Escherichia coli producing constitutive AmpC. ESBL-negative, carbapenemase-negative.

AmpC beta-lactamases are widely distributed among members of different phyla including several enterobacteria, and pseudomonads, which are the most frequent producers of this type of enzyme in the clinical setting. When enterobacteria overproduce the AmpC beta-lactamase they may become resistant to broad-spectrum penicillins, cephalosporins (except cefepime and cefpirome), beta-lactam/beta-lactamase inhibitor combinations, and aztreonam.

Two mechanisms account for AmpC activity in E.coli:
1. Acquisition of plasmid-carried ampC genes.
2. Mutations in the ampC promoter and attenuator regions resulting in ampC overexpression.

AmpC enzymes are active on penicillins but even more active on cephalosporins and can hydrolyze cephamycins (cefoxitin, cefotetatn), oxyiminocephalosporins (ceftazidime, cefotaxime, ceftriaxone) and monobactams such as aztreonam (at a rate <1% of that of benzylpenicillin).

AmpC vs. ESBL Key Differences

• AmpC-producing E.coli is resistatn to cefoxitin, while ESBL-producing strains are usually susceptible.
• ESBL enzymes are inhibited by clavulanic acid, while AmpC enzymes are not.
• Both require carbapenems in severe infections, but cefepime may be an option for some AmpC strains.