Induction of chromosomal beta-lactamase expression in enterobacteria.

Enterobacteria and Pseudomonas express evolutionarily related chromosomally encoded beta-lactamases. These enzymes have a high affinity for modern cephalosporins, and beta-lactamase overproduction is the most important factor in the development of resistance against such drugs. In some species the beta-lactamase is produced constitutively at a low level, and mutations to high expression occur only infrequently. In other species beta-lactamase synthesis can be induced by beta-lactams, and mutation to constitutive overexpression of the enzyme is a frequent event. We discuss the current knowledge concerning the genetic basis of the two different modes of beta-lactamase regulation, as well as the mechanisms through which a high level of beta-lactamase synthesis is reached.