In vivo development of antimicrobial resistance in Pseudomonas aeruginosa strains isolated from the lower respiratory tract of Intensive Care Unit patients with nosocomial pneumonia and receiving antipseudomonal therapy.

Pseudomonas aeruginosa causes severe nosocomial pneumonia in Intensive Care Unit (ICU) patients, with an increased prevalence of multiresistant strains. We examined the impact of the use of antipseudomonal antibiotic(s) on the susceptibility of P. aeruginosa isolated from ICU patients with clinically suspected hospital-acquired pneumonia collected in five teaching hospitals (110 non-duplicate initial isolates; 62 clonal pairs of initial and last isolates during treatment). Minimum inhibitory concentrations (MICs) were determined for amikacin, ciprofloxacin, meropenem, piperacillin/tazobactam (TZP), cefepime and ceftazidime (used in therapy) as well as five reporter antibiotics (aztreonam, colistin, gentamicin, piperacillin and ticarcillin) using Clinical and Laboratory Standards Institute (CLSI) methodology. Susceptibility was assessed according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) and CLSI breakpoints. Resistance rates prior to treatment exceeded 25% for cefepime, ceftazidime, piperacillin, ticarcillin and aztreonam (EUCAST and CLSI) and for gentamicin, TZP and colistin (EUCAST only). The highest rates of cross-resistance were noted for ceftazidime and cefepime and the lowest rate for amikacin. Mean MIC values were systematically higher in isolates from patients previously exposed (1 month) to the corresponding antibiotic. For clonal pairs, a systematic increase in MIC between initial and last isolates (significant for amikacin, cefepime, meropenem and TZP) was noted. There was a significant correlation between the use of antibiotics (adjusted for respective proportional use of each drug) and loss of susceptibility at the population level when using EUCAST breakpoints. The high level of resistance of P. aeruginosa in ICU patients with nosocomial pneumonia as well as its further increase during treatment severely narrows the already limited therapeutic options. Further observational studies and the development of early diagnosis for resistant isolates are warranted.