PURPOSE: In individuals with cystic fibrosis (CF) experiencing respiratory exacerbations, response to antimicrobial therapy does not appear to be associated with the susceptibilities of the causative Pseudomonas aeruginosa (PA) strain. We wished to examine whether heterogeneity in antibiotic resistance among PA populations infecting these individuals might contribute to this phenomenon.
METHODS: Two consecutive respiratory samples were obtained from 20 CF patients infected with PA (40 total samples), and antibiotic susceptibilities were determined on 5 colonies from each specimen (200 total isolates). For comparison, conventional morphotype testing was performed.
RESULTS: Significant heterogeneity in antibiotic MICs was noted when 5 colonies were examined. In 37 of the 40 specimens (93%), at least one isolate differed (by two or more dilutions) from the rest in its MIC to at least one antibiotic. In 23 of 40 specimens (58%), at least one isolate differed in its MIC to at least two antibiotics. Heterogeneity in susceptibilities was greatest with penicillins, cephalosporins, and fluoroquinolones. This heterogeneity resulted in susceptibility changes (from resistant to susceptible or vice versa) in at least one isolate in 29 of the 40 specimens (73%). When compared to a conventional approach in which susceptibilities were independently determined only on representative colonies of different morphotypes from the same specimens, these results indicated that in 15 of 33 specimens (45%) the conventional approach reported susceptibility to at least one antibiotic for which resistant isolates were present. DNA genotyping demonstrated that in the majority of specimens the heterogeneity was within isolates of a single genotype and not due to the presence of multiple strains. Even in 26 respiratory specimens containing only a single genotype of PA, the conventional approach reported sensitivity when resistance was present in 12 (46%) cases.
CONCLUSION: Significant heterogeneity exists in the antibiotic susceptibility of PA isolates within a single respiratory specimen from CF patients.
CLINICAL IMPLICATIONS: Conventional approaches to determining antibiotic susceptibilities may be insufficient to adequately sample the phenotypic heterogeneity present even within single strains of PA infecting individuals with CF.
DISCLOSURE: Alan Hauser, None.