Effect of pH and CO2 on in vitro susceptibility of Pseudomonas cepacia to beta-lactams.

Inhibition of Pseudomonas cepacia (but not Pseudomonas aeruginosa) by beta-lactams was decreased in 5% CO2 in air compared with air alone. The effect of CO2 and pH (range, 6.0 to 8.0) on beta-lactam susceptibility, beta-lactamase expression, and outer membrane proteins was studied in isolates recovered from the sputum of children with cystic fibrosis. Incubation in 5% CO2 decreased the activity of piperacillin, piperacillin/tazobactam, and ceftazidime, although isolates were still clinically sensitive (minimum inhibitory concentrations < 16 mg/L). Cefpirome activity was markedly decreased from a minimum inhibitory concentration of 2.0 to greater than 64 mg/L. On highly buffered 3-(N-morpholino)-propane sulfonic acid media, beta-lactam susceptibility was eliminated at pH greater 7.5. A 2- to 13-fold increase in beta-lactamase activity was demonstrated after growth in 5% CO2 compared with basal aerobic levels for 13 of 15 clinical isolates. beta-Lactamase activity did not vary significantly with pH. Addition of imipenem to media (2.0 mg/L) resulted in hyperproduction of beta-lactamase (180-fold). Isoelectric points varied with cultural conditions, and all beta-lactamases detected were inhibited by clavulanate and tazobactam. Significant hydrolysis of piperacillin and ceftazidime could not be demonstrated. A 36-kD porin was present at all pH tested. Thus, our strains of Pseudomonas cepacia were markedly affected by cultural conditions not normally used in standardized susceptibility tests. However, such conditions may be encountered in the pathologically altered infected lung in cystic fibrosis.