Background: Staphylococcus aureus (SA) is one of the most important pathogens in hospital-acquired infections. About 40% of SA clinical isolates are resistant to methicillin. We have noticed that patients colonized with methicillin-susceptible S. aureus (MSSA) do not get methicillin-resistant S. aureus (MRSA) infections. The purpose of this study was to compare how frequently SA infections are MRSA infections in MSSA carriers, MRSA carriers, and non-carriers of SA.
Methods: Adult patients hospitalized at the Cleveland Clinic Health System from 2008 to 2015, who were tested for SA colonization during the hospitalization were screened. The first hospitalization per patient was included. Patients were classified as MRSA carriers, MSSA carriers, and non-carriers, based on their first nasal SA test result. For patients who had SA bacteremia (SAB) or a non-bacteremic SA infection (NBSAI) during the same hospitalization, the association of carrier status and MRSA infection was examined.
Results: Of 90891 patients identified, 5774 (6%) were MRSA carriers and 12510 (14%) MSSA carriers. Among 873 patients with SAB, 241 (93%) of the 260 SABs in MRSA carriers (reference group) were MRSA bacteremia compared with 17 (7%) of 230 in MSSA carriers (OR 0.006, 99% CI 0.003 0.012, p-value 2.32 × 10-48) and 203 (53%) of 383 in non-carriers (OR 0.089, 99% CI 0.052 0.144, p-value 1.044 × 10-20). Among 1126 patients with NBSAI, 309 (91%) of the 341 NBSAIs in MRSA carriers (reference group) were MRSA infections compared with 25 (9%) of 286 in MSSA carriers (OR 0.010, 99% CI 0.006 0.017, p-value 4.73 × 10-61) and 226 (45%) of 499 in non-carriers (OR 0.086, 99% CI 0.056 0.127, p-value 1.10 × 10-32). A Monte Carlo simulation with 1000 trials simulating corrections for false positive and false negative nasal MRSA tests found very similar results. The odds of a SA infection being an MRSA infection was much lower in MSSA carriers than in MRSA carriers across all subgroups of age, sex, hospital, length of stay, and year (figure).
Conclusion: SA infections in MSSA carriers are 100 times less likely to be MRSA infections than are SA infections in MRSA carriers. Patients colonized with MSSA rarely get MRSA infections. This fact can be used to support an antimicrobial stewardship policy of actively discouraging empiric coverage for MRSA infection in MSSA carriers.
Figure. Subgroup analysis
S. Gordon, None