Determination of intracellular and extracellular β-lactamase activities of Pseudomonas aeruginosa after exposure to β-lactams in vitro and in vivo

β-Lactamase production in Pseudomonas aeruginosa was determined in in-vitro models and in rat pouch infection models after exposure to ceftazidime, imipenem, and piperacillin. Exposure of 28 P. aeruginosa strains to 1/4 minimum inhibitory concentration (MIC) of ceftazidime, imipenem, and piperacillin for 24h enhanced intracellular β-lactamase activities in 14, 22, and 6 strains, respectively, of the 28 clinical strains tested, and enhanced extracellular β-lactamase activities which were not detected without exposure to antibiotics, in 7, 23, and 1 of the 28 strains, respectively. Extracellular β-lactamase activity from P. aeruginosa S-1278, producing an inducible β-lactamase, scarcely increased after exposure to ceftazidime and piperacillin 24h after incubation, while the activity increased after exposure to imipenem over the range of 1/8 to 8 MIC. In the rat granuloma pouch models infected with P. aeruginosa S-1278, ceftazidime and piperacillin, after single administration (20mg/kg) and serial administration (20mg/kg per day × 3 days), did not enhance extracellular β-lactamase activities. However, the activities were enhanced with single and serial administrations of imipenem, and levels over 10mU/ml were detected until the third day. The β-lactamase activity, similar to the activity found in rat pouches after serial administration of imipenem, inactivated various cephalosporins. In conclusion, extracellular β-lactamase activity was detected both in vitro and in vivo after exposure to a good inducer, and extracellular β-lactamase remained at infection site at levels that could inactivate cephalosporins.