Heterogeneity of Beta-Lactamase Production in Pseudomonas maltophilia, a Nosocomial Pathogen

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Abstract
Twenty Pseudomonas maltophilia isolates were examined for susceptibility to β-lactam antibiotics, including carbapenems, and for β-lactamase production. All the isolates were resistant to imipenem (MICs 64–512 mg/l) and to a lesser extent, to meropenem (MICs 16–256 mg/l). None of the isolates produced significant amounts of β-lactamase without induction. Among the β-lactams studied imipenem proved to be the most potent inducer; meropenem was a weaker inducer. Interestingly, 6-amino-penicil-lanic acid, even in concentrations up to 100 mg/l, entirely lacked induction activity. In any case enzyme production was drug concentration dependent and transient. Isoelectric focusing revealed 6 different enzymes distinguished by their different isoelectric points (pH 6.2, 8.3, 8.5, 9.0, 9.2 and 9.4). This suggested the lack of a unique β-lactamase profile in P. maltophilia. Addition of 5 mM cyclic AMP (cAMP) or 0.5 mM cAMP-N6, O2-dioctanoyl (a lipophilic derivative) resulted in a marked drop of β-lactamase induction by imipenem as compared to the control assay. Monitoring of carbapenem hydrolysis by cell-free supernatants revealed inactivation of both carbapenems. Meropenem was inactivated about 5 times more rapidly than imipenem. Our studies revealed that β-lactamase production in P. maltophilia as well as growth kinetics were influenced to a considerable extent by the nutrient medium employed.