Methicillin-susceptible Staphylococcus aureus in Community-Acquired Pneumonia: Risk Factors and Outcomes

Background: The epidemiology of Staphylococcus aureus is changing, with several studies reporting a decline in methicillin-resistant S. aureus (MRSA) infection and an increase in methicillin-susceptible S. aureus (MSSA) infection. We aimed to describe and compare the prevalence, clinical features, and risk factors of community-acquired pneumonia (CAP) caused by MSSA with that caused by Streptococcus pneumoniae. Methods: This was a prospective observational cohort study of consecutive adults with CAP and a definitive etiology that were enrolled between January 2004 and December 2018. Patients were divided into groups with MSSA CAP and pneumococcal CAP for analysis. Results: A microbial etiology was established in 1,548 patients with CAP, of which 52 cases were caused by MSSA and 734 were caused by S. pneumoniae. MSSA affected 1% of all patients hospitalized with CAP, and 3% when a microbiological diagnosis was obtained. The presence of fever was independently associated with a lower risk of MSSA CAP. Patients with CAP due to MSSA had higher 30-day mortality than patients with pneumococcal CAP, both before and after adjustment for potential confounders. MSSA infection, older age, inhaled corticosteroid use, lymphopenia, acute renal failure, acute respiratory distress syndrome, and septic shock were associated with 30-day mortality. Conclusion: MSSA CAP was associated with worse outcomes compared with pneumococcal CAP in our cohort. The absence of fever was associated with an increased risk of MSSA CAP. Methicillin-susceptibility in S. aureus strains causing CAP should not be regarded as a marker of lesser severity.