Recovery of antimicrobial-resistant Pseudomonas aeruginosa from sputa of cystic fibrosis patients by culture on selective media

Objectives: To assess the utility of direct plating of whole sputa onto selective media as a means of identifying antimicrobial resistance in strains of Pseudomonas aeruginosa from the sputa of patients with cystic fibrosis (CF). Methods: A total of 45 sputum samples from CF patients were cultured onto conventional culture media for isolation of P. aeruginosa and were also cultured directly onto Iso-Sensitest agar plates containing each of 10 antimicrobials incorporated at a 'breakpoint' concentration. A representative of each colonial type (morphotype) recovered from both routine media and selective media was tested for its susceptibility to 10 antimicrobials using a standard agar dilution MIC technique. Results: Of the samples shown to contain resistant strains, the proportion (%) detected using routine media and selective media, respectively, was: 42 and 100 for amikacin, 57 and 100 for gentamicin, 54 and 100 for tobramycin, 88 and 77 for aztreonam, 62 and 90 for ceftazidime, 70 and 97 for meropenem, 61 and 100 for piperacillin/tazobactam, 90 and 86 for temocillin, 66 and 100 for ticarcillin/clavulanic acid, and 80 and 90 for ciprofloxacin resistance. The increased rates of isolation on selective media were statistically significant (P < 0.05) for amikacin, gentamicin, tobramycin, meropenem, piperacillin/tazobactam and ticarcillin/clavulanic acid. Conclusions: For most antimicrobials, selection of colonies from conventional media for antimicrobial susceptibility testing provided a considerable underestimation of resistance in P. aeruginosa. The use of selective media for the culture of whole sputum was effective for the detection of resistant isolates of P. aeruginosa.