Association Between Resistance in Pseudomonas aeruginosa Isolates and Antibiotic Exposure in the Previous Six Months

Background: Study of risk factors for colonization and/or infection with antimicrobial-resistant bacteria are often limited by consideration of a restricted number of variables such as use of a single antibiotic. 

Objective: To assess frequency of prior antibiotic use as a risk factor for subsequent isolation of P. aeruginosa resistant to that antibiotic and frequency of use of one antibiotic to subsequent isolation of an isolate resistant to a different antibiotic.

Methods: We studied the relationship between prior, recent exposure to antibiotics and the subsequent presence of antibiotic resistant-Pseudomonas aeruginosa (within six months following antibiotic exposure) among inpatients, over a recent 18 month period.  Susceptibility to amikacin, tobramycin, gentamicin, ceftazidime, cefepime, ciprofloxacin, levofloxacin, piperacillin/tazobactam, aztreonam, imipinem, meropenem, and ticarcillin/clavulanate were determined by standard methods by the Clinical Microbiology Laboratory.  Percentages were calculated to represent the frequency of patients who received a particular antibiotic within the past 180 days. 

Results: A number of striking results were found between prior antibiotic use and current resistance. As examples, 62% of current ciprofloxacin resistant isolates were associated with ciprofloxacin use within the prior 180 days.  Fifty-six percent of imipinem resistant isolates occurred having prior meropenem exposure, and 62% of imipinem resistant isolates occurred having prior piperacillin/tazobactam exposure.  Piperacillin/tazobactam exposure was also present in 62% of tobramycin resistant isolates.

Conclusions: Not only is antibiotic resistance in P. aeruginosa associated with prior, recent exposure to that antibiotic, but also to prior, recent exposure to other antipseudomonal agents. This observation has implications for empiric antibiotic selection when P. aeruginosa is considered as a potential or proven pathogen.