Identification of Small Molecules Exhibiting Oxacillin Synergy through a Novel Assay for Inhibition of vraTSR Expression in Methicillin-Resistant Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) strains that are resistant to all forms of penicillin have become an increasingly common and urgent problem threatening human health. They are responsible for a wide variety of infectious diseases ranging from minor skin abscesses to life-threatening severe infections. The vra operon that is conserved among S. aureus strains encodes a three-component signal transduction system (vraTSR) that is responsible for sensing and responding to cell wall stress. We developed a novel and multifaceted assay to identify compounds that potentiate the activity of oxacillin, essentially restoring efficacy of oxacillin against MRSA, and performed high-throughput screening (HTS) to identify oxacillin potentiators. HTS of 13,840 small-molecule compounds from an antimicrobial-focused Life Chemicals library, using the MRSA cell-based assay, identified three different inhibitor scaffolds. Checkerboard assays for synergy with oxacillin, reverse transcriptase PCR (RT-PCR) assays against vraR expression, and direct confirmation of interaction with VraS by surface plasmon resonance (SPR) further verified them to be viable hit compounds. A subsequent structure-activity relationship (SAR) study of the best scaffold with diverse analogs was utilized to improve potency and provides a strong foundation for further development.

Can Methicillin-resistant Staphylococcus aureus Silently Travel From the Gut to the Wound and Cause Postoperative Infection? Modeling the "Trojan Horse Hypothesis".

OBJECTIVE: To determine whether intestinal colonization with methicillin-resistant Staphylococcus aureus (MRSA) can be the source of surgical site infections (SSIs). BACKGROUND: We hypothesized that gut-derived MRSA may cause SSIs via mechanisms in which circulating immune cells scavenge MRSA from the gut, home to surgical wounds, and cause infection (Trojan Horse Hypothesis). METHODS: MRSA gut colonization was achieved by disrupting the microbiota with antibiotics, imposing a period of starvation and introducing MRSA via gavage. Next, mice were subjected to a surgical injury (30% hepatectomy) and rectus muscle injury and ischemia before skin closure. All wounds were cultured before skin closure. To control for postoperative wound contamination, reiterative experiments were performed in mice in which the closed wound was painted with live MRSA for 2 consecutive postoperative days. To rule out extracellular bacteremia as a cause of wound infection, MRSA was injected intravenously in mice subjected to rectus muscle ischemia and injury. RESULTS: All wound cultures were negative before skin closure, ruling out intraoperative contamination. Out of 40 mice, 4 (10%) developed visible abscesses. Nine mice (22.5%) had MRSA positive cultures of the rectus muscle without visible abscesses. No SSIs were observed in mice injected intravenously with MRSA. Wounds painted with MRSA after closure did not develop infections. Circulating neutrophils from mice captured by flow cytometry demonstrated MRSA in their cytoplasm. CONCLUSIONS: Immune cells as Trojan horses carrying gut-derived MRSA may be a plausible mechanism of SSIs in the absence of direct contamination.

Impact of Staphylococcus aureus USA300 Colonization and Skin Infections on Systemic Immune Responses in Humans.

Staphylococcus aureus is both a commensal and a pathogen, and USA300, a strain that is usually methicillin-resistant but can sometimes be methicillin-susceptible, has been causing skin and soft tissue infections (SSTIs) in epidemic proportions among otherwise healthy individuals. Although many people are colonized with S. aureus strains, including some with USA300, few of these colonized individuals develop SSTIs. This prompts the hypothesis that infections may develop in individuals with somewhat reduced innate and/or adaptive immune responses to S. aureus, either because prior S. aureus colonization has dampened such responses selectively, or because of more globally reduced immune reactivity. In this study, we analyzed the S. aureus colonization status and PBMC responses to innate and adaptive stimuli in 72 patients with SSTIs and 143 uninfected demographically matched controls. Contrary to the hypothesis formulated, PBMCs from infected patients obtained at the time of infection displayed enhanced innate cytokine production upon restimulation compared with PBMCs from controls, a difference that disappeared after infection resolution. Notably, PBMCs from patients infected with a documented USA300 SSTI displayed greater innate cytokine production than did those from patients infected with documented non-USA300 genotypes. Moreover, colonization with USA300 in infected patients, regardless of their infecting strain, correlated with increased production of IL-10, IL-17A, and IL-22 compared with patients colonized with non-USA300 subtypes. Thus, our results demonstrate that infected patients associated with USA300 either as an infecting strain, or as a colonizing strain, have systemic immune responses of greater magnitude than do those associated with other S. aureus subtypes.

Intrahost Evolution of Methicillin-Resistant Staphylococcus aureus USA300 Among Individuals With Reoccurring Skin and Soft-Tissue Infections.

BACKGROUND: Methicillin-resistant Staphylococcus aureus (MRSA) USA300 is the leading cause of MRSA infections in the United States and has caused an epidemic of skin and soft-tissue infections. Recurrent infections with USA300 MRSA are common, yet intrahost evolution during persistence on an individual has not been studied. This gap hinders the ability to clinically manage recurrent infections and reconstruct transmission networks. METHODS: To characterize bacterial intrahost evolution, we examined the clinical courses of 4 subjects with 3-6 recurrent USA300 MRSA infections, using patient clinical data, including antibiotic exposure history, and whole-genome sequencing and phylogenetic analysis of all available MRSA isolates (n = 29). RESULTS: Among sequential isolates, we found variability in diversity, accumulation of mutations, and mobile genetic elements. Selection for antimicrobial-resistant populations was observed through both an increase in the number of plasmids conferring multidrug resistance and strain replacement by a resistant population. Two of 4 subjects had strain replacement with a genetically distinct USA300 MRSA population. DISCUSSIONS: During a 5-year period in 4 subjects, we identified development of antimicrobial resistance, intrahost evolution, and strain replacement among isolates from patients with recurrent MRSA infections. This calls into question the efficacy of decolonization to prevent recurrent infections and highlights the adaptive potential of USA300 and the need for effective sampling.

Proteomic Identification of saeRS-Dependent Targets Critical for Protective Humoral Immunity against Staphylococcus aureus Skin Infection.

Recurrent Staphylococcus aureus skin and soft tissue infections (SSTIs) are common despite detectable antibody responses, leading to the belief that the immune response elicited by these infections is not protective. We recently reported that S. aureus USA300 SSTI elicits antibodies that protect against recurrent SSTI in BALB/c but not C57BL/6 mice, and in this study, we aimed to uncover the specificity of the protective antibodies. Using a proteomic approach, we found that S. aureus SSTI elicited broad polyclonal antibody responses in both BALB/c and C57BL/6 mice and identified 10 S. aureus antigens against which antibody levels were significantly higher in immune BALB/c serum. Four of the 10 antigens identified are regulated by the saeRS operon, suggesting a dominant role for saeRS in protection. Indeed, infection with USA300Δsae failed to protect against secondary SSTI with USA300, despite eliciting a strong polyclonal antibody response against antigens whose expression is not regulated by saeRS. Moreover, the antibody repertoire after infection with USA300Δsae lacked antibodies specific for 10 saeRS-regulated antigens, suggesting that all or a subset of these antigens are necessary to elicit protective immunity. Infection with USA300Δhla elicited modest protection against secondary SSTI, and complementation of USA300Δsae with hla restored protection but incompletely. Together, these findings support a role for both Hla and other saeRS-regulated antigens in eliciting protection and suggest that host differences in immune responses to saeRS-regulated antigens may determine whether S. aureus infection elicits protective or nonprotective immunity against recurrent infection.

Staphylococcus aureus skin infection recurrences among household members: an examination of host, behavioral, and pathogen-level predictors.

BACKGROUND: Many patients suffer from recurrent Staphylococcus aureus infections, but there are few data examining recurrence predictors. METHODS: We followed adults and children after treatment for S. aureus skin infections and their household contacts in Los Angeles and Chicago. We surveyed subjects for S. aureus body colonization, household fomite contamination, and behavioral and clinical factors at baseline and 3 and 6 months later. Using repeated measures modeling, we examined host, pathogen, behavioral, and clinical factors associated with recurrence. RESULTS: Among 330 index subjects, 182 (55%) were infected with an isolate of the USA300 methicillin-resistant S. aureus (MRSA) genetic background. Recurrences occurred in 39% by month 3 and 51% by month 6. Among 588 household contacts, 10% reported a skin infection by month 3 and 13% by month 6. Among index subjects, recurrence was associated with (P < .05) Los Angeles site, diabetes, recent hospitalization, recent skin infection, recent cephalexin use, and household S. aureus or MRSA fomite contamination; recurrence was inversely associated with recent contact sports participation. In the multivariate model, independent predictors of recurrence in index patients were recent hospitalization, household MRSA fomite contamination, and lack of recent contact sports participation. Among household contacts, independent predictors of subsequent skin infection were Chicago site, antibiotic use in the prior year, and skin infection in the prior 3 months. CONCLUSIONS: In our longitudinal study, patients with a S. aureus skin infection were more likely to suffer a recurrence if household fomites were MRSA contaminated. Interventions to prevent recurrence may be enhanced by decontamination of household fomites.

Staphylococcus aureus metabolic adaptations during the transition from a daptomycin susceptibility phenotype to a daptomycin nonsusceptibility phenotype.

Staphylococcus aureus is a major cause of nosocomial and community-acquired infections. The success of S. aureus as a pathogen is due in part to its many virulence determinants and resistance to antimicrobials. In particular, methicillin-resistant S. aureus has emerged as a major cause of infections and led to increased use of the antibiotics vancomycin and daptomycin, which has increased the isolation of vancomycin-intermediate S. aureus and daptomycin-nonsusceptible S. aureus strains. The most common mechanism by which S. aureus acquires intermediate resistance to antibiotics is by adapting its physiology and metabolism to permit growth in the presence of these antibiotics, a process known as adaptive resistance. To better understand the physiological and metabolic changes associated with adaptive resistance, six daptomycin-susceptible and -nonsusceptible isogenic strain pairs were examined for changes in growth, competitive fitness, and metabolic alterations. Interestingly, daptomycin nonsusceptibility coincides with a slightly delayed transition to the postexponential growth phase and alterations in metabolism. Specifically, daptomycin-nonsusceptible strains have decreased tricarboxylic acid cycle activity, which correlates with increased synthesis of pyrimidines and purines and increased carbon flow to pathways associated with wall teichoic acid and peptidoglycan biosynthesis. Importantly, these data provided an opportunity to alter the daptomycin nonsusceptibility phenotype by manipulating bacterial metabolism, a first step in developing compounds that target metabolic pathways that can be used in combination with daptomycin to reduce treatment failures.

High Staphylococcus aureus colonization prevalence among patients with skin and soft tissue infections and controls in an urban emergency department.

Staphylococcus aureus is a commensal species that can also be a formidable pathogen. In the United States, an epidemic of community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infections has been occurring for the last 15 years. In the context of a study in which we identified patients with skin and soft tissue infections (SSTIs) and randomized them to receive one of two antimicrobial treatment regimens, we assessed S. aureus colonization in the nares, throat, and perianal skin on the day of enrollment and 40 days after therapy. We compared the prevalence of colonization between the SSTI patients and an uninfected control population. A total of 144 subjects and 130 controls, predominantly African American, participated in this study, and 116 returned for a 40-day follow-up visit. Of the SSTI patients, 76% were colonized with S. aureus at enrollment, as were 65% of the controls. Patients were more likely than the controls to be colonized with USA300 MRSA (62/144 [43.1%] versus 11/130 [8.5%], respectively; P < 0.001). The nares were not the most common site of colonization. The colonization prevalence diminished somewhat after antibiotic treatment but remained high. The isolates that colonized the controls were more likely than those in the patients to be methicillin-susceptible S. aureus (MSSA) (74/84 [88.1%] versus 56/106 [52.8%], respectively; P < 0.001). In conclusion, the prevalence of S. aureus colonization among SSTI patients was high and often involved USA300 MRSA. The prevalence diminished somewhat with antimicrobial therapy but remained high at the 40-day follow-up visit. Control subjects were also colonized at a high prevalence but most often with a genetic background not associated with a clinical infection in this study. S. aureus is a commensal species and a pathogen. Plans for decolonization or eradication should take this distinction into account.

Pediatric Staphylococcus aureus Isolate Genotypes and Infections from the Dawn of the Community-Associated Methicillin-Resistant S. aureus Epidemic Era in Chicago, 1994 to 1997.

Widespread infections with community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA) have occurred in the United States with the dissemination of the USA300 strain beginning in 2000. We examined 105 isolates obtained from children treated at the University of Chicago from 1994 to 1997 (75 methicillin-susceptible S. aureus [MSSA] and 30 MRSA isolates) in order to investigate for possible evidence of USA300 during this period. Infections were defined epidemiologically based on medical record review. The isolates underwent multilocus sequence typing (MLST), as well as assays for the Panton-Valentine leukocidin (PVL) genes, the protein A gene (spa), and arcA and opp3, proxy markers for the arginine catabolic mobile element (ACME), characteristic of USA300 MRSA. MRSA isolates also underwent staphylococcal cassette chromosome mec (SCCmec) typing and pulsed-field gel electrophoresis (PFGE) subtyping. MSSA isolates belonged to 17 sequence type (ST) groups. The 12 epidemiologically defined CA-MRSA infection isolates were either ST1 (n = 4) or ST8 (n = 8). They belonged to 3 different PFGE types: USA100 (n = 1), USA400 (n = 5), and USA500 (n = 6). Among the CA-MRSA infection isolates, 8 (67%) were PVL(+). None of the MRSA or MSSA isolates contained arcA or opp3. Only one MRSA isolate was USA300 by PFGE. This was a health care-associated (HA) MRSA isolate, negative for PVL, that carried SCCmec type II. USA300 with its characteristic features was not identified in the collection from the years 1994 to 1997.

Staphylococcus aureus bacteremia at 5 US academic medical centers, 2008-2011: significant geographic variation in community-onset infections.

BACKGROUND: The incidence of community-onset (CO) methicillin-resistant Staphylococcus aureus (MRSA) bacteremia rose from the late 1990s through the 2000s. However, hospital-onset (HO) MRSA rates have recently declined in the United States and Europe. METHODS: Data were abstracted from infection prevention databases between 1 January 2008 and 31 December 2011 at 5 US academic medical centers to determine the number of single-patient blood cultures positive for MRSA and methicillin-susceptible S. aureus (MSSA) per calendar year, stratified into CO and HO infections. RESULTS: Across the 5 centers, 4171 episodes of bacteremia were identified. Center A (Los Angeles, California) experienced a significant decline in CO-MRSA bacteremia rates (from a peak in 2009 of 0.42 to 0.18 per 1000 patient-days in 2011 [P = .005]), whereas CO-MSSA rates remained stable. Centers B (San Francisco, California), D (Chicago, Illinois), and E (Raleigh-Durham, North Carolina) experienced a stable incidence of CO-MRSA and CO-MSSA bacteremia. In contrast, at center C (New York, New York), the incidence of CO-MRSA increased >3-fold (from 0.11 to 0.34 cases per 1000 patient-days [P < .001]). At most of the sites, HO-MRSA decreased and HO-MSSA rates were stable. USA300 accounted for 52% (104/202) of genotyped MRSA isolates overall, but this varied by center, ranging from 35% to 80%. CONCLUSIONS: CO-MRSA rates and the contribution of USA300 MRSA varied dramatically across diverse geographical areas in the United States. Enhanced infection control efforts are unlikely to account for such variation in CO infection rates. Bioecological and clinical explanations for geographical differences in CO-MRSA bacteremia rates merit further study.

Genomic and transcriptomic differences in community acquired methicillin resistant Staphylococcus aureus USA300 and USA400 strains.

BACKGROUND: Staphylococcus aureus is a human pathogen responsible for substantial morbidity and mortality through its ability to cause a number of human infections including bacteremia, pneumonia and soft tissue infections. Of great concern is the emergence and dissemination of methicillin-resistant Staphylococcus aureus strains (MRSA) that are resistant to nearly all ß-lactams. The emergence of the USA300 MRSA genetic background among community associated S. aureus infections (CA-MRSA) in the USA was followed by the disappearance of USA400 CA-MRSA isolates. RESULTS: To gain a greater understanding of the potential fitness advantages and virulence capacity of S. aureus USA300 clones, we performed whole genome sequencing of 15 USA300 and 4 USA400 clinical isolates. A comparison of representative genomes of the USA300 and USA400 pulsotypes indicates a number of differences in mobile genome elements. We examined the in vitro gene expression profiles by microarray hybridization and the in vivo transcriptomes during lung infection in mice of a USA300 and a USA400 MRSA strain by performing complete genome qRT-PCR analysis. The unique presence and increased expression of 6 exotoxins in USA300 (12- to 600-fold) compared to USA400 may contribute to the increased virulence of USA300 clones. Importantly, we also observed the up-regulation of prophage genes in USA300 (compared with USA400) during mouse lung infection (including genes encoded by both prophages ΦSa2usa and ΦSa3usa), suggesting that these prophages may play an important role in vivo by contributing to the elevated virulence characteristic of the USA300 clone. CONCLUSIONS: We observed differences in the genetic content of USA300 and USA400 strains, as well as significant differences of in vitro and in vivo gene expression of mobile elements in a lung pneumonia model. This is the first study to document the global transcription differences between USA300 and USA400 strains during both in vitro and in vivo growth.

Roles of two large serine recombinases in mobilizing the methicillin-resistance cassette SCCmec.

Methicillin-resistant Staphylococcus aureus (MRSA) emerged via acquisition of a mobile element, staphylococcal cassette chromosome mec (SCCmec). Integration and excision of SCCmec is mediated by an unusual site-specific recombination system. Most variants of SCCmec encode two recombinases, CcrA and CcrB, that belong to the large serine family. Since CcrA and CcrB are always found together, we sought to address their specific roles. We show here that CcrA and CcrB can carry out both excisive and integrative recombination in Escherichia coli in the absence of any host-specific or SCCmec-encoded cofactors. CcrA and CcrB are promiscuous in their substrate choice: they act on many non-canonical pairs of recombination sites in addition to the canonical ones, which may explain tandem insertions into the SCCmec attachment site. Moreover, CcrB is always required, but CcrA is only required if one of the four half-sites is present. Recombinational activity correlates with DNA binding: CcrA recognizes only that half-site, which overlaps a conserved coding frame on the host chromosome. Therefore, we propose that CcrA serves as a specificity factor that emerged through modular evolution to enable recognition of a bacterial recombination site that is not an inverted repeat.

Hand and nasal carriage of discordant Staphylococcus aureus isolates among urban jail detainees.

In 928 Dallas County Jail detainees, nasal carriage of Staphylococcus aureus was found in 32.8% (26.5% methicillin-susceptible Staphylococcus aureus [MSSA] and 6.3% methicillin-resistant S. aureus [MRSA]), and hand carriage was found in 24.9% (20.7% MSSA and 4.1% MRSA). Among MRSA nasal carriers, 41% had hand MRSA carriage; 29% with hand MRSA carriage had no nasal S. aureus carriage. The prevalence of carriage was not associated with duration of the jail stay up to 180 days.

VraT/YvqF is required for methicillin resistance and activation of the VraSR regulon in Staphylococcus aureus.

Staphylococcus aureus infections caused by strains that are resistant to all forms of penicillin, so-called methicillin-resistant S. aureus (MRSA) strains, have become common. One strategy to counter MRSA infections is to use compounds that resensitize MRSA to methicillin. S. aureus responds to diverse classes of cell wall-inhibitory antibiotics, like methicillin, using the two-component regulatory system VraSR (vra) to up- or downregulate a set of genes (the cell wall stimulon) that presumably facilitates resistance to these antibiotics. Accordingly, VraS and VraR mutations decrease resistance to methicillin, vancomycin, and daptomycin cell wall antimicrobials. vraS and vraR are encoded together on a transcript downstream of two other genes, which we call vraU and vraT (previously called yvqF). By producing nonpolar deletions in vraU and vraT in a USA300 MRSA clinical isolate, we demonstrate that vraT is essential for optimal expression of methicillin resistance in vitro, whereas vraU is not required for this phenotype. The deletion of vraT also improved the outcomes of oxacillin therapy in mouse models of lung and skin infection. Since vraT expressed in trans did not complement a vra operon deletion, we conclude that VraT does not inactivate the antimicrobial. Genome-wide transcriptional microarray experiments reveal that VraT facilitates resistance by playing a necessary regulatory role in the VraSR-mediated cell wall stimulon. Our data prove that VraTSR comprise a novel three-component regulatory system required to facilitate resistance to cell wall agents in S. aureus. We also provide the first in vivo proof of principle for using VraT as a sole target to resensitize MRSA to ß-lactams.

Comparing pulsed-field gel electrophoresis with multilocus sequence typing, spa typing, staphylococcal cassette chromosome mec (SCCmec) typing, and PCR for panton-valentine leukocidin, arcA, and opp3 in methicillin-resistant Staphylococcus aureus isolates at a U.S. Medical Center.

Methicillin-resistant Staphylococcus aureus (MRSA) has become a common cause of skin infections and invasive infections in community dwellers in the United States since the late 1990s. Isolates characterized as USA300 by pulsed-field gel electrophoresis (PFGE) are the predominant strain type in these infections. USA100 and USA500 strains commonly cause health care-associated infections. We compared PFGE with a number of other methods of genotyping in a sample of 149 clinical MRSA isolates from the University of Chicago Medical Center. The 5 USA500 isolates yielded 3 spa types and 2 multilocus sequence types (MLSTs). Among the 24 USA100 isolates, 21 (88%) were of spa type t002, 19 (79%) were of ST5, 2 carried arcA and opp3, and 1 was Panton-Valentine leukocidin positive (PVL(+)). Among the 102 USA300 isolates, 96 (94%) were of ST8 and 94 (92%) were of spa type t008. The combination of traits that provided the best sensitivity (98%), specificity (97%), positive predictive value (PPV) (99%), and negative predictive value (NPV) (95%) for identifying USA300 isolates were the presence of the arcA gene and the presence of the PVL genes (area under the curve, 0.980; 95% confidence interval [CI], 0.955 to 1.0). PFGE did not delineate a homogeneous group of MRSA genetic backgrounds, as documented for other typing methods, particularly for USA500 and USA100 pulsotypes. Documenting the presence of arcA and PVL genes by PCR was an efficient and accurate means of identifying USA300 in a collection of MRSA isolates in which USA300 is common. None of the tested genotyping methods provided an accurate means of identifying the next most common PFGE-based backgrounds, USA100 and USA500.

Asymptomatic carriage of sequence type 398, spa type t571 methicillin-susceptible Staphylococcus aureus in an urban jail: a newly emerging, transmissible pathogenic strain.

Sequence type 398 (ST398) Staphylococcus aureus, frequently carried by livestock, has caused severe human infections and often carries transmissible antibiotic resistance genes. Among methicillin-susceptible S. aureus isolates colonizing Dallas County Jail detainees, 13.2% were ST398, spa type t571, and were genetically similar to human colonization isolates from New York, Chicago, and the Dominican Republic.

CodY deletion enhances in vivo virulence of community-associated methicillin-resistant Staphylococcus aureus clone USA300.

The Staphylococcus aureus global regulator CodY responds to nutrient availability by controlling the expression of target genes. In vitro, CodY represses the transcription of virulence genes, but it is not known if CodY also represses virulence in vivo. The dominant community-associated methicillin-resistant S. aureus (CA-MRSA) clone, USA300, is hypervirulent and has increased transcription of global regulators and virulence genes; these features are reminiscent of a strain defective in CodY. Sequence analysis revealed, however, that the codY genes of USA300 and other sequenced S. aureus isolates are not significantly different from the codY genes in strains known to have active CodY. codY was expressed in USA300, as well as in other pulsotypes assessed. Deletion of codY from a USA300 clinical isolate resulted in modestly increased expression of the global regulators agr and saeRS, as well as the gene encoding the toxin alpha-hemolysin (hla). A substantial increase (>30-fold) in expression of the lukF-PV gene, encoding part of the Panton-Valentine leukocidin (PVL), was observed in the codY mutant. All of these expression differences were reversed by complementation with a functional codY gene. Moreover, purified CodY protein bound upstream of the lukSF-PV operon, indicating that CodY directly represses expression of lukSF-PV. Deletion of codY increased the virulence of USA300 in necrotizing pneumonia and skin infection. Interestingly, deletion of lukSF-PV from the codY mutant did not attenuate virulence, indicating that the hypervirulence of the codY mutant was not explained by overexpression of PVL. These results demonstrate that CodY is active in USA300 and that CodY-mediated repression restrains the virulence of USA300.

Staphylococcus aureus colonization among household contacts of patients with skin infections: risk factors, strain discordance, and complex ecology.

BACKGROUND: The USA300 methicillin resistant Staphylococcus aureus (MRSA) genetic background has rapidly emerged as the predominant cause of community-associated S. aureus infections in the U.S. However, epidemiologic characteristics of S. aureus household transmission are poorly understood. METHODS: We performed a cross-sectional study of adults and children with S. aureus skin infections and their household contacts in Los Angeles and Chicago. Subjects were surveyed for S. aureus colonization of the nares, oropharynx, and inguinal region and risk factors for S. aureus disease. All isolates underwent genetic typing. RESULTS: We enrolled 1162 persons (350 index patients and 812 household members). The most common infection isolate characteristic was ST8/SCCmec IV, PVL+ MRSA (USA300) (53%). S. aureus colonized 40% (137/350) of index patients and 50% (405/812) of household contacts. A nares-only survey would have missed 48% of S. aureus and 51% of MRSA colonized persons. Sixty-five percent of households had >1 S. aureus genetic background identified and 26% of MRSA isolates in household contacts were discordant with the index patients' infecting MRSA strain type. Factors independently associated (P < .05) with the index strain type colonizing household contacts were recent skin infection, recent cephalexin use, and USA300 genetic background. CONCLUSIONS: In our study population, USA300 MRSA appeared more transmissible among household members compared with other S. aureus genetic backgrounds. Strain distribution was complex; >1 S. aureus genetic background was present in many households. S. aureus decolonization strategies may need to address extra-nasal colonization and the consequences of eradicating S. aureus genetic backgrounds infrequently associated with infection.

MRSA USA300 at Alaska Native Medical Center, Anchorage, Alaska, USA, 2000-2006.

To determine whether methicillin-resistant Staphylococcus aureus (MRSA) USA300 commonly caused infections among Alaska Natives, we examined clinical MRSA isolates from the Alaska Native Medical Center, Anchorage, during 2000-2006. Among Anchorage-region residents, USA300 was a minor constituent among MRSA isolates in 2000-2003 (11/68, 16%); by 2006, USA300 was the exclusive genotype identified (10/10).

Improved oxacillin treatment outcomes in experimental skin and lung infection by a methicillin-resistant Staphylococcus aureus isolate with a vraSR operon deletion.

Methicillin-resistant Staphylococcus aureus (MRSA) strains are major pathogens causing infections of the skin and soft tissues and more serious, life-threatening diseases, including sepsis and necrotizing pneumonia. The vraSR operon encodes the key regulatory system that modulates the stress response of S. aureus elicited upon exposure to cell wall antibiotics. Mutation of vraS and vraR results in decreased oxacillin resistance in vitro. We investigated the effect of oxacillin treatment in experimental models employing a clinical USA300 MRSA strain (strain 923) and an isogenic vraSR deletion mutant (strain 923-M23). In a murine model of S. aureus necrotizing pneumonia, animals were treated with oxacillin, beginning 15 min after inoculation. Among mice infected with mutant strain 923-M23, oxacillin treatment significantly improved survival compared with saline treatment, whereas oxacillin treatment had no effect in mice infected with strain 923. Similarly, treatment with oxacillin decreased the bacterial burden among animals infected with strain 923-M23 but not among animals infected with strain 923. In a murine skin infection model, oxacillin eliminated the development of dermonecrosis among 923-M23-infected mice and decreased the bacterial burden in the lesions, but not among strain 923-infected mice. We conclude that deletion of the vraSR operon allowed an oxacillin regimen to be effective in murine models of MRSA pneumonia and skin infection. These findings provide proof-of-principle for development of a new antibiotic that could restore the usefulness of oxacillin against MRSA by inhibiting VraS or VraR.