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

Objective: Bacterial respi- ratory tract colonization predisposes critically ill patients to intensive care unit (ICU)-acquired infections. It is unclear to what extent systemic antibiotics affect colonization per- sistence. Persistence of respiratory tract colonization, and the effects of systemic antibiotics hereon, were de- termined 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 coloniza- tion and antibiotic effects hereon were analyzed for six groups of pathogens: Pseudomonas aeruginosa, Acineto- bacter species, Enterobacteriaceae, Staphylococcus aureus, Streptococ- cus 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 an- tibiotics were quantified as relative risk (RR) of bacterial persistence in the absence of effective antibiotics. Measurements and results: Persis- tence of colonization differed signif- icantly between pathogens, ranging from 4 days (median) for H. influen- zae and Strep. pneumoniae to 8d ays for P. aeruginosa. Systemic antibi- otics were administered on 7,102 (61%) of patient days. Antibiotic use was associated with non-persistence for all pathogens, except Acinetobac- ter species and P. aeruginosa .R R 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.