Impact of Methicillin Resistance on Outcomes of Staphylococcus aureus Bacteremia at a U.S. Center, 2023–2024

Abstract Background Methicillin-resistant Staphylococcus aureus (MRSA) bacteremia has historically been associated with worse outcomes than methicillin-susceptible S. aureus (MSSA), but MRSA epidemiology has evolved and contemporary data are mixed. We evaluated whether methicillin resistance is independently associated with S. aureus bacteremia (SAB) outcomes. Methods We conducted a retrospective cohort study of adults with a first-episode SAB at two academic hospitals from January 17, 2023 to January 16, 2024. The primary outcome of in-hospital mortality/hospice discharge was compared between MRSA and MSSA. Secondary outcomes included intensive care unit (ICU) admission, 7-day readmission, metastatic complications, infective endocarditis, and recurrent bacteremia. Multivariable logistic regression was used to estimate adjusted odds ratios (aORs), with propensity score matching as sensitivity analysis. Results Among 329 patients with SAB, 140 (42.6%) had MRSA and 189 (57.4%) had MSSA. Compared with MSSA, MRSA was associated with younger age, lower body mass index, higher injection drug use prevalence, more skin and soft tissue sources, and lower illness severity. After covariate adjustment, methicillin resistance was not associated with mortality/hospice discharge (aOR 1.03, 95% CI 0.49–2.12), ICU admission (aOR 1.09, 95% CI 0.61–1.94), 7-day readmission (aOR 1.51, 95% CI 0.51–4.67), metastatic complications (aOR 0.98, 95% CI 0.47–2.05), infective endocarditis (aOR 1.02, 95% CI 0.42–2.45), or recurrent bacteremia (aOR 2.43, 95% CI 0.92–6.76). Findings were consistent in propensity-matched analyses. Conclusions In this contemporary academic cohort, methicillin resistance was associated with distinct patient characteristics but not worse short-term outcomes. Host factors, illness severity, and infection source may be more important prognostic determinants.