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. T he effect of CO2 and pH (range, 6.0 to 8.0) on beta-lactam susceptibil ity, beta-lactamase expression, and outer membrane proteins was studie d in isolates recovered from the sputum of children with cystic fibros is. Incubation in 5% CO2 decreased the activity of piperacillin, piper acillin/tozobactam, and ceftazidime, although isolates were still clin ically sensitive (minimum inhibitory concentrations < 16 mg/L). Cefpir ome activity was markedly decreased from a minimum inhibitory concentr ation of 2.0 to greater than 64 mg/L. On highly buffered 3-(N-morpholi no)-propane sulfonic acid media, beta-lactam susceptibility was elimin ated at pH greater 7.5. A 2- to 13-fold increase in beta-lactamase act ivity was demonstrated after growth in 5% CO2 compared with basal aero bic 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). Isoelect ric points varied with cultural conditions, and all beta-lactamases de tected were inhibited by clavulanate and tazobactam. Significant hydro lysis of piperacillin and ceftazidime could not be demonstrated. A 36- kD porin was present at all pH tested. Thus, our strains of Pseudomona s cepacia were markedly affected by cultural conditions not normally u sed in standardized susceptibility tests. However, such conditions may be encountered in the pathologically altered infected lung in cystic fibrosis.