarino A.V.
      • et al.
      Non-classical immunity controls microbiota impact on skin immunity and tissue repair.
      ). In contrast, epicutaneous inoculation with Corynebacterium accolens promoted skin inflammation through the activation of long-lasting skin T cells (
      • Ridaura V.K.
      • Bouladoux N.
      • Claesen J.
      • Chen Y.E.
      • Byrd A.L.
      • Constantinides M.G.
      • et al.
      Contextual control of skin immunity and inflammation by Corynebacterium.
      ). In addition, isolated S. aureus strains colonizing the skin of humans with AD induced more skin inflammation than laboratory strains isolated from other body sites (
      • Byrd A.L.
      • Deming C.
      • Cassidy S.K.B.
      • Harrison O.J.
      • Ng W.I.
      • Conlan S.
      • et al.
      Staphylococcus aureus and Staphylococcus epidermidis strain diversity underlying pediatric atopic dermatitis.
      ). Alternatively, Candida albicans was applied to the skin to interrogate a role for cutaneous sensory neurons in host defense (
      • Kashem S.W.
      • Riedl M.S.
      • Yao C.
      • Honda C.N.
      • Vulchanova L.
      • Kaplan D.H.
      Nociceptive sensory fibers drive interleukin-23 production from CD301b+ dermal dendritic cells and drive protective cutaneous immunity.
      ). Despite the usefulness of the swab epicutaneous model, it is generally done with multiple bacterial (or fungal) applications that might not fully replicate the normal colonization of commensal microbes on human skin.

      Human skin xenograft model

      Because of the inherent differences between human and mouse skin, human skin xenografts can be used to validate and translate the findings in mouse models to human skin (
      • Parker D.
      Humanized mouse models of Staphylococcus aureus infection.
      ). To prevent graft rejection, human skin biopsies are sutured onto immunodeficient mice that include NSG (NOD.Cg-Prkdcscid IL2rgtm1Wjl), NOG (NOD.cg-Prkdcscid IL2rgtm1Sug), and NRG (NOD.Cg-Rag1tm1Mom IL2rgtm1Wj) mice, all of which lack T, B, and NK cells (
      • Kenney L.L.
      • Shultz L.D.
      • Greiner D.L.
      • Brehm M.A.
      Humanized mouse models for transplant immunology.
      ). Moreover, it is possible to perform human skin xenografts in combination with engraftment of CD34+ stem cells (allowing the development of human immune cells in the same mice) to provide the new in vivo capability to study the human immune system in the context of a human skin infection (
      • Brehm M.A.
      • Racki W.J.
      • Leif J.
      • Burzenski L.
      • Hosur V.
      • Wetmore A.
      • et al.
      Engraftment of human HSCs in nonirradiated newborn NOD-scid IL2rγ null mice is enhanced by transgenic expression of membrane-bound human SCF.
      ). There are numerous advantages for the use of human skin, including healthy samples that are readily available and engrafted skin tissue with epidermal and dermal layers and vascularized skin that closely resembles normal human skin. For example,
      • Soong G.
      • Paulino F.
      • Wachtel S.
      • Parker D.
      • Wickersham M.
      • Zhang D.
      • et al.
      Methicillin-resistant Staphylococcus aureus adaptation to human keratinocytes.
      demonstrated that toxin-deficient, agr-mutants of S. aureus are able to persist on the human skin by stimulating autophagy. In addition, epicutaneously swabbed S. aureus on human skin xenografts led to local production of IL-8, which induced neutrophil migration to the skin to promote bacterial clearance (
      • Schulz A.
      • Jiang L.
      • de Vor L.
      • Ehrström M.
      • Wermeling F.
      • Eidsmo L.
      • et al.
      Neutrophil recruitment to noninvasive MRSA at the stratum corneum of human skin mediates transient colonization.
      ). Studies involving human skin xenograft infections are not widely used and thus represent an exciting opportunity in the dermatology field to translate the immunologic findings from mouse skin infection models to human skin.

      Conclusion

      Mouse models of skin infection remain the most commonly used model of skin infections owing to their relatively inexpensive experimental costs as well as the opportunity to take advantage of genetically engineered mice and in vivo optical imaging techniques. Currently, a great variety of skin infection models and genetically engineered mice are readily available, which serve as extremely valuable tools for noninvasive and longitudinal monitoring of the underlying immune responses and host–pathogen interactions that occur during skin infections. Mouse skin infection models will continue to be essential for better understanding of skin immunologic responses in different contexts, including skin colonization, impetiginization, abscesses, and wounds as well as in the setting of diseases such as AD and diabetes. Unfortunately, mouse models cannot completely replicate the pathogenesis of the human disease. Therefore, these limitations need to be considered when translating the results to cutaneous immune responses in human skin (summarized in Table 1). Further advancements in humanized skin xenografts in immunocompromised mice are continually being developed to help validate and improve the discrepancies between the species.

      Conflict of Interest

      LSM is a full-time employee at Janssen Research and Development and may own Johnson & Johnson stock and stock options. He has received grant support from AstraZeneca, MedImmune (a subsidiary of AstraZeneca), Pfizer, Boehringer Ingelheim, Regeneron Pharmaceuticals, and Moderna Therapeutics. He is a shareholder of Noveome Biotherapeutics, was a paid consultant for Almirall and Janssen Research and Development, and was on the scientific advisory board of Integrated Biotherapeutics, which are all developing therapeutics against infections (including S. aureus and other pathogens) and/or inflammatory conditions. NKA has received grant support from Pfizer.

      Multiple Choice Questions

      • 1.
        Which of the following would be the most immunologically relevant purpose to reinfect mice in a skin infection model?
        • A.
          To study primary T-cell responses to skin infection
        • B.
          To examine memory T-cell responses to skin infection
        • C.
          To study innate immune responses during initial skin infection
        • D.
          To study polymicrobial infections
      • 2.
        Which of the genetically engineered mouse strains can be used to monitor cytokine expression kinetics during skin infections?
        • A.
          IL-17A/F knockout mouse
        • B.
          Mouse with specific IL-17A/F deletion in T cells
        • C.
          IL-17A-tdTomato/IL-17F-GFP dual-color reporter mice
        • D.
          All of the above
      • 3.
        Which of the following skin infection models has the potential to be used with human skin xenografts?
        • A.
          Epicutaneous model
        • B.
          Intradermal model
        • C.
          Wound model
        • D.
          All of the above
      • 4.
        The epicutaneous skin infection model replicates which type of skin inflammation?
        • A.
          Atopic dermatitis
        • B.
          Psoriasis
        • C.
          Vitiligo
        • D.
          Alopecia areata
      • 5.
        Which bacterium is the leading cause of skin infections in humans?
        • A.
          Staphylococcus epidermidis
        • B.
          Pseudomonas aeruginosa
        • C.
          Staphylococcus aureus
        • D.
          Corynebacterium accolens

      Acknowledgments

      This work was funded in part by grants R01AR073665 (LSM and NKA) and R01AR069502 (LSM and NKA) from the US National Institutes of Health (NIH). The content is solely the responsibility of the authors and does not necessarily represent the official views of the US NIH. We apologize to the many researchers in the field whose work could not be included because of space and citation limitations by the journal for this research techniques made simple manuscript.

      Author Contributions

      Writing - Original Draft Preparation: CY, NKA; Writing - Review and Editing: CY, NKA, LSM

      Detailed Answers

      • 1.
        Which of the following would be the most immunologically relevant purpose to reinfect mice in a skin infection model?
      • CORRECT ANSWER: B. To examine memory T-cell responses to skin infection
      • Memory T cells are involved in the secondary response to skin infection.
      • 2.
        Which of the genetically engineered mouse strains can be used to monitor cytokine expression kinetics during skin infections?
      • CORRECT ANSWER: C. IL-17A-tdTomato/IL-17F-GFP dual-color reporter mice
      • The IL-17A-tdTomato/IL-17F-GFP dual-color reporter mouse allows for in vivo visualization of IL-17A and IL-17F with an In Vivo Imaging System.
      • 3.
        Which of the following skin infection models has the potential to be used with human skin xenografts?
      • CORRECT ANSWER: D. All of the above
      • Human skin xenografts can be adapted to work with any of the skin infection models.
      • 4.
        The epicutaneous skin infection model replicates which type of skin inflammation?
      • CORRECT ANSWER: A. Atopic dermatitis
      • The epicutaneous model replicates Staphylococcus aureus colonization and skin inflammation on atopic dermatitis skin.
      • 5.
        Which bacterium is the leading cause of skin infections in humans?
      • CORRECT ANSWER: C. Staphylococcus aureus
      • S. aureus is the leading cause of skin and soft tissue infections in humans.

      Supplementary Material

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