Comparative evaluation of selective media for isolation of Pseudomonas cepacia from cystic fibrosis patients and environmental sources.

Abstract

Pseudomonas cepacia has recently emerged as an important pathogen affecting cystic fibrosis (CF) patients. We evaluated three selective media to assess their comparative potential for identification of patients colonized with P. cepacia and for efficacy of detection of P. cepacia in environmental fluids. Test organisms included P. cepacia isolates from CF patients (10 each from two CF centers), non-CF patients (10 isolates), and environmental sources (10 isolates). Microbiologic assays were done by the membrane filter procedure; filters were placed on P. cepacia medium (PCM), OFPBL, TB-T, MacConkey agar (MAC), and blood agar (BA) or Standard Methods (SM) sugar, and colonies were counted after incubation at 30 or 35 degrees C for 72 h. Mean recovery efficiencies (MREs) (mean CFU/ml on selective media compared with CFU/ml on BA controls) for environmental and non-CF P. cepacia and patient isolates from one CF center showed a rank order of PCM greater than OFPBL greater than TB-T; for isolates from a second CF center, a rank order of PCM greater than TB-T greater than OFPBL was obtained. MREs for CF center isolates were generally lower than for non-CF patients or environmental isolates on P. cepacia-selective media. With MAC, the MREs for each group of CF isolates were extremely low (14 and 2%) compared with those for non-CF patient (47%) or environmental (84%) isolates. In laboratory and field studies, PCM and OFPBL showed good selectivity against bacteria commonly associated with CF patient respiratory secretions. These findings show that selective media should be used in clinical settings where P. cepacia is sought. With environmental fluids from CF centers, P. cepacia-selective media showed low selectivity against a variety of gram-negative water bacteria and appeared to afford little advantage over SM agar for isolating P. cepacia from environmental samples.