Methicillin-Susceptible Staphylococcus aureus as a Predominantly Healthcare-Associated Pathogen: A Possible Reversal of Roles?

Methicillin-resistant Staphylococcus aureus (MRSA) strains have become common causes of skin and soft tissue infections (SSTI) among previously healthy people, a role of methicillin-susceptible (MSSA) isolates before the mid-1990s. We hypothesized that, as MRSA infections became more common among S. aureus infections in the community, perhaps MSSA infections had become more important as a cause of healthcare-associated infection.We compared patients, including children and adults, with MRSA and MSSA infections at the University of Chicago Medical Center (UCMC) from all clinical units from July 1, 2004-June 30, 2005; we also compared the genotypes of the MRSA and MSSA infecting bacterial strains.Compared with MRSA patients, MSSA patients were more likely on bivariate analysis to have bacteremia, endocarditis, or sepsis (p = 0.03), to be an adult (p = 0.005), to be in the intensive care unit (21.9% vs. 15.6%) or another inpatient unit (45.6% vs. 40.7%) at the time of culture. MRSA (346/545) and MSSA (76/114) patients did not differ significantly in the proportion classified as HA-S. aureus by the CDC CA-MRSA definition (p = 0.5). The genetic backgrounds of MRSA and MSSA multilocus sequence type (ST) 1, ST5, ST8, ST30, and ST59 comprised in combination 94.5% of MRSA isolates and 50.9% of MSSA isolates. By logistic regression, being cared for in the Emergency Department (OR 4.6, CI 1.5-14.0, p = 0.008) was associated with MRSA infection.Patients with MSSA at UCMC have characteristics consistent with a health-care-associated infection more often than do patients with MRSA; a possible role reversal has occurred for MSSA and MRSA strains. Clinical MSSA and MRSA strains shared genotype backgrounds

Methicillin-Susceptible <i>Staphylococcus aureus</i> as a Predominantly Healthcare-Associated Pathogen: A Possible Reversal of Roles?

Background: Methicillin-resistant Staphylococcus aureus (MRSA) strains have become common causes of skin and soft tissue infections (SSTI) among previously healthy people, a role of methicillin-susceptible (MSSA) isolates before the mid-1990s. We hypothesized that, as MRSA infections became more common among S. aureus infections in the community, perhaps MSSA infections had become more important as a cause of healthcare-associated infection.Methods: We compared patients, including children and adults, with MRSA and MSSA infections at the University of Chicago Medical Center (UCMC) from all clinical units from July 1, 2004-June 30, 2005; we also compared the genotypes of the MRSA and MSSA infecting bacterial strains.Results: Compared with MRSA patients, MSSA patients were more likely on bivariate analysis to have bacteremia, endocarditis, or sepsis (p = 0.03), to be an adult (p = 0.005), to be in the intensive care unit (21.9% vs. 15.6%) or another inpatient unit (45.6% vs. 40.7%) at the time of culture. MRSA (346/545) and MSSA (76/114) patients did not differ significantly in the proportion classified as HA-S. aureus by the CDC CA-MRSA definition (p = 0.5). The genetic backgrounds of MRSA and MSSA multilocus sequence type (ST) 1, ST5, ST8, ST30, and ST59 comprised in combination 94.5% of MRSA isolates and 50.9% of MSSA isolates. By logistic regression, being cared for in the Emergency Department (OR 4.6, CI 1.5-14.0, p = 0.008) was associated with MRSA infection.Conclusion: Patients with MSSA at UCMC have characteristics consistent with a health-care-associated infection more often than do patients with MRSA; a possible role reversal has occurred for MSSA and MRSA strains. Clinical MSSA and MRSA strains shared genotype backgrounds.</p

Presence of CDC risk factors for HA-MRSA^* among patients with MSSA and MRSA infections.

<p>*Absence of these risk factors comprise the CDC case definition for community-associated MRSA infections. Denominators for HA-MRSA risk factors exclude those interviewed patients who answered that that they did not know information requested of them and those patients about whom risk factor information could not be determined from chart review. For all 659 patients it was determined whether MRSA had been isolated from them at UCH since 1994, but for 295 patients, it could not be determined if MRSA had been isolated from them at another health care facility. The information regarding a stay in a long-term care facility was determined only for those patients lacking another health-care risk factor.</p><p><u>Abbreviations</u>: <i>HA-</i>, health care associated; <i>MRSA</i>, methicillin-resistant <i>Staphylococcus aureus</i>; <i>MSSA</i>, methicillin-susceptible <i>Staphylococcus aureus</i>.</p

(a) Logistic regression Model 1. including variables demonstrating significant association with MRSA infection on bivariate analysis (<i>n</i> = 184); (b) Logistic regression Model 2. same as Model 1. <i>excluding</i> variable for ever been incarcerated (<i>n</i> = 455).

<p>*African American (<i>n</i> = 139) compared with all others of known race (<i>n</i> = 45).</p><p>†Public Insurance or uninsured (<i>n</i> = 131) compared with privately insured (<i>n</i> = 53).</p><p>‡Indicates odds ratio for every additional person in the household.</p><p>∫African American (<i>n</i> = 353) compared with all others of known race (<i>n</i> = 102).</p><p>¶Public Insurance or uninsured (<i>n</i> = 333) compared with privately insured (<i>n</i> = 122).</p><p><u>Abbreviations</u>: <i>SSTI</i>, skin or soft tissue infection.</p

Demographic and clinical characteristics of patients with MSSA and MRSA infections.

<p>*Includes abdominal abscess, toxic shock syndrome, cholecystitis, conjunctivitis, peritonitis, empyema, neurosurgical device infection, uncertain site of culture, and upper respiratory infection.</p><p><u>Abbreviations</u>: <i>MRSA</i>, methicillin-resistant <i>Staphylococcus aureus</i>; <i>MSSA</i>, methicillin-susceptible <i>Staphylococcus aureus</i>.</p

MLST of MRSA and MSSA isolates causing infections at UCMC.

<p>*CC1 was subsumed by the CC15 by the MLST administrators.</p><p>†<i>slv</i>, single locus variant.</p><p><u>Abbreviations</u>: <i>MLST</i>, multilocus sequence typing; <i>MRSA</i>, methicillin-resistant <i>Staphylococcus aureus</i>; <i>MSSA</i>, methicillin-susceptible <i>Staphylococcus aureus</i>.</p

Percent of MSSA and MRSA isolates from UCMC, July 1, 2004-June 30, 2005, belonging to ST1, ST5, ST8, ST30, ST59, and other genetic backgrounds.

<p>Percent of MSSA and MRSA isolates from UCMC, July 1, 2004-June 30, 2005, belonging to ST1, ST5, ST8, ST30, ST59, and other genetic backgrounds.</p

MRSA USA300 at Alaska Native Medical Center, Anchorage, Alaska, USA, 2000–2006

To determine whether methicillin-resistant Staphylococcus aureus (MRSA) USA300 commonly caused infections among Alaska Natives, we examined clinical MRSA isolates from the Alaska Native Medical Center, Anchorage, during 2000–2006. Among Anchorage-region residents, USA300 was a minor constituent among MRSA isolates in 2000–2003 (11/68, 16%); by 2006, USA300 was the exclusive genotype identified (10/10)