Background: Methicillin-resistant Staphylococcus aureus (MRSA) infections are associated with severe necrotizing syndromes and high mortality, accounting for 20% to 40% of all hospital-acquired pneumonia and ventilator-associated pneumonia. There is insufficient data on comparative outcomes by specific strain characteristics in MRSA isolates in patients with pneumonia.
Methods: Clinical outcomes of patients with USA600 (ST45) MRSA strain were compared with other sequence types in MRSA pneumonia from 5 different hospitals in a retrospective case-control study. Patients were identified through review of microbiology laboratory records and through the Improving Medicine through Pathway Assessment of Critical Therapy in Hospital-Acquired Pneumonia database. Pulsed-field gel electrophoresis of SmaI-digested genomic DNA was performed on all isolates using a CHEF-DR III (BioRad). Pulsed-field gel electrophoresis patterns were compared using BioNumerics software (Applied Maths).
Results: Two hundred fifty-one consecutive patients with MRSA pneumonia were evaluable for an all-cause 28-day mortality and 14-day failure outcome. Prevalence of USA600 was 8% (21). Laboratory characteristics of USA600 isolates were 100% (21) Panton-Valentine leukocidin toxin negative, agr I type, 67% (14) heteroresistant to vancomycin, and higher rates of SCCmec type II versus SCCmec type IVa, 95% versus 5%. Twenty-eight–day mortality rates were USA600 (52.4%), USA100 (ST5; 29%), and USA300 (ST8; 32%); and 14-day failure rates were USA600 (50%), USA100 (37%), and USA300 (27%), respectively.
Conclusions: This is the first comparative observational study of its kind, with evidence of a much higher failure rate (>50%) within patients with USA600 MRSA pneumonia. The high mortality rate in this subset warrants further investigation of factors of this emerging strain that may predict mortality and failure outcomes.