Several studies, often performed without the benefit of genetic fingerprinting of the bacterial isolates, found several identifiable risk factors (e.g., hospitalization within one year, nursing home residence, hemodialysis, or placement of a long-term intravascular device) that were associated with community-onset MRSA infection or colonization [11–13]. However, this scenario has changed dramatically with the emergence of two MRSA strains, USA300 and USA400. While several well-described outbreaks involving USA300 (e.g., in prisons and sports teams) and USA400 (e.g., in postpartum women and maternity units) have been reported [7–10], risk factors for acquisition of these strains in the general population are largely unknown. In Atlanta, patients with skin and soft tissue infections with the USA300/400 strains were more likely to be black and female when compared to patients with infections due to MSSA . The USA300 type predominated in this study, and the medical center served a largely black and indigent population . In Minnesota, patients with cultures with CA-MRSA were more likely to be younger, nonwhite, and of lower socioeconomic status when compared to patients with hospital-acquired strains of MRSA . In a multicenter study involving patients from Atlanta, Baltimore, and Minnesota, patients with CA-MRSA were likely to have several underlying conditions (e.g., tobacco use, prior skin infections, diabetes mellitus, asthma, and HIV infection) and were of lower socioeconomic status; isolates in this report were not fingerprinted . In a nationwide survey examining rates of nasal colonization, S. aureus was more common in men, those with asthma, and in subjects < 65 years of age; blacks and Mexicans had lower colonization rates when compared to whites. Risk factors for MRSA colonization included age > 65 years, female sex, underlying diabetes mellitus, and residence in a long-term care facility; Hispanics were less likely than whites to be colonized with MRSA . However, approximately half of the MRSA isolates in the last study possessed SCCmec II, suggesting that many were hospital-associated strains.
As the boundary between cases with nosocomial and community-associated MRSA becomes hazy, it is increasingly apparent that future epidemiological studies will require thorough characterization of the bacterial isolates. In this report, only 35% of our isolates with the antibiotic phenotype suggestive of CA-MRSA (MRSA susceptible to clindamycin) belonged to the USA300/400 types. In addition, only 62% of isolates with SCCmec type IV belonged to the USA300/400 types; whether the other isolates represent CA-MRSA strains unique to our region requires further investigation.
In this report, we performed a population-based analysis of CA-MRSA in Brooklyn, NY using all S. aureus isolates identified in hospital microbiology laboratories. By itself, Brooklyn would rank as the fourth largest city in the United States, and has an extremely heterogeneous population. In this urban setting, we found a higher prevalence of USA300 strains in neighborhoods with several distinguishing characteristics. Neighborhoods with a higher prevalence of USA300 had a greater proportion of blacks, Hispanics, females, and children, and had measures indicative of a disadvantaged socioeconomic status. As more households had ≥ 3 persons in the high prevalence neighborhoods, crowded living conditions are likely an important contributing factor for the spread of the USA300 strain. Although racial and ethnic risk factors have been noted in other studies of CA-MRSA [2–4], it remains to be determined if these features are causal in nature or just reflect lower socioeconomic status (and crowded living conditions).
Our results are in stark contrast to a prior study examining epidemiology of Streptococcus pneumoniae in Brooklyn . In that report, the western region of the city (identified with the lower prevalence of USA300) had a higher rate of penicillin-resistant S. pneumoniae, and was attributed to greater access to healthcare (and antimicrobial agents). Indeed, increased antibiotic consumption has been postulated as a protective factor against CA-MRSA in certain populations . It is evident that in a large urban setting, these two resistant community pathogens do not share similar epidemiological characteristics.