Influence of previous exposure to antibiotic therapy on the susceptibilitypattern of Pseudomonas aeruginosa bacteremic isolates

Many patients who present with Pseudomonas aeruginosa bacteremia have been previously exposed to antibiotics. To assess whether resistance of bacterem ic strains to antipseudomonal antibiotics (piperacillin, ceftazidime, imipe nem, ciprofloxacin, or aminoglycosides) is associated with previous exposur e to these drugs, a case-control study including 267 cases of P. aeruginosa bacteremia was conducted. Twenty-five percent of the episodes had been pre ceded by the exposure to an antipseudomonal antibiotic. Eighty-one strains were resistant to at least 1 antibiotic; 186 were susceptible to all drugs. Via univariate analysis, the risks of resistance to ceftazidime and imipen em were found to be significantly associated with previous receipt of these agents. Using multivariate analysis, exposure to any antipseudomonal antib iotic as a monotherapy was found to be associated with an increased risk of subsequent resistance to itself (odds ratio, 2.5; P = .006). Therefore, cl inicians should avoid readministering previously prescribed antibiotics whe n initiating empiric therapies for possible P. aeruginosa bacteremia, espec ially when they have been given as monotherapies.