Regulation of inducible AmpC beta-lactamase expression among Enterobacteriaceae

AmpC ss-lactamases are active-site serine enzymes that are primarily cephalosporinases. In many gram negative organisms, including Enterobacter spp.,Citrobacter freundii, Serratia marcescens, Morganella morganii and Pseudomonas aeruginosa, the expression of chromosomal ampC genes is low but inducible in response to ss-lactams and other stimuli. The current working model for AmpC induction requires exposure of bacterial cells to ss-lactam drugs or other stimuli and is linked to the cell wall recycling pathway. Induction of ampC appears to involve several gene products associated with this pathway. These gene products include AmpR, AmpD, and AmpG. In addition, anhydro forms of cell wall precursor muropeptides are believed to act as cofactors for AmpC induction. These cofactors bind to the DNA binding protein, AmpR, and define the role of AmpR as activator. Recent debate has ensued in the literature as to the identification of the precursor muropeptide involved in the activation process. Two candidate muropeptides include 1,6-anhydro-N-acetylmuramic acid L-Ala-D-Glu-meso-diaminopimelic acid (anhydro-MurNAc-tripeptide) and anhydro-MurNAc-L-Ala-D-Glu-meso-diaminopimelic acid- D-Ala-D-Ala (pentapeptide). The intent of this review is to address the general mechanism involved in AmpC induction. In doing so, the genes and gene products required for the process of AmpC induction are described. In addition, we review the data addressing cell wall recycling as it relates to AmpC induction.