Community-associated Methicillin-resistant S. Aureus Clone Research Articles

Australian Group on Antimicrobial Resistance (AGAR) Australian Staphylococcus aureus Surveillance Outcome Program (ASSOP) Bloodstream Infection Annual Report 2022.

From 1 January to 31 December 2022, fifty-five institutions across Australia participated in the Australian Staphylococcus aureus Surveillance Outcome Program (ASSOP). The aim of ASSOP 2022 was to determine the proportion of Staphylococcus aureus bacteraemia (SAB) isolates in Australia that were antimicrobial resistant, with particular emphasis on susceptibility to methicillin and on characterisation of the molecular epidemiology of the methicillin-resistant isolates. A total of 3,214 SAB episodes were reported, of which 77.5% were community-onset. Overall, 15.0% of S. aureus were methicillin resistant. The 30-day all-cause mortality associated with methicillin-resistant SAB was 21.4%, which was significantly different to the 16.8% all-cause mortality associated with methicillin-susceptible SAB (p = 0.02). With the exception of the β-lactams and erythromycin, antimicrobial resistance in methicillin-susceptible S. aureus was rare. However, in addition to the β-lactams, approximately 31% of methicillin-resistant S. aureus (MRSA) were resistant to ciprofloxacin; 30% to erythromycin; 13% to tetracycline; 11% to gentamicin; and 2% to co-trimoxazole. One MRSA isolate, with a daptomycin MIC of 1.5 mg/L, harboured the A302V mprF and A23V cls2 mutations. When applying the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, teicoplanin resistance was detected in one MRSA isolate. Resistance to vancomycin or linezolid was not detected. Resistance to non-β-lactam antimicrobials was largely attributable to the healthcare-associated MRSA (HA-MRSA) clone ST22-IV [2B] (EMRSA-15), and to the community-associated MRSA (CA-MRSA) clone ST45-V [5C2&5] which has acquired resistance to multiple antimicrobials including ciprofloxacin, clindamycin, erythromycin, gentamicin, and tetracycline. The ST22-IV [2B] (EMRSA-15) clone is the predominant HA-MRSA clone in Australia. Nonetheless, 86% of methicillin-resistant SAB episodes were due to CA-MRSA clones. Although polyclonal, approximately 72% of CA-MRSA clones were characterised as ST93-IV [2B] (Queensland clone); ST5-IV [2B]; ST45-V [5C2&5]; ST1-IV [2B]; ST30-IV [2B]; ST97-IV [2B]; ST953-IV [2B]; and ST8-IV [2B]. As CA-MRSA is well established in the Australian community, it is important to monitor antimicrobial resistance patterns in community- and healthcare-associated SAB as this information will guide therapeutic practices in treating S. aureus bacteraemia.

Australian Group on Antimicrobial Resistance (AGAR) Australian Staphylococcus aureus Surveillance Outcome Program (ASSOP).

From 1 January to 31 December 2021, forty-eight institutions around Australia participated in the Australian Staphylococcus aureus Surveillance Outcome Programme (ASSOP). The aim of ASSOP 2021 was to determine the proportion of Staphylococcus aureus bacteraemia (SAB) isolates in Australia that were antimicrobial resistant, with particular emphasis on susceptibility to methicillin and on characterisation of the molecular epidemiology of the methicillin-resistant isolates. A total of 2,928 SAB episodes were reported, of which 78.4% were community-onset. Overall, 16.9% of S. aureus isolates were methicillin resistant. The 30-day all-cause mortality associated with methicillin-resistant SAB was 15.0%, which was not significantly different from the 14.4% all-cause mortality associated with methicillin-susceptible SAB (p = 0.7). With the exception of the β-lactams and erythromycin, antimicrobial resistance in methicillin-susceptible S. aureus was rare. However, in addition to the β-lactams, approximately 36% of methicillin-resistant S. aureus (MRSA) were resistant to ciprofloxacin; 30% to erythromycin; 15% to tetracycline; 16% to gentamicin; and 3% to co-trimoxazole. When applying the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, teicoplanin resistance was detected in three S. aureus isolates. Resistance to vancomycin or linezolid was not detected. Resistance to non-β-lactam antimicrobials was largely attributable to the healthcare-associated MRSA (HA-MRSA) clone ST22-IV [2B] (EMRSA-15), and the community-associated MRSA (CA-MRSA) clone ST45-V [5C2&5] which has acquired multiple antimicrobial resistance determinants including ciprofloxacin, erythromycin, clindamycin, gentamicin and tetracycline. The ST22-IV [2B] (EMRSA-15) clone is the predominant HA-MRSA clone in Australia. Nonetheless, 85% of methicillin-resistant SAB episodes were due to CA-MRSA clones. Although polyclonal, approximately 68% of CA-MRSA clones were characterised as ST93-IV [2B] (Queensland clone); ST45-V [5C2&5]; ST5-IV [2B]; ST1-IV [2B]; ST30-IV [2B]; and ST97-IV [2B]. As CA-MRSA is well established in the Australian community, it is important to monitor antimicrobial resistance patterns in community- and healthcare-associated SAB as this information will guide therapeutic practices in treating S. aureus bacteraemia.

MRSA in Humans, Pets and Livestock in Portugal: Where We Came from and Where We Are Going.

Over the years, molecular typing of methicillin-resistant S. aureus (MRSA) has allowed for the identification of endemic MRSA strains and pathogenic strains. After reaching a peak of predominance in a given geographic region, MRSA strains are usually replaced by a new strain. This process is called clonal replacement and is observed worldwide. The worldwide spread of hospital-associated MRSA (HA-MRSA), community-associated MRSA (CA-MRSA) and livestock-associated MRSA (LA-MRSA) clones over the last few decades has allowed this microorganism to be currently considered a pandemic. In Portugal, most HA-MRSA infections are associated with EMRSA-15 (S22-IV), New York/Japan (ST5-II) and Iberian (ST247-I) clones. Regarding the strains found in the community, many of them are frequently associated with the hospital environment, namely the Pediatric, Brazilian and Iberian clones. On the other hand, a strain that is typically found in animals, MRSA clonal complex (CC) 398, has been described in humans as colonizing and causing infections. The ST398 clone is found across all animal species, particularly in farm animals where the economic impact of LA-MRSA infections can have disastrous consequences for industries. In contrast, the EMRSA-15 clone seems to be more related to companion animals. The objective of this review is to better understand the MRSA epidemiology because it is, undoubtedly, an important public health concern that requires more attention, in order to achieve an effective response in all sectors.

Australian Group on Antimicrobial Resistance (AGAR) Australian Staphylococcus aureus Sepsis Outcome Programme (ASSOP) Annual Report 2020.

From 1 January to 31 December 2020, forty-nine institutions around Australia participated in the Australian Staphylococcus aureus Sepsis Outcome Programme (ASSOP). The aims of ASSOP 2020 were to determine the proportion of Staphylococcus aureus bacteraemia (SAB) isolates in Australia that were antimicrobial resistant, with particular emphasis on susceptibility to methicillin; and to characterise the molecular epidemiology of the methicillin-resistant isolates. A total of 2,734 SAB episodes were reported, of which 79.7% were community-onset. Of S. aureus isolates, 17.6% were methicillin resistant. The 30-day all-cause mortality associated with methicillin-resistant SAB was 14.2%, which was not significantly different from the 13.3% mortality associated with methicillin-susceptible SAB (p = 0.6). With the exception of the β-lactams and erythromycin, antimicrobial resistance in methicillin-susceptible S. aureus was rare. However, in addition to the β-lactams, approximately 35% of methicillin-resistant S. aureus (MRSA) were resistant to erythromycin, 33% to ciprofloxacin, 13% to tetracycline, 13% to gentamicin and 4% to co-trimoxazole. When applying the European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, teicoplanin resistance was detected in four S. aureus isolates. Resistance was not detected for vancomycin and linezolid. Resistance to non-beta-lactam antimicrobials was largely attributable to two healthcare-associated MRSA (HA-MRSA) clones: ST22-IV [2B] (EMRSA-15) and ST239-III [3A] (Aus-2/3 EMRSA). The ST22-IV [2B] (EMRSA-15) clone is the predominant HA-MRSA clone in Australia. However, 85% percent of methicillin-resistant SAB isolates were community-associated MRSA (CA-MRSA) clones. Although polyclonal, approximately 77% of CA-MRSA clones were characterised as: ST93-IV [2B] (Queensland CA-MRSA); ST5-IV [2B]; ST45-V [5C2&5]; ST1-IV [2B]; ST30-IV [2B]; ST8-IV [2B]; and ST97-IV [2B]. The CA-MRSA clones, in particular ST45-V [5C2&5], have acquired multiple antimicrobial resistance determinants including ciprofloxacin, erythromycin, clindamycin, gentamicin and tetracycline. The multi-resistant ST45-V [5C2&5] clone accounted for 11.0% of CA-MRSA. As CA-MRSA is well established in the Australian community, it is important to monitor antimicrobial resistance patterns in community- and healthcare-associated SAB as this information will guide therapeutic practices in treating S. aureus sepsis.

Staphylococcus aureus Pneumonia in the Community.

Staphylococcus aureus is an emergent etiology of community-acquired pneumonia (CAP) over the past 2 decades, with severe community-acquired pneumonia (SCAP) caused by methicillin-resistant S. aureus (MRSA) leading to critical illness and death. S. aureus colonization is associated with a high incidence of pneumonia. Panton-Valentine leukocidin (PVL) is one of the most important virulence factors of S. aureus associated with serious complications. In recent years, community-associated MRSA (CA-MRSA) clones that caused infections in young adults and healthy individuals with no exposure to health care settings and no classical risk factors have emerged. Clinical features at admission including concurrent influenza infection, hemoptysis, multilobar infiltrates, and neutropenia should suggest S. aureus CAP. Sputum Gram stains, cultures (or tracheobronchial aspirates or bronchoalveolar lavage in mechanically ventilated patients), polymerase chain reaction (nasopharyngeal or oropharyngeal or lower respiratory tract specimens), and two sets of blood cultures should be obtained from patients presenting with severe S. aureus CAP. For CAP due to methicillin-susceptible S. aureus, first-line therapy is usually cefazolin, oxacillin, or ceftaroline. For CA-MRSA pneumonia, linezolid is recommended. If vancomycin or teicoplanin are used, combination with clindamycin or rifampicin should be considered in cases of PVL-positive MRSA CAP.

Seasonal H3N2 influenza A virus fails to enhance Staphylococcus aureus co-infection in a non-human primate respiratory tract infection model

Staphylococcus aureus community-acquired pneumonia is often associated with influenza or an influenza-like syndrome. Morbidity and mortality due to methicillin-resistant S. aureus (MRSA) or influenza and pneumonia, which includes bacterial co-infection, are among the top causes of death by infectious diseases in the United States. We developed a non-lethal influenza A virus (IAV) (H3N2)/S. aureus co-infection model in cynomolgus macaques (Macaca fascicularis) to test the hypothesis that seasonal IAV infection predisposes non-human primates to severe S. aureus pneumonia. Infection and disease progression were monitored by clinical assessment of animal health; analysis of blood chemistry, nasal swabs, and X-rays; and gross pathology and histopathology of lungs from infected animals. Seasonal IAV infection in healthy cynomolgus macaques caused mild pneumonia, but unexpectedly, did not predispose these animals to subsequent severe infection with the community-associated MRSA clone USA300. We conclude that in our co-infection model, seasonal IAV infection alone is not sufficient to promote severe S. aureus pneumonia in otherwise healthy non-human primates. The implication of these findings is that comorbidity factors in addition to IAV infection are required to predispose individuals to secondary S. aureus pneumonia.

Evolution of methicillin-resistant Staphylococcus aureus towards increasing resistance

To elucidate the evolutionary history of Staphylococcus aureus clonal complex (CC) 8, which encompasses several globally distributed epidemic lineages, including hospital-associated methicillin-resistant S. aureus (MRSA) and the highly prevalent community-associated MRSA clone USA300. We reconstructed the phylogeny of S. aureus CC8 by mutation discovery at 112 genetic housekeeping loci from each of 174 isolates, sampled on five continents between 1957 and 2008. The distribution of antimicrobial resistance traits and of diverse mobile genetic elements was investigated in relation to the isolates' phylogeny. Our analyses revealed the existence of nine phylogenetic clades within CC8. We identified at least eight independent events of methicillin resistance acquisition in CC8 and dated the origin of a methicillin-resistant progenitor of the notorious USA300 clone to the mid-1970s. Of the S. aureus isolates in our collection, 88% carried plasmidic rep gene sequences, with up to five different rep genes in individual isolates and a total of eight rep families. Mapping the plasmid content onto the isolates' phylogeny illustrated the stable carriage over decades of some plasmids and the more volatile nature of others. Strikingly, we observed trends of increasing antibiotic resistance during the evolution of several lineages, including USA300. We propose a model for the evolution of S. aureus CC8, involving a split into at least nine phylogenetic lineages and a subsequent series of acquisitions and losses of mobile genetic elements that carry diverse virulence and antimicrobial resistance traits. The evolution of MRSA USA300 towards resistance to additional antibiotic classes is of major concern.

Reduced susceptibility to vancomycin in isogenic Staphylococcus aureus strains of sequence type 59: tracking evolution and identifying mutations by whole-genome sequencing

Vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA) phenotypes are increasingly reported in methicillin-resistant S. aureus (MRSA) strains of distinct genetic backgrounds. This study tracked genetic evolution during the development of vancomycin non-susceptibility in a prevalent Asian community-associated MRSA clone of sequence type (ST) 59. ST59 strains were consecutively isolated from a patient who failed chemotherapy for a septic knee over 15 months. The genetic mutations associated with the VISA phenotype were identified by whole-genome sequencing of two strains, which had the vancomycin-susceptible S. aureus (VSSA) and VISA phenotypes. The mutations were subsequently screened in other strains. By correlating the accumulated mutations with vancomycin susceptibility, genetic evolution was tracked at the whole-genome scale. Nine non-synonymous mutations and two steps of genetic evolution were identified during the development of the VISA phenotype. The first step involved a nonsense mutation in agrC and point mutations at five other loci, which were associated with the VSSA-to-hVISA conversion. Mutations of rpoB and fusA following the use of rifampicin and fusidic acid were identified in the second step of evolution, which corresponded to the development of dual resistance to rifampicin and fusidic acid and the conversion of hVISA to VISA. In vivo genetic evolution of S. aureus occurred in stepwise order during the development of incremental vancomycin non-susceptibility and was related to the use of antimicrobial agents.

Community-associated methicillin-resistant Staphylococcus aureus in children treated in Uruguay

Staphylococcus aureus produces a variety of diseases among children, ranging from skin and soft tissue infections to invasive life-threatening diseases. Since 1990, an increasing number of diseases produced by community-associated methicillin-resistant S. aureus (CA-MRSA) isolates have been reported. The aim of this study was to describe the importance and the microbiological characteristics of S. aureus isolates recovered from children treated at the Hospital Pediátrico del Centro Hospitalario "Pereira Rossell" (HP-CHPR); focusing on invasive diseases caused by CA-MRSA isolates, as well as some clinical aspects of the diseases they have produced. One hundred and twenty-five S. aureus isolates recovered from the HP-CHPR between 2003 and 2006 from children with invasive (n=89) and superficial diseases (n=36) were included. Genotypic and phenotypic characteristics of S. aureus isolates and relevant clinical aspects of each child were studied. CA-MRSA isolates accounted for 73% of all S. aureus recovered from invasive (mainly bone and joint) infections, pneumonia and bacteraemia. The most common CA-MRSA strain recovered from invasive (n=65) and superficial (n=36) diseases had the following features: pulsotype A (type USA1100), SCCmec cassette type IV, Panton-Valentine Leukocidin genes positive, susceptibility to trimethoprim-sulfamethoxazole without the inducible macrolide-lincosamide-streptogramin B (iMLSB) resistance phenotype. No association between genotypic characteristics of invasive CA-MRSA isolates and clinical outcomes was found. CA-MRSA isolates produced a wide spectrum of invasive diseases in a public paediatric hospital between 2003 and 2006. Microbiologic characterization suggests the spread of an adapted CA-MRSA clone lacking erm genes.

A third case of USA300 community-associated methicillin-resistantStaphylococcus aureusinfection in Korea

To the Editor, The global epidemiology of Staphylococcus aureus has changed over the past decade, and this has been characterized by the emergence of community-associated methicillin-resistant S. aureus (CA-MRSA), which has spread to hospitals. Several CA-MRSA clones have emerged in various geographic locations, some of which have become widespread worldwide through intercontinental transmission. The most successful pandemic CA-MRSA clone is the Panton-Valentine leukocidin (PVL)-positive sequence type (ST) 8-MRSA-SCCmec IV (USA300), which originated in the USA. At present, USA300 or closely related variants have been reported in many countries across five continents [1]. In Korea, a distinct CA-MRSA clone, ST72-MRSA-IV, emerged and has been spreading in both communities and hospitals [2]. Nevertheless, in Korea, only two reported infections caused by the pandemic clone USA300 have occurred to date [3,4]. Here, we report the third confirmed infection caused by the USA300 CA-MRSA strain in Korea. An 82-year-old female patient, who had no history of international travel, presented with a palpable tender lesion in the suprapubic area on October 17, 2009. She had hypertension for 30 years. Four months earlier, she underwent percutaneous coronary intervention for unstable angina. She developed a painful ulcer in the suprapubic area 2 months earlier, which worsened and was complicated by erythema and purulent discharge beginning 20 days earlier. On admission, she was hypothermic (35.6℃), and had a pulse rate of 77 beats/min, blood pressure of 149/74 mmHg, and respiratory rate of 20 breaths/min. The erythema in her suprapubic area was 10 cm in diameter and had a crust with purulent discharge at the center. Laboratory tests showed 19,260 leukocytes/µL, hemoglobin 12.5 g/dL, platelets 262,000/µL, aspartate transaminase 29 U/L, alanine aminotransferase 27 U/L, lactate dehydrogenase 535 IU/L, blood urea nitrogen 29 mg/dL, and creatinine 1.07 mg/dL. Pelvic magnetic resonance imaging suggested an immature subcutaneous abscess in the suprapubic area with surrounding inflammation (Fig. 1). Cefazolin was administered empirically after requesting cultures of the purulent discharge. These grew MRSA, which was susceptible to ciprofloxacin, gentamicin, clindamycin, rifampicin, cotrimoxazole, and tetracycline. The antibiotic regimen was changed to vancomycin, and surgical drainage with debridement of necrotic tissues was performed. All of the symptoms and signs of the subcutaneous abscess subsided after vancomycin injection for 7 days and subsequent use of oral cotrimoxazole for 12 days. Figure 1 A contrast-enhanced T2-weighted magnetic resonance image with fat saturation shows an enhancing lesion in the subcutaneous layer of the suprapubic area. The molecular characteristics of the isolated MRSA strain were compared with those of reference strains. Multilocus sequence typing, SCCmec typing, and staphylococcal protein A (spa) typing were performed, as described previously [4]. The presence of the lukF-PV and lukS-PV genes encoding the components of the PVL toxin was screened, as described previously. The isolate was determined to be PVL-positive ST8-MRSA-IV and belonged to spa type t008. Pulsed-field gel electrophoresis (PFGE) was performed, as described previously. The PFGE patterns were analyzed using the GelCompar II software (Applied Maths, Sint-Martens-Latem, Belgium) and compared to two reference strains: the SMC USA300-1 strain, a clinical isolate reported twice in Korea [4], and the USA300 reference strain (NRS384), which was obtained from the Network on Antimicrobial Resistance in Staphylococcus aureus supported under National Institute of Allergy and Infectious Diseases/National Institutes of Health contract #HHSN2722 0070 0055C. The PFGE patterns of the isolate (SMC USA300-2) showed 81.0% similarity to the USA300 reference strain and 93.3% similarity to the SMC USA300-1 strain, indicating that they are closely related (Fig. 2). Figure 2 Pulsed-field gel electrophoresis (PFGE) patterns of the methicillin-resistant Staphylococcus aureus isolate from the patient and control stains. The analysis of the PFGE patterns showed that this isolate (SMC USA300-2) was the same pulsotype as the USA300 ... This is the third confirmed infection in Korea caused by the USA300 clone. While the first Korean case of USA300 reported in 2008 was in a patient who had traveled to Hawaii [2], the second and current cases had no overseas travel history, which suggests the autochthonous acquisition of the USA300 clone in Korea. Autochthonous acquisition of the USA300 clone has also been documented in some European countries, Canada, and Japan [5]. Interestingly, the PFGE pattern of the MRSA strain from this case was more similar to that of the second Korean strain than to the USA300 reference strain, which also supports the possibility of autochthonous acquisition of the USA300 clone in Korea. It also implies that our case and the second case were caused by autochthonous acquisition of the USA300 clone. In summary, we report the third confirmed infection caused by the USA300 clone in a Korean resident, possibly acquired autochthonously.

Community-associated Methicillin-resistant Staphylococcus aureus Skin Infections in the Tropics

Skin and soft tissue infections (SSTI) caused by Staphylococcus aureus are very common, particularly in children, in tropical regions. The proportion of S aureus SSTI caused by community-associated methicillin-resistant S aureus (CA-MRSA) varies according to region, but is up to 25% in some areas. There are diverse CA-MRSA clones, including several that harbor Panton-Valentine leukocidin. Key predisposing factors for staphylococcal infections are scabies infestation, overcrowding, poor hygiene, and inadequate water supplies. In the setting of a community outbreak of staphylococcal SSTI, interventions intended to improve personal and community hygiene are likely to be the most practical, effective, and achievable. Options for oral treatment of clinical infections caused by CA-MRSA include clindamycin and trimethoprim-sulfamethoxazole. Although rapid diagnostics are now available, and 2 vaccines have reached clinical trials, neither of these is likely to be of use in tropical, developing regions in the near future.

Panton-Valentine Leukocidin–Positive MRSA, Shanghai

To the Editor: The development of methicillin resistance in community strains of Staphylococcus aureus is a notable step in the evolution of this pathogen. Unlike their equivalents in the hospital environment, community-associated methicillin-resistant S. aureus (CA-MRSA) strains tend to cause infections in children and young adults who have few known healthcare risks (1). CA-MRSA strains usually possess the Panton-Valentine leukocidin (PVL) genes and staphylococcal cassette chromosome (SCC) mec type IV or V (1,2). We studied 72 S. aureus isolates (49 MRSA and 23 methicillin-susceptible [MSSA]) by pulsed-field gel electrophoresis and by SCCmec, staphylococcal protein A (spa), and multilocus sequence typing (1,3). These isolates were recovered from clinical specimens (52 respiratory specimens, 9 wound, 4 urine, 2 blood, and 5 other body fluids) from 72 patients treated in 5 district hospitals in Shanghai, People’s Republic of China, during October 2005 through January 2007. The isolates were randomly chosen. In the hospitals, ≈1,000 S. aureus isolates were recovered annually during the time period of our study. The 5 hospitals are estimated to serve a population of 3.4 million, equivalent to one fourth of the total population in Shanghai. Hospital D is a children’s hospital. The other 4 hospitals (A, B, C, and E) have all the major clinical specialties, emergency departments, and outpatient clinics. The isolates were identified as S. aureus by Gram stain, latex agglutination (Slide StaphPlus; bioMerieux, Marcy l’Etoile, France), and tube coagulase, mannitol, ornithine, and deoxyribonuclease reactions (1,4). Methicillin resistance in the isolates was detected by cefoxitin disc screening and confirmed by mecA PCR (1,4). For patients with PVL-positive MRSA, the computerized discharge records in the hospitals were retrospectively reviewed to ascertain demographic and clinical information. A MRSA case was considered to be community associated if it was isolated from an outpatient or within 2 days of a patient’s hospitalization. Exclusion criteria included a history of hospitalization for illness (except birth), surgery, or dialysis in the previous year or the presence of indwelling catheters or other medical devices (1). Conversely, healthcare-associated MRSA was defined by isolation >2 days after hospitalization or presence of any of the aforementioned healthcare risks. PVL genes were detected in 9 (18.4%) of the 49 MRSA isolates (Table) and 4 (17.3%) of the 23 MSSA isolates. The 9 MRSA case-patients included 8 infants with pneumonia and 1 adult with prostatitis. Pulsed-field gel electrophoresis clustered 8 of the 9 PVL-positive MRSA isolates into 2 groups: 6 isolates as SH100 and 2 isolates as SH200. Strains of SH100 were spa type/MLST-SCCmec type t318/ST30-IV or t318/ST1114-V, and SH200 strains had t1376/ST88-V. Similar to the PVL-positive MRSA isolates, a limited number of spa types were found among the 40 PVL-negative MRSA isolates. These were t037/ST239-III (n = 19), t002/ST5-II (n = 14), t030/ST239-III (n = 5), t459/ST239-III (n = 1), and t1764/ST88-IV (n = 1). Table Epidemiologic and microbiologic characteristics for Panton-Valentine leukocidin–positive MRSA infections in 9 case-patients, Shanghai, People’s Republic of China, 2006* In contrast, spa and sequence types (STs) among the 23 MSSA isolates were highly diverse. There were 20 spa types and 14 STs, giving a total of 20 distinct patterns. Three patterns (t091/ST7, t3388/ST630, t3389/ST15) had 2 isolates, and 17 patterns (t002/ST5, t1077/ST121, t127/ST1, t1376/ST88, t189/ST188, t2024/ST30, t2092/ST121, t2207/ST1206, t2471/ST25, t258/ST25, t3383/ST20, t3386/ST630, t377/ST630, t437/ST1205, t548/ST5, t701/ST6, t796/ST7) had 1 isolate only. The 4 PVL-positive MSSA isolates were t1376/ST88, t2471/ST25, t258/ST25, and t3383/ST20. Mupirocin resistance rates among the PVL-positive and PVL-negative MRSA isolates were 33.3% (3/9) and 7.5% (3/40), respectively (p = 0.07). All MSSA isolates were susceptible to mupirocin. It is notable that of the 9 PVL-positive MRSA isolates, 8 of them came from hospital D and all were from children 1–4 months of age. Others have noted that the epidemiology of MRSA differs for children and adults (1,2,7,8). Molecular typing showed that the PVL-positive CA-MRSA isolates were attributed to 2 clones with genotypes t318/ST30-IV (or t318/ST1114-V) and t1376/ST88-V. Detection of t318/ST30 strains in 4 patients with healthcare-associated infections suggested hospital transmission of this CA-MRSA clone, corroborating reports elsewhere (9). Worldwide, ST30 is a common CA-MRSA genetic lineage (1,2). Besides t318, strains related to the ST30 clone have been reported to be spa types t019, t021, and t1273 (2). ST88 PVL-positive MRSA is relatively less common but has been found in Wenzhou (People’s Republic of China), Bangladesh, Belgium, and Nigeria (2,7,8,10). Because the number of isolates tested in this study is relatively small, no firm conclusion could be drawn on the prevalence of PVL-positive CA-MRSA among S. aureus isolates. Nonetheless, our findings agree with previous reports that the genotypes of MSSA isolates are more diverse than are those for PVL-positive and -negative MRSA isolates and that genotypes for some CA-MRSA strains are shared by a few of the MSSA strains (1).