Effects of systemic antibiotic therapy on bacterial persistence in the respiratory tract of mechanically ventilated patients

Objective: Bacterial respiratory tract colonization predisposes critically ill patients to intensive care unit (ICU)-acquired infections. It is unclear to what extent systemic antibiotics affect colonization persistence. Persistence of respiratory tract colonization, and the effects of systemic antibiotics hereon, were determined in a cohort of ICU patients. Design: Clinical and microbiological data were collected from 715 admitted mechanically ventilated ICU patients with bacterial growth documented in respiratory tract samples. First day of colonization, persistence of colonization and antibiotic effects hereon were analyzed for six groups of pathogens: Pseudomonas aeruginosa, Acinetobacter species, Enterobacteriaceae, Staphylococcus aureus, Streptococcus pneumoniae and Haemophilus influenzae. Systemic antibiotics were grouped into 'effective' and 'ineffective' antibiotics, based on in-vitro susceptibility data for the relevant bacteria. The effects of antibiotics were quantified as relative risk (RR) of bacterial persistence in the absence of effective antibiotics. Measurements and results: Persistence of colonization differed significantly between pathogens, ranging from 4 days (median) for H. influenzae and Strep. pneumoniae to 8 days for P. aeruginosa. Systemic antibiotics were administered on 7,102 (61%) of patient days. Antibiotic use was associated with non-persistence for all pathogens, except Acinetobacter species and P. aeruginosa. RR for non-persistence (as compared to ineffective or no antibiotics) ranged from 3.1 (95% CI 1.4-6.6) for H. influenzae to 0.5 (0.3-1.0) for Acinetobacter species. Conclusions: In mechanically ventilated patients, persistence dynamics of bacterial respiratory tract colonization, and the effects of (in-vitro) effective antibiotics hereon, are pathogen-specific.