title: Population dynamics of nasal strains of methicillin-resistant Staphylococcus aureus--and their relation to community-associated disease activity.
abstract: BACKGROUND
  Nasal carriage of methicillin-resistant Staphylococcus aureus (MRSA) plays a key role in the epidemiology and pathogenesis of disease. The purpose of this study was to determine the characteristics and dynamics of nasal strains of MRSA, as well as their relation to community-associated disease activity.
  
  
  METHODS
  This study is a cross-sectional survey and molecular epidemiologic analysis of nasal colonization by S. aureus in homeless and runaway youths, an underserved population at high risk for staphylococcal disease.
  
  
  RESULTS
  Of the 308 study participants, 27.6% carried S. aureus, and 6.2% carried MRSA. Subgroups of individuals with increased MRSA carriage rates were also at highest risk for community-associated MRSA infection; these subgroups included individuals with either HIV infection or AIDS, injection drug users, patients with abscesses, and those recently hospitalized. Multilocus sequence typing and pulsed-field gel electrophoresis identified 2 genotypes--ST59:P (USA1000) and ST8:S (USA300)--that accounted for 84.2% (16/19) of the MRSA isolates carried. The genotypes were distinct from nosocomial genotypes endemic in the hospital, although they originated from individuals with prior exposure to health care.
  
  
  CONCLUSIONS
  Comparison of MRSA strains from asymptomatic carriers versus concurrently collected community-associated clinical strains from patients treated at local health-care facilities allowed for the identification of 3 population dynamics of nasal strains of MRSA: (1) endemic clones--for example, ST8:C and ST59:P--sustained asymptomatic carriage and infection over prolonged periods; (2) an epidemic clone, ST8:S, demonstrated enhanced capacity for rapid transmission and widespread infections; and (3) an outbreak clone, ST30:Z (USA1100), was highly infectious but exhibited poor asymptomatic transmission.
authors.affiliation: Department of Medicine, Division of Infectious Diseases, University of California, San Francisco, California, USA.
authors.forename: Erica S
authors.initials: ES
authors.lastname: Pan
authors.forename: Binh An
authors.initials: BA
authors.lastname: Diep
authors.forename: Edwin D
authors.initials: ED
authors.lastname: Charlebois
authors.forename: Colette
authors.initials: C
authors.lastname: Auerswald
authors.forename: Heather A
authors.initials: HA
authors.lastname: Carleton
authors.forename: George F
authors.initials: GF
authors.lastname: Sensabaugh
authors.forename: Françoise
authors.initials: F
authors.lastname: Perdreau-Remington
mesh_terms: D000293:Adolescent; D000328:Adult; D017714:Community-Acquired Infections / Q000453:epidemiology / Q000382:microbiology*; D003430:Cross-Sectional Studies; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D016521:Electrophoresis, Gel, Pulsed-Field; D005260:Female; D005838:Genotype; D006703:Ill-Housed Persons; D006801:Humans; D008297:Male; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D009297:Nasal Mucosa / Q000382:microbiology*; D012495:San Francisco / Q000453:epidemiology; D017422:Sequence Analysis, DNA; D013203:Staphylococcal Infections / Q000453:epidemiology / Q000382:microbiology*; D013211:Staphylococcus aureus / Q000235:genetics / Q000254:growth & development / Q000472:pathogenicity / Q000502:physiology*
chemical_list: D004269:DNA, Bacterial