Genetic basis of induction and overproduction of chromosomal class I beta-lactamase in nonfastidious gram-negative bacilli.

Pseudomonas and several species of gram-negative enterobacteria produce low levels of chromosomally encoded class I beta-lactamase. The level of synthesis can be greatly increased by the addition of beta-lactam antibiotics into the growth medium. Moreover, mutants overproducing the enzyme arise at a high frequency in these species. The beta-lactamase is encoded by the chromosomal ampC gene, and its induction is governed by the ampR regulatory gene, which encodes an activator of ampC transcription. The ampD gene acts, probably indirectly, as a repressor of beta-lactamase synthesis, and it is in this gene that mutations leading to enzyme overproduction are located.