Derepressed beta-lactamase production as a mediator of high-level beta-lactam resistance in Pseudomonas cepacia

A mechanism of high-level resistance to readily and poorly hydrolyzable beta-lactam substrates in Pseudomonas cepacia was identified using a hypersusceptible, beta-lactamase-inducible, non-CF clinical isolate, 75-26, and a drug-resistant mutant derived from this strain. Inoculation of 75-26 onto agar containing 16 micrograms/ml of ceftazidime produced a stable, beta-lactam-resistant mutant at a frequency of 1.7 x 10(-5). Baseline beta-lactamase production by a representative mutant isolate (75-26z) was almost 40-fold greater than the parent. Both strains produced major beta-lactamase bands with isoelectric points of 6.9, 7.8, 8.1, 8.5, and 9.2 by isoelectric focusing. Compared with the parental strain, multiple satellite bands associated with the major beta-lactamase bands were present in the mutant. Growth of an indicator strain, E. coli ATCC 25922, was inhibited by ceftazidime and piperacillin but was not inhibited after the compounds were preincubated with the beta-lactamase preparation from 75-26z. Preincubation of ceftazidime with beta-lactamase, followed by the addition of a beta-lactamase inhibitor, inhibited the growth of the indicator strain; piperacillin failed to inhibit growth of the indicator strain in a similar experiment. One mechanism of high-level resistance to both poorly and readily hydrolyzable beta-lactam substrates in P. cepacia is derepressed chromosomal beta-lactamase production.