Antibiotic pressure is a major risk factor for rectal colonization by multidrug-resistant Pseudomonas aeruginosa in critically ill patients.

The intestinal reservoir is central to the epidemiology of Pseudomonas aeruginosa, but the dynamics of intestinal colonization by different phenotypes have been poorly described. To determine the impact of antimicrobial exposure on intestinal colonization by multidrug-resistant (MDR) and extensively drug-resistant (XDR) P. aeruginosa, we screened intensive care unit (ICU) patients for rectal colonization on admission and at weekly intervals. During an 18-month study period, 414 ICU patients were enrolled, of whom 179 (43%) were colonized; 112 (63%) of these were identified at ICU admission and 67 (37%) during their ICU stay. At 10 days after ICU admission, the probabilities of carriage were 44%, 24%, and 24% for non-MDR, MDR-non-XDR, and XDR P. aeruginosa strains, respectively (log rank, 0.02). Pulsed-field gel electrophoresis showed 10 pairs of non-MDR P. aeruginosa and subsequent MDR-non-XDR strains isolated from the same patients to be clonally identical and another 13 pairs (8 MDR-non-XDR and 5 XDR) to be unrelated. There was one specific clone between the 8 MDR-non-XDR strains and an identical genotype in the 5 XDR isolates. The Cox regression analysis identified MDR P. aeruginosa acquisition as associated with the underlying disease severity (adjusted hazard ratio [aHR], 1.97; 95% confidence interval [CI], 1.22 to 3.18; P = 0.006) and prior use of fluoroquinolones (aHR, 1.02; 95% CI, 1.00 to 1.04; P = 0.039), group 2 carbapenems (aHR, 1.03; 95% CI, 1.00 to 1.07; P = 0.041), and ertapenem (aHR, 1.08; 95% CI, 1.02 to 1.14; P = 0.004). The epidemiology of MDR P. aeruginosa is complex, and different clusters may coexist. Interestingly, ertapenem was found to be associated with the emergence of MDR isolates.