Mixed morphotype susceptibility testing of Pseudomonas aeruginosa from patients with cystic fibrosis

Disk diffusion antibiograms were determined for mixtures of Pseudomonas aeruginosa morphotypes isolated from the sputum of patients with cystic fibrosis (CF). The results were compared with the predicted susceptibility patterns derived from the antibiograms of individual morphotypes within the mixture. Fifty separate cultures, each yielding two, three, or four distinct morphotypes of Pseudomonas aeruginosa were evaluated. Overall, the correlation between observed and predicted results was 92.2% with only 2.9% of all observations leading to major disagreements in susceptibility. These data suggest that mixed morphotype susceptibility is potentially a useful method to monitor the collective resistance of colonizing strains of Pseudomonas aeruginosa from the respiratory tract of patients with CF.