National survey of susceptibility to antimicrobials amongst clinical isolates of Pseudomonas aeruginosa.

Between September and December 1993, each of 24 hospitals in the UK collected up to 100 consecutive clinical isolates of Pseudomonas aeruginosa and sent these to the London Hospital Medical College (LHMC). Of 2184 cultures received, 1991 contained viable P. aeruginosa. Minimum inhibitory concentrations (MICs) of antimicrobials were determined by agar dilution. The frequencies of resistance to low breakpoints were as follows: gentamicin, MIC > 2 mg/L, 11.7%; amikacin, MIC > 4 mg/L, 10.5%, carbenicillin, MIC > 128 mg/L, 11.7%; azlocillin, MIC> 16 mg/L, 10.9%; ceftazidime, MIC > 4 mg/L, 9.6%; ciprofloxacin, MIC > 1 mg/L, 8.1%; imipenem, MIC > 4 mg/L 2.5% and meropenem, MIC > 4 mg/L, 1.1%. Resistance to each antimicrobial except amikacin was commoner among the 134 isolates from patients in intensive care units (ICUs) than amongst the 1042 isolates from other in-patients (P < 0.01). Resistance to penicillins and ceftazidime, though not to other agents, was rarer among the 797 isolates from out-patients than amongst those from non-ICU in-patient (P < 0.01). Compared to a similar study in 1982, during which 1866 isolates had been examined, the frequency of resistance to the aminoglycosides increased (P < 0.05) as had those to the penicillins and ceftazidime (P < 0.01). Ciprofloxacin and the carbapenems were not tested in 1982. Cross-resistance patterns suggested that the increases in resistance to aminoglycosides and beta-lactams were largely a reflection of greater numbers of isolates with barrier or efflux mechanisms and were not due to an increase in isolates with antibiotic-degrading enzymes. The participating hospitals mostly employed Stokes' disc diffusion method and, when the results were compared to the MICs determined at the LHMC, fewer than 9% of the isolates reported as susceptible were found to be resistant. However, up to 72% of those reported by the hospitals as resistant were found to be susceptible.