Accuracy and cost of antibiotic susceptibility testing of mixed morphotypes of Pseudomonas aeruginosa.

Chronic Pseudomonas aeruginosa colonization of the lower respiratory tract of patients with cystic fibrosis frequently results in pulmonary exacerbations requiring treatment with antimicrobial agents. Multiple morphotypes with different antibiotic susceptibilities are often isolated from a single sputum sample. Determination of MICs of antibiotics for each sputum morphotype is used to guide therapy but is time-consuming and expensive. We explored an alternative assay for determining MICs for all P. aeruginosa morphotypes cultured from a homogenized sputum sample. We sought correlations of those MICs with the MIC for the most resistant morphotypes tested separately. The MICs determined for a mixture of morphotypes correctly predicted the highest MICs (+/- one dilution) determined for isolated morphotypes 73.5% of the time. The MIC for the mixed morphotypes correctly predicted susceptibility in 90.4% of samples. In contrast, determination of the MIC for the mixture of morphotypes correctly predicted resistance in only 57.0%. For sputa containing susceptible isolates, testing the mixed culture may provide adequate susceptibility data with significant laboratory time and cost savings. However, for sputa with resistant strains, the traditional method of testing isolated morphotypes should still be used.