Dependence of induction of enterobacterial AmpC β-lactamase on cell-wall peptidoglycan, as demonstrated in Proteus mirabilis and its wall-less protoplast L-form

The mobilizable plasmid pMD101 (ampR, ampC) was constructed by inserting cloned ampC, the structural gene for the chromosomal AmpC β-lactamase of Citrobacter freundii, and the closely linked ampR encoding the transcriptional regulator essential for enzyme induction, into the broad host-range plasmid pKT231. Plasmid pMD101 was transconjugated into Proteus mirabilis VI and its isogenic, cell-wall-less protoplast L-form LVI. AmpC β-lactamase was expressed constitutively from cloned ampR and ampC in bacteria and in some L-form protoplasts. However, induction of the enzyme by β-lactam antibiotics occurred only in bacterial cells and not in the cell-wall- and peptidoglycan-deficient L-form. In agreement with current models, induction of AmpC β-lactamase is thought to be initiated by an induction signal arising from the metabolic disturbance of cell-wall peptidoglycan.