RESEARCH Community-Acquired Methicillin-Resistant Staphylococcus aureus, Finland Saara Salmenlinna,* Outi Lyytikäinen,* and Jaana Vuopio-Varkila* Methicillin-resistant Staphylococcus aureus (MRSA) is no longer only hospital acquired. MRSA is defined as community acquired if the MRSA-positive specimen was obtained outside hospital settings or within 2 days of hospital admission, and if it was from a person who had not been hospitalized within 2 years before the date of MRSA isolation. To estimate the proportion of community-acquired MRSA, we analyzed previous hospitalizations for all MRSA-positive persons in Finland from1997 to 1999 by using data from the National Hospital Discharge Register. Of 526 MRSA-positive persons, 21% had community-acquired MRSA. Three MRSA strains identified by phage typing, pulsed-field gel electrophoresis, and ribotyping were associated with community acquisition. None of the strains were multiresistant, and all showed an mec hypervariable region hybridization pattern A (HVR type A). None of the epidemic multiresistant hospi- tal strains were prevalent in nonhospitalized persons. Our population-based data suggest that community- acquired MRSA may also arise de novo, through horizontal acquisition of the mecA gene. M an important cause of nosocomial infections world- ethicillin-resistant Staphylococcus aureus (MRSA) is wide. Recent studies suggest that the epidemiology of MRSA may be changing, as the isolation of MRSA is no longer lim- ited to hospitalized patients or persons with predisposing risk factors (1–4). However, the prevalence of MRSA colonization in healthy persons in the community has been shown to be low, even when MRSA is highly endemic in hospital settings (5). Nosocomial MRSA strains in the community, including nursing homes and other nonacute-care facilities, may be transmitted by discharged patients and health-care workers (6,7). Whether strains of MRSA in the community also arise de novo, as a consequence of horizontal acquisition of the mecA gene, is unclear. The transfer of mecA DNA to a suscep- tible S. aureus strain has occurred in vitro (8) and recently in a hospitalized patient during antibiotic treatment (9). The mech- anism of transfer of mec DNA from a donor to a recipient is not completely understood. However, the excision and integra- tion of the mec DNA from and to the chromosome are appar- ently catalyzed by cassette chromosome recombinases A and B (Ccr A and B) coded by mec-associated genes (ccrA and B), with homology to the invertase-resolvase family of DNA recombinases (10). In Finland, the prevalence of MRSA has remained low, although several hospital epidemics have occurred in the last decade (11). We recently recognized two distinct groups of MRSA, one representing multiresistant epidemic strains and the other only β-lactam-resistant strains. These two groups also showed differences in ribotypes and mec determinant pro- files (12). The aim of this study was to estimate the proportion of community-acquired MRSA by analyzing the hospital con- tacts of persons from whom MRSA was found from 1997 to 1999. We also compared the MRSA isolates in persons with *National Public Health Institute, Helsinki, Finland and without hospital contact in terms of strain type (deter- mined by phage typing, pulsed-field gel electrophoresis [PFGE], and ribotyping), antibiotic resistance, and mec deter- minant profile. Materials and Methods Surveillance and Typing Scheme of MRSA Finnish microbiology laboratories report (generally elec- tronically) all MRSA isolations to the National Infectious Dis- ease Register at the National Public Health Institute (KTL). The KTL records the date, source of specimen, and the patient’s birth date, sex, and place of treatment. Using this information and a time interval of 36 months, multiple isola- tions from the same person are deleted from the database. The microbiology laboratories also send the MRSA isolates to the Laboratory of Hospital Bacteriology at KTL for further analy- sis. Phage typing, PFGE, and antimicrobial drug susceptibility testing were performed as described (11). In brief, phage typ- ing was performed with the universal set of phages (13) at 1x and 100x routine test dilutions, both with and without heat treatment of bacteria (14). Antimicrobial drug susceptibilities were tested by the disk diffusion method according to guide- lines recommended by the National Committee for Clinical Laboratory Standards (NCCLS). The antibiotics tested were oxacillin, ampicillin, penicillin, cephalexin, cefuroxime, gen- tamicin, tobramycin, erythromycin, clindamycin, chloram- phenicol, ciprofloxacin, rifampicin, fusidic acid, mupirocin, and vancomycin. MICs of oxacillin were determined by E-test (AB Biodisk, Solna, Sweden) according to manufacturer’s instructions. If the oxacillin MIC was <64 µ g/mL, methicillin resistance was verified with the MRSA screen test (Denka Seiken, Japan) or mecA-polymerase chain reaction (15). For PFGE, genomic DNA prepared in agarose blocks was digested with SmaI restriction endonuclease, and chromosomal 602 Emerging Infectious Diseases • Vol. 8, No. 6, June 2002