Community-associated Methicillin-resistant Staphylococcus aureus Isolates and Healthcare-Associated Infections

Methicillin-resistant Staphylococcus aureus (MRSA) is the most frequently identified antimicrobial drug–resistant pathogen in US hospitals (1). The epidemiology of infections caused by MRSA is rapidly changing. In the past 10 years, infections caused by this organism have emerged in the community. The 2 MRSA clones in the United States most closely associated with community outbreaks, USA400 (MW2 strain, ST1 lineage) and USA300, often contain pvl genes and, more frequently, have been associated with skin and soft tissue infections (2,3). Outbreaks of community-associated (CA)–MRSA infections have been reported in correctional facilities, among athletic teams, among military recruits, in newborn nurseries, and among men who have sex with men (4–7). CA-MRSA infections now appear to be endemic in many urban regions and cause most CA–S. aureus infections (5,6,8–10). CA-MRSA isolates were first recognized by distinct resistance profiles of antimicrobial drugs that lacked resistance to older antimicrobial drugs (11–13). Several groups have noted these distinct susceptibility patterns appearing in isolates from hospitalized patients. Denis et al. noted that since 1995, MRSA isolates in Belgian hospitals were losing resistance to older antimicrobial drugs such as gentamicin and clindamycin (14). A Spanish hospital experienced a decrease in gentamicin-resistant MRSA isolates (from 97% in 1998 to 20% in 2002) and a simultaneous increase in MRSA isolates carrying the SCCmec type IV cassette (from 0% prevalence in 2000 to 23% prevalence in 2002) (15). A French group noted a similar finding in their hospitals over an 11-year period and found a correlation between isolates that contained SCCmec type IV and susceptibility profiles to ≥3 antimicrobial drugs (16). However, these investigations did not distinguish between cultures obtained from patients hospitalized with CA infection and those with hospital-associated (HA) infections. Thus, it is unclear whether these trends in decreased antimicrobial drug resistance and increased number of MRSA isolates that contained SCCmec type IV were due to increased hospitalization of patients with CA-MRSA infections or to an increased prevalence of isolates containing SCCmec type IV among HA-MRSA isolates. Some MRSA strains associated with CA infection have been noted to cause HA infections. Outbreaks of HA infections caused by isolates containing SCCmec type IV have been reported from Australia and the United States. Affected populations have included postpartum women and patients undergoing prosthetic joint replacement (17–19). Another recent report demonstrated that CA strains had emerged as a substantial cause of HA bloodstream infections (20). However, these reports are anecdotal, and data examining temporal trends are lacking. At our institution, which is located in an area in which CA-MRSA infections are endemic, we have noted a large increase in HA infections caused by MRSA isolates that, by assessment of antibiotic susceptibility patterns, appear to carry the SCCmec type IV element (e.g., susceptible to gentamicin, clindamycin, and trimethoprim-sulfamethoxazole) (6,10,21). The aim of this study was to quantify this trend over a 6-year period.