title: Protective effect of methicillin-susceptible Staphylococcus aureus carriage against methicillin-resistant S. aureus acquisition in nursing homes: a prospective cross-sectional study.
abstract: OBJECTIVE
  To evaluate whether an ecologic inverse association exists between methicillin-susceptible Staphylococcus aureus (MSSA) prevalence and methicillin-resistant S. aureus (MRSA) prevalence in nursing homes.
  
  
  METHODS
  We conducted a secondary analysis of a prospective cross-sectional study of S. aureus prevalence in 26 nursing homes across Orange County, California, from 2008-2011. Admission prevalence was assessed using bilateral nares swabs collected from all new residents within 3 days of admission until 100 swabs were obtained. Point prevalence was assessed from a representative sample of 100 residents. Swab samples were plated on 5% sheep blood agar and Spectra MRSA chromogenic agar. If MRSA was detected, no further tests were performed. If MRSA was not detected, blood agar was evaluated for MSSA growth. We evaluated the association between MRSA and MSSA admission and point prevalence using correlation and linear regression testing.
  
  
  RESULTS
  We collected 3,806 total swabs. MRSA and MSSA admission prevalence were not correlated (r = -0.40, P = .09). However, MRSA and MSSA point prevalence were negatively correlated regardless of whether MSSA prevalence was measured among all residents sampled (r = -0.67, P = .0002) or among those who did not harbor MRSA (r = -0.41, P = .04). This effect persisted in regression models adjusted for the percentage of residents with diabetes (β = -0.73, P = .04), skin lesions (β = -1.17, P = .002), or invasive devices (β = -1.4, P = .0006).
  
  
  CONCLUSIONS
  The inverse association between MRSA and MSSA point prevalence and minimal association on admission prevalence suggest MSSA carriage may protect against MRSA acquisition in nursing homes. The minimal association on admission prevalence further suggests competition may occur during nursing home stays.
authors.affiliation: Health Policy Research Institute, University of California Irvine School of Medicine, Irvine, California.
authors.forename: Rupak
authors.initials: R
authors.lastname: Datta
authors.forename: Victor
authors.initials: V
authors.lastname: Quan
authors.forename: Diane
authors.initials: D
authors.lastname: Kim
authors.forename: Ellena M
authors.initials: EM
authors.lastname: Peterson
authors.forename: Courtney
authors.initials: C
authors.lastname: Reynolds
authors.forename: Hildy
authors.initials: H
authors.lastname: Meyers
authors.forename: Michele
authors.initials: M
authors.lastname: Cheung
authors.forename: Susan S
authors.initials: SS
authors.lastname: Huang
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002140:California / Q000453:epidemiology; D002353:Carrier State / Q000453:epidemiology / Q000276:immunology / Q000382:microbiology; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000276:immunology; D008875:Middle Aged; D009296:Nasal Cavity / Q000382:microbiology; D009735:Nursing Homes / Q000706:statistics & numerical data; D015995:Prevalence; D011446:Prospective Studies; D013203:Staphylococcal Infections / Q000453:epidemiology / Q000276:immunology / Q000382:microbiology*; D013211:Staphylococcus aureus / Q000276:immunology