Persistence ofstaphylococcus aureusin lower respiratory tract in patients undergoing mechanical ventilation

Introduction: Staphylococcus aureus is frequently isolated in respiratory samples both as colonizer and causing lower respiratory tract infection (LRTI) and may persist despite treatment with risk of developing resistance. Objective: To investigate the correlation of persistent isolation of S.aureus with clinical and microbiological variables in patients undergoing mechanical ventilation treated as LRTI. Methods: Clinico-epidemiological data were collected from patients with isolation of S.aureus in tracheal aspirate. Persistence was considered when lasted more than 3 days despite treatment adjusted to susceptibility profile. Strains were phenotypically and genotypically characterized. Results: During the study period (3 years) S.aureus was isolated from 122 patients (82,3% nosocomial), stratified by CPIS as 28 pneumonia,43 tracheobronchitis and 51 bronchial colonization. Among the 91 patients that received antimicrobial treatment after first isolation, persistence was documented in 32 cases (35,2%)(8,9 days).None of the clinical variables correlated with persistence except ictus of ischemic origin (but only 5 cases).Regarding strains characteristics, there was no correlation with cloxacillin resistance (18%MRSA), clonal complex and presence of selected virulence factors. However, agrI allele (p=0,002) and absence of TSST-1 gene (p=0,025) were more frequent in persistent strains. Conclusion: Persistent isolation of S.aureus is frequent, without correlation to clinical variables, prognosis,nor resistance pattern.However, some genotypical features of S.aureus might be related, suggesting its adaptation to the lower respiratory tract allowing its survival.SEPAR054/2011;FIS PI13/01418.