Variable antibiotic susceptibility in populations of Pseudomonas aeruginosa infecting patients with bronchiectasis

To investigate variability in colony morphology and antibiotic susceptibility in populations of Pseudomonas aeruginosa from sputa of patients with bronchiectasis without cystic fibrosis (CF) compared with P. aeruginosa isolated from patients with CF, and from other infections as controls. P. aeruginosa was cultured from 31 patients with non-CF bronchiectasis, 24 with CF, 7 ventilated patients and 9 skin swabs. Four colonies of each morphotype of P. aeruginosa were tested for susceptibility to 12 antibiotics by disc diffusion. The variability in susceptibility between the isolates in each patient's population of P. aeruginosa was investigated. The classic morphotype of P. aeruginosa was cultured from control samples with an average variation in zone size of 2 mm (range 0-4 mm) for the four colonies tested. Non-CF bronchiectasis sputa contained 1-3 colonial morphotypes of P. aeruginosa; the average difference between the largest and smallest zone sizes found in all examples of the morphotypes present in each sample varied from 3 mm (1-9 mm) for colistin to 8 mm (0-24 mm) for piperacillin/tazobactam. CF sputa contained 2-6 morphotypes of P. aeruginosa with a wider variation of susceptibility. There was variation between bacteria of the same morphotype from non-CF bronchiectasis and CF sputa. Phenotypic variation in colonial form and antibiotic susceptibility is not unique to chronic infection in CF but is also found in non-CF bronchiectasis. This questions the use of current susceptibility testing methods for the complex populations of bacteria found in chronic lung infection.