Systemic dissemination and cutaneous damage in a mouse model of staphylococcal skin infections

In vitro and in vivo antibacterial studies of volatile oil from Atractylodis Rhizoma against Staphylococcus pseudintermedius and multidrug resistant Staphylococcus pseudintermedius strains from canine pyoderma

ETHNOPHARMACOLOGICAL RELEVANCE

Atractylodis Rhizoma is extensively employed in Traditional Chinese Medicine for the treatment of skin and gastrointestinal ailments. Its active components have been proven to demonstrate numerous beneficial properties, including antibacterial, antiviral, anti-inflammatory, anti-tumor, and anti-ulcer activities. Furthermore, the volatile oil from Atractylodis Rhizoma (VOAR) has been reported to effectively inhibit and eradicate pathogens such as Staphylococcus aureus, Escherichia coli and Candida albicans. Of particular concern is Staphylococcus pseudintermedius, the predominant pathogen responsible for canine pyoderma, whose increasing antimicrobial resistance poses a serious public health threat. VOAR merits further investigation regarding its antibacterial potential against Staphylococcus pseudintermedius.

AIM OF THE STUDY

The study aims to verify the in vitro antibacterial activity of VOAR against Staphylococcus pseudintermedius. And a superficial skin infection model in mice was established to assess the in vivo therapeutic effect of VOAR.

MATERIALS AND METHODS

Thirty strains of S. pseudintermedius were isolated from dogs with pyoderma, and the drug resistance was analyzed by disc diffusion method. The Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of VOAR were determined through the broth dilution method. The growth curve of bacteria in a culture medium containing VOAR was monitored using a UV spectrophotometer. Scanning electron microscopy was employed to observe the effects of VOAR on the microstructure of S. pseudintermedius. The impact of VOAR on the antibiotic resistance of S. pseudintermedius was assessed using the disc diffusion method. Twenty mice were randomly divided into four groups: the control group, the physiological saline group, the VOAR group, and the amikacin group. With the exception of the control group, the skin barrier of mice was disrupted by tap stripping, and the mice were subsequently inoculated with S. pseudintermedius to establish a superficial skin infection model. The modeled mice were treated with normal saline, VOAR, and amikacin for 5 days. Following the treatment period, the therapeutic effect of each group was evaluated based on the measures of body weight, skin symptoms, tissue bacterial load, tissue IL-6 content, and histopathological changes.

RESULTS

The MIC and MBC of VOAR against 30 clinical isolates of S. pseudintermedius were found to be 0.005425% and 0.016875%, respectively. VOAR could exhibit the ability to delay the entry of bacteria into the logarithmic growth phase, disrupt the bacterial structure, and enhance the antibacterial zone in conjunction with antibiotic drugs. In the superficial skin infection model mice, VOAR significantly reduced the scores for skin redness (P < 0.0001), scab formation (P < 0.0001), and wrinkles (P < 0.0001). Moreover, VOAR markedly reduced the bacterial load (P < 0.001) and IL-6 content (P < 0.0001) in the skin tissues of mice. Histopathological observations revealed that the full-layer skin structure in the VOAR group was more complete, with clearer skin layers, and showed significant improvement in inflammatory cell infiltration and fibroblast proliferation compared to other groups.

CONCLUSION

The results demonstrate that VOAR effectively inhibits and eradicates Staphylococcus pseudintermedius in vitro while also enhancing the pathogen's sensitivity to antibiotics. Moreover, VOAR exhibits a pronounced therapeutic effect in the superficial skin infection model mice.

Efficacy of a Novel Antibacterial Agent Exeporfinium Chloride, (XF-73), Against Antibiotic-Resistant Bacteria in Mouse Superficial Skin Infection Models

Background

The number of incidences of antimicrobial resistance is rising continually, necessitating new and effective antibacterial drugs. The present study aimed to assess the in vitro and in vivo activity of XF-73 against antibiotic-resistant Staphylococcus aureus (S. aureus) isolates and to investigate the potential mechanism of action of XF-73.

Methods

The in vitro antibacterial activity of XF-73 and comparator antibacterial drugs, (mupirocin, fusidine, retapamulin, vancomycin, erythromycin, linezolid and daptomycin), against S. aureus (both antibiotic sensitive and resistant strains) was assessed using a broth microdilution method. Two different superficial Staphylococcal skin infection murine models were established to study the in vivo efficacy of XF-73 against antibiotic-resistant strains. The effect of XF-73 on the ultrastructure and cellular morphology of S. aureus was studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Results

The MICs (minimum inhibitory concentration) determined by the broth microdilution method for XF-73 demonstrated that the compound had a high potency against S. aureus isolates with varying susceptibility to the study drugs. Also, the antibacterial activity of XF-73 was superior or similar to most of the tested antibacterial drugs. We also found that the XF-73 dermal formulation significantly inhibited S. aureus survival in both the murine skin tape-stripping and suture superficial skin infection models, maintained a consistently high inhibitory capacity against the antibiotic-resistant strains tested and was significantly more effective than mupirocin ointment, a commonly used antibiotic for the treatment of skin infections. The morphological studies using TEM suggest that XF-73 had a rapid (2 minute) bacterial cell wall disruption activity, with longer incubation (10 minute) subsequently causing membrane damage. SEM analysis demonstrated that this cell wall and cell membrane disruption did not lead to disintegration of the plasma membrane, and did not cause bacterial cell lysis.

Conclusion

Therefore, XF-73 may be an effective drug alternative to combat multi-drug-resistant skin infections in the clinical setting.

Study on skin infection model of Staphylococcus aureus based on analytic hierarchy process and Delphi method

Purpose

Infectious skin diseases are a type of inflammatory skin lesions caused by pathogenic microorganisms. Because of the uncertainty of methodology, the skin infection model usually have low replication rate and lack of good evaluation system. We aimed to establish multi-index and comprehensive evaluation method for Staphylococcus aureus (S.aureus) skin-infection models through Analytic hierarchy process (AHP) and Delphi method, and screen high quality animal models through it.

Materials and methods

Firstly, the evaluation indicators of skin infection were collected basing on literature research. The weight of the evaluation indicators were decided according to AHP and Delphi method. Then different ulcer models (mouse or rat) infected by S. aureus were selected as the research objects.

Results

The evaluation indicators were classified into four groups of criteria (including ten sub-indicators) and given different weights, physical sign changes (0.0518), skin lesion appearance (0.2934), morphological observation (0.3184), etiological examination (0.3364). Through the evaluation system, we screened and found that the mouse ulcer model which caused by a round wound and 1.0 × 10¹⁰ CFU/mL (0.1 mL) bacterial concentration got the highest comprehensive score, and also found that the model which caused by a 1.5 cm-round wound and 1.0 × 10¹⁰ CFU/mL (0.2 mL) maybe the best rat ulcer model.

Conclusions

This study has established an evaluation system based on AHP and Delphi method, also provided the best skin ulcer models selected by this system, the models are suitable for disease research and drug development research of skin ulcer.

Impetigo Animal Models: A Review of Their Feasibility and Clinical Utility for Therapeutic Appraisal of Investigational Drug Candidates

Impetigo (school sores), a superficial skin infection commonly seen in children, is caused by the gram-positive bacteria Staphylococcus aureus and/or Streptococcus pyogenes. Antibiotic treatments, often topical, are used as the first-line therapy for impetigo. The efficacy of potential new antimicrobial compounds is first tested in in vitro studies and, if effective, followed by in vivo studies using animal models and/or humans. Animal models are critical means for investigating potential therapeutics and characterizing their safety profile prior to human trials. Although several reviews of animal models for skin infections have been published, there is a lack of a comprehensive review of animal models simulating impetigo for the selection of therapeutic drug candidates. This review critically examines the existing animal models for impetigo and their feasibility for testing the in vivo efficacy of topical treatments for impetigo and other superficial bacterial skin infections.

Thymoquinone-Loaded Polymeric Films and Hydrogels for Bacterial Disinfection and Wound Healing

The purpose of this study was to synthesize and characterize novel biocompatible topical polymeric film and hydrogel systems that have the potential to deliver the antibacterial agent thymoquinone (TQ) directly to the skin target site to manage the local wound infection and thereby wound healing. The polyvinyl pyrrolidone (PVP) matrix-type films containing TQ were prepared by the solvent casting method. In vitro skin permeation studies on human cadaver skin produced a mean flux of 2.3 µg TQ/cm²/h. Human keratinocyte monolayers subjected to a scratch wound (an in vitro wound healing assay) showed 85% wound closure at day 6 in the TQ group (100 ng/mL TQ) as compared to 50% in the vehicle control group (p = 0.0001). In a zone-of-inhibition (ZOI) assay, TQ-containing films and hydrogels completely wiped out Staphylococcus aureus in 10 cm diameter Tryptic Soy Agar plates while 500 µg/mL gentamicin containing filters gave 10 mm of ZOI. In an ex vivo model, TQ-containing films eradicated bacterial colonization on human cadaver skin. Furthermore, in a full-thickness wound infection model in mice, TQ-containing films showed significant activity in controlling Staphylococcus aureus infection, thereby disinfecting the skin wound. In summary, TQ-containing PVP films and hydrogels developed in this study have the potential to treat and manage wound infections.

A murine model of Staphylococcus aureus infected chronic diabetic wound: A new tool to develop alternative therapeutics

Diabetic wound infection is a frequent complication that may result in limb amputation. To develop new treatment strategies in response to increasing bacterial resistance, animal models are needed. We created a diabetic mouse model with chronically infected wounds. Diabetes was induced using streptozotocin, and wounds were performed using a biopsy punch, and then infected with a clinical strain of Staphylococcus aureus. Chronification was reached by delaying healing thanks to chemical products (aminotriazole and mercaptosuccinic acid). Overall survival, as well as clinical, bacteriological and immunological data in skin, blood and spleens were collected at days 1, 7, and 14 after wounding. After a transient bacteremia proved by bacteria presence in spleen and kidneys in the first days after wounding, infected mice showed a chronic infection, with a bioburden impairing the healing process, and bacteria persistence compared to control mice. Infected mice showed gradual increasing skin levels of IL-17A compared to control mice that resulted in an IL-17/IFN-γ inbalance, pointing out a localized Th17 polarization of the immune response. Whether infected or not, the skin level of IL-10 decreased dramatically at days 1 and 7 after wounding, with an increase observed only in the control mice at day 14. After a decrease at day 1 in both groups, spleen IL-10 showed a rather steady level at days 7 and 14 in the control group compared with the decrease observed in the infected group. The spleen IL-10/IFN-γ ratio showed a systemic inflammatory response with Th1 polarization. Therefore, this model provides useful data to study wound healing. It is easy to reproduce, affordable and offers clinical and biological tools to evaluate new therapeutics.

Berberine-Incorporated Shape Memory Fiber Applied as a Novel Surgical Suture

The surgical suture has long been used to reconnect the injured tissues to restore their structure and function. However, its utility remains challenging in many areas, such as surgical site infections and minimally invasive surgeries. Herein, we report a novel surgical suture that possesses both antibacterial activity and shape memory effect to address these issues. In detail, natural antibacterial berberine was incorporated directly into the spinning solution of shape memory polyurethane with a near body transition temperature, and then berberine-containing polyurethane (BP) fibers were prepared by a facile one-step wet-spinning method for surgical suture. The prepared BP fibers were micro-sized and characterized by their transition temperature, morphology, water contact angles, mechanical properties, in vitro shape memory effect, drug release, and antibacterial activity. The results showed that with the increasing amount of the incorporated berberine, the transition temperatures of the fibers were not significantly affected, remains at near body temperature, while the contact angles of the fibers were significantly decreased and the mechanical properties of the fibers were significantly weakened. The optimized fiber was selected to evaluate the cytotoxicity and in vivo biocompatibility before in vivo shape memory effect and wound healing capacity in a mouse skin suture-wound model was tested. Besides the shape memory effect, it was demonstrated that the fiber is capable of antibacterial activity and anti-inflammatory effect, and promoting wound healing. The mechanism of the antibacterial activity and anti-inflammatory effect of the fiber was discussed. Overall, it is expected that by the berberine added to the fiber for surgical suture, it will be more popular and extend the utility of the sutures in a wide range of clinical applications.

TNF overproduction impairs epithelial staphylococcal response in hyper IgE syndrome

Autosomal dominant hyper IgE syndrome (AD-HIES), or Job's syndrome, is a primary immune deficiency caused by dominant-negative mutations in STAT3. Recurrent Staphylococcus aureus skin abscesses are a defining feature of this syndrome. A widely held hypothesis that defects in peripheral Th17 differentiation confer this susceptibility has never been directly evaluated. To assess the cutaneous immune response in AD-HIES, we induced suction blisters in healthy volunteers (HVs) and patients with AD-HIES and then challenged the wound with lethally irradiated bacteria. We show that cutaneous production of IL-17A and IL-17F was normal in patients with AD-HIES. Overproduction of TNF-α differentiated the responses in AD-HIES from HVs. This was associated with reduced IL-10 family signaling in blister-infiltrating cells and defective epithelial cell function. Mouse models of AD-HIES recapitulated these aberrant epithelial responses to S. aureus and involved defective epithelial-to-mesenchymal transition (EMT) rather than a failure of bacterial killing. Defective responses in mouse models of AD-HIES and primary keratinocyte cultures from patients with AD-HIES could be reversed by TNF-α blockade and by drugs with reported modulatory effects on EMT. Our results identify these as potential therapeutic approaches in patients with AD-HIES suffering S. aureus infections.

Staphylococcus aureus penetrate the interkeratinocyte spaces created by skin-infiltrating neutrophils in a mouse model of impetigo

BACKGROUND

Impetigo is a bacterial skin disease characterized by intraepidermal neutrophilic pustules. Previous studies have demonstrated that exfoliative toxin producing staphylococci are isolated in the cutaneous lesions of human and canine impetigo. However, the mechanisms of intraepidermal splitting in impetigo remain poorly understood.

OBJECTIVE

To determine how staphylococci penetrate the living epidermis and create intraepidermal pustules in vivo using a mouse model of impetigo.

METHODS

Three Staphylococcus aureus strains harbouring the etb gene and three et gene negative strains were epicutaneously inoculated onto tape-stripped mouse skin. The skin samples were subjected to time course histopathological and immunofluorescence analyses to detect intraepidermal neutrophils and infiltrating staphylococci. To determine the role of neutrophils on intraepidermal bacterial invasion, cyclophosphamide (CPA) was injected intraperitoneally into the mice to cause leucopenia before the inoculation of etb gene positive strains.

RESULTS

In mice inoculated with etb gene positive S. aureus, intraepidermal pustules resembling impetigo were detected as early as 4 h post-inoculation (hpi). Neutrophils in the epidermis were detected from 4 hpi, whereas intraepidermal staphylococci was detected from 6 hpi. The dimensions of the intraepidermal clefts created in mice inoculated with etb gene positive strains at 6 hpi were significantly larger than those in mice inoculated with et gene negative strains. In CPA treated mice, staphylococci or neutrophils were not detected in the deep epidermis until 6 hpi.

CONCLUSION

Our findings indicate that intraepidermal neutrophils play an important role in S. aureus invasion into the living epidermis in a mouse model of impetigo.

Pathogenesis of Staphylococcus aureus Bloodstream Infections

Staphylococcus aureus, a Gram-positive bacterium colonizing nares, skin, and the gastrointestinal tract, frequently invades the skin, soft tissues, and bloodstreams of humans. Even with surgical and antibiotic therapy, bloodstream infections are associated with significant mortality. The secretion of coagulases, proteins that associate with and activate the host hemostatic factor prothrombin, and the bacterial surface display of agglutinins, proteins that bind polymerized fibrin, are key virulence strategies for the pathogenesis of S. aureus bloodstream infections, which culminate in the establishment of abscess lesions. Pathogen-controlled processes, involving a wide spectrum of secreted factors, are responsible for the recruitment and destruction of immune cells, transforming abscess lesions into purulent exudate, with which staphylococci disseminate to produce new infectious lesions or to infect new hosts. Research on S. aureus bloodstream infections is a frontier for the characterization of protective vaccine antigens and the development of immune therapeutics aiming to prevent disease or improve outcomes.

Perforin-2 is essential for intracellular defense of parenchymal cells and phagocytes against pathogenic bacteria

Perforin-2 (MPEG1) is a pore-forming, antibacterial protein with broad-spectrum activity. Perforin-2 is expressed constitutively in phagocytes and inducibly in parenchymal, tissue-forming cells. In vitro, Perforin-2 prevents the intracellular replication and proliferation of bacterial pathogens in these cells. Perforin-2 knockout mice are unable to control the systemic dissemination of methicillin-resistant Staphylococcus aureus (MRSA) or Salmonella typhimurium and perish shortly after epicutaneous or orogastric infection respectively. In contrast, Perforin-2-sufficient littermates clear the infection. Perforin-2 is a transmembrane protein of cytosolic vesicles -derived from multiple organelles- that translocate to and fuse with bacterium containing vesicles. Subsequently, Perforin-2 polymerizes and forms large clusters of 100 Å pores in the bacterial surface with Perforin-2 cleavage products present in bacteria. Perforin-2 is also required for the bactericidal activity of reactive oxygen and nitrogen species and hydrolytic enzymes. Perforin-2 constitutes a novel and apparently essential bactericidal effector molecule of the innate immune system.

The νSaα Specific Lipoprotein Like Cluster (lpl) of S. aureus USA300 Contributes to Immune Stimulation and Invasion in Human Cells

All Staphylococcus aureus genomes contain a genomic island, which is termed νSaα and characterized by two clusters of tandem repeat sequences, i.e. the exotoxin (set) and 'lipoprotein-like' genes (lpl). Based on their structural similarities the νSaα islands have been classified as type I to IV. The genomes of highly pathogenic and particularly epidemic S. aureus strains (USA300, N315, Mu50, NCTC8325, Newman, COL, JH1 or JH9) belonging to the clonal complexes CC5 and CC8 bear a type I νSaα island. Since the contribution of the lpl gene cluster encoded in the νSaα island to virulence is unclear to date, we deleted the entire lpl gene cluster in S. aureus USA300. The results showed that the mutant was deficient in the stimulation of pro-inflammatory cytokines in human monocytes, macrophages and keratinocytes. Purified lipoprotein Lpl1 was further shown to elicit a TLR2-dependent response. Furthermore, heterologous expression of the USA300 lpl cluster in other S. aureus strains enhanced their immune stimulatory activity. Most importantly, the lpl cluster contributed to invasion of S. aureus into human keratinocytes and mouse skin and the non-invasive S. carnosus expressing the lpl gene cluster became invasive. Additionally, in a murine kidney abscess model the bacterial burden in the kidneys was higher in wild type than in mutant mice. In this infection model the lpl cluster, thus, contributes to virulence. The present report is one of the first studies addressing the role of the νSaα encoded lpl gene cluster in staphylococcal virulence. The finding that the lpl gene cluster contributes to internalization into non-professional antigen presenting cells such as keratinocytes highlights the lpl as a new cell surface component that triggers host cell invasion by S. aureus. Increased invasion in murine skin and an increased bacterial burden in a murine kidney abscess model suggest that the lpl gene cluster serves as an important virulence factor.

Highly pathogenic and epidemic Staphylococcus aureus strains carry a pathogenicity island in their genome that contains a cluster of lipoprotein-encoding genes termed lpl. As the role lpl in virulence is still unclear, we deleted the entire lpl cluster in the community-acquired methicillin-resistant S. aureus (CA-MRSA) USA300 and found that the mutant was defective in stimulation of pro-inflammatory cytokines in human immune cells. Moreover, the major finding highlighted in this study is that the lpl cluster contributes to invasion into non-professional phagocytes such as epithelial cells and keratinocytes. Furthermore, the lpl-dependent increase in invasive activity, most likely, accounts for the enhanced bacterial burden observed in a murine kidney abscess model. In general, internalization of a pathogen into host epithelial cells shields the pathogen from immune defense and antibiotic treatment. However, further investigation is needed to clarify whether the increased ability to invade host cells is responsible for the potent disseminative activity and hypervirulent phenotype characterizing the νSaα type I island expressing S. aureus strains, including the USA300 CA-MRSA strain.

Mouse models for infectious diseases caused by Staphylococcus aureus

Staphylococcus aureus - a commensal of the human skin, nares and gastrointestinal tract - is also a leading cause of bacterial skin and soft tissue infection (SSTIs), bacteremia, sepsis, peritonitis, pneumonia and endocarditis. Antibiotic-resistant strains, designated MRSA (methicillin-resistant S. aureus), are common and represent a therapeutic challenge. Current research and development efforts seek to address the challenge of MRSA infections through vaccines and immune therapeutics. Mice have been used as experimental models for S. aureus SSTI, bacteremia, sepsis, peritonitis and endocarditis. This work led to the identification of key virulence factors, candidate vaccine antigens or immune-therapeutics that still require human clinical testing to establish efficacy. Past failures of human clinical trials raised skepticism whether the mouse is an appropriate model for S. aureus disease in humans. S. aureus causes chronic-persistent infections that, even with antibiotic or surgical intervention, reoccur in humans and in mice. Determinants of S. aureus evasion from human innate and adaptive immune responses have been identified, however only some of these are relevant in mice. Future research must integrate these insights and refine the experimental mouse models for specific S. aureus diseases to accurately predict the failure or success for candidate vaccines and immune-therapeutics.

Effect of thioridazine on experimental cutaneous staphylococcal infections

BACKGROUND

Some non-antibiotic drugs, such as the phenothiazine antipsychotic agents, may have antimicrobial activity.

MATERIALS AND METHODS

We sought to determine the in vivo antimicrobial effects of the phenothiazine thioridazine in two mouse models of Staphylococcus aureus skin infection.

RESULTS

Thioridazine significantly suppressed dissemination from skin to spleen and kidney after inoculation of the skin surface. However, the drug did not affect infection parameters in the skin itself. Thioridazine did suppress the size of abscesses produced when the bacteria were injected intradermally. On the other hand, using the cutaneous abscess model we were not able to demonstrate synergistic activity between thioridazine and the β-lactam drug cefazolin against methicillin-resistant S. aureus, as previously demonstrated in vitro.

CONCLUSION

The phenothiazine drug thioridazine has in vivo antimicrobial activity against certain S. aureus skin infections, although the previously-demonstrated reversal of methicillin resistance by this agent may not be readily evident in vivo.

Direct translocation of staphylococci from the skin surface to deep organs

BACKGROUND

Staphylococcus aureus can invade the bloodstream and cause bacteremic infections, but this organism frequently produces serious deep infections without bacteremia or an identifiable portal of entry.

METHODS

We used experimental cutaneous S. aureus infections in mice to determine if the bacteria could reach deep organs without travel through the bloodstream.

RESULTS

After skin surface application the bacteria rapidly distributed to lymph nodes, spleen, kidneys and other organs. In these animals, blood cultures were negative, dissemination was more efficient after surface application than injection near dermal blood vessels, and kidney bacterial localization sites were unlike those of bacteremic infections. Whereas normal mice eventually cleared bacteria from the deep sites, those with prolonged immunosuppression became moribund from these infections; they also had negative blood cultures and kidney localization not consistent with hematogenous dissemination. Bacteria were also found in the intervening abdominal wall outside the spleen and kidney sites, suggesting direct movement of the organisms from the skin surface through connecting tissues.

CONCLUSIONS

Although capable of hematogenous dissemination, S. aureus can also spread from skin to deep organs by a non-bacteremic process. In this case the bacteria appear to migrate directly from the skin surface to the deep organs below.

In vivo bioluminescence imaging to evaluate systemic and topical antibiotics against community-acquired methicillin-resistant Staphylococcus aureus-infected skin wounds in mice

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) frequently causes skin and soft tissue infections, including impetigo, cellulitis, folliculitis, and infected wounds and ulcers. Uncomplicated CA-MRSA skin infections are typically managed in an outpatient setting with oral and topical antibiotics and/or incision and drainage, whereas complicated skin infections often require hospitalization, intravenous antibiotics, and sometimes surgery. The aim of this study was to develop a mouse model of CA-MRSA wound infection to compare the efficacy of commonly used systemic and topical antibiotics. A bioluminescent USA300 CA-MRSA strain was inoculated into full-thickness scalpel wounds on the backs of mice and digital photography/image analysis and in vivo bioluminescence imaging were used to measure wound healing and the bacterial burden. Subcutaneous vancomycin, daptomycin, and linezolid similarly reduced the lesion sizes and bacterial burden. Oral linezolid, clindamycin, and doxycycline all decreased the lesion sizes and bacterial burden. Oral trimethoprim-sulfamethoxazole decreased the bacterial burden but did not decrease the lesion size. Topical mupirocin and retapamulin ointments both reduced the bacterial burden. However, the petrolatum vehicle ointment for retapamulin, but not the polyethylene glycol vehicle ointment for mupirocin, promoted wound healing and initially increased the bacterial burden. Finally, in type 2 diabetic mice, subcutaneous linezolid and daptomycin had the most rapid therapeutic effect compared with vancomycin. Taken together, this mouse model of CA-MRSA wound infection, which utilizes in vivo bioluminescence imaging to monitor the bacterial burden, represents an alternative method to evaluate the preclinical in vivo efficacy of systemic and topical antimicrobial agents.

Subcutaneous infection of methicillin resistant Staphylococcus aureus (MRSA)

MRSA is a worldwide threat to public health, and MRSA skin and soft-tissue infections now account for more than half of all soft-tissue infections in the United States. Among soft-tissue infections, myositis, pyomyositis, and necrotizing fasciitis have been increasingly reported in association with MRSA arising from the community. To understand the interplay between MRSA and host immunity leading to more severe infection, the availability of animal models is critical, permitting the study of host and bacterial factors. Several infection models have been introduced to assess the pathogenesis of S. aureus during superficial skin infection. Here, we describe a subcutaneous infection model that examines the skin, subcutaneous, and muscle pathologies.

Noninvasive in vivo imaging to evaluate immune responses and antimicrobial therapy against Staphylococcus aureus and USA300 MRSA skin infections

Staphylococcus aureus skin infections represent a significant public health threat because of the emergence of antibiotic-resistant strains such as methicillin-resistant S. aureus (MRSA). As greater understanding of protective immune responses and more effective antimicrobial therapies are needed, a S. aureus skin wound infection model was developed in which full-thickness scalpel cuts on the backs of mice were infected with a bioluminescent S. aureus (methicillin sensitive) or USA300 community-acquired MRSA strain and in vivo imaging was used to noninvasively monitor the bacterial burden. In addition, the infection-induced inflammatory response was quantified using in vivo fluorescence imaging of LysEGFP mice. Using this model, we found that both IL-1α and IL-1β contributed to host defense during a wound infection, whereas IL-1β was more critical during an intradermal S. aureus infection. Furthermore, treatment of a USA300 MRSA skin infection with retapamulin ointment resulted in up to 85-fold reduction in bacterial burden and a 53% decrease in infection-induced inflammation. In contrast, mupirocin ointment had minimal clinical activity against this USA300 strain, resulting in only a 2-fold reduction in bacterial burden. Taken together, this S. aureus wound infection model provides a valuable preclinical screening method to investigate cutaneous immune responses and the efficacy of topical antimicrobial therapies.

Clearance of experimental cutaneous Staphylococcus aureus infections in mice

Staphylococcal skin infections are quite common in human patients. These infections often clear spontaneously, but may also progress locally and/or disseminate to cause serious and sometimes fatal deep infections. The present studies were undertaken to examine the clearance phase of experimental cutaneous Staphylococcus aureus infections in a mouse model system. Previous work in this system has shown that staphylococci applied to the skin rapidly disseminate to the spleen and kidney. In the present experiments the bacteria were found to persist at the skin infection site at a time (8 days after inoculation) when they had disappeared from the spleen and kidney. Examination of the infected skin at earlier times revealed rapid (within 6 h) invasion into the stratum corneum, stratum Malpighii, and dermis, but subsequent redistribution of bacteria (at 1-2 days) to more superficial sites, particularly crusts located just above the skin surface. The crusts seen in these infections were of two distinct types, which were termed type 1 and type 2. Type 1 crusts appeared first, consisted of bacteria, inflammatory cells, and debris, and developed over an intact epidermis. Type 2 crusts arose from the process of dermal necrosis previously reported to take place at 2 days in this model system. In the latter situation the bacteria were not really cleared from the epidermis and dermis; rather those layers were transformed into a superficial crust that contained the bacteria. Deep hair follicle infections in the dermis were found in these infections, but they did not persist and did not seem to be a reservoir for organisms in the dermis. Resolution of these experimental infections appeared to involve redistribution of invading bacteria to more superficial locations in crusts above the skin surface, marked proliferation of the epidermis, loss of the bacteria-laden crusts from the skin, and eventual healing of the cutaneous damage.