ampC

AmpC refers to a family of beta-lactamase enzymes that confer resistance to a broad range of beta-lactam antibiotics. They are Ambler class C beta-lactamases found in many Gram-negative bacteria and can be encoded on the chromosome or on plasmids. Chromosomal ampC genes are common in several Enterobacterales species and are often inducible, while plasmid-mediated ampC genes (pAmpC) can spread between different species more readily.

AmpC enzymes hydrolyze cephalosporins and cephamycins (for example, cefoxitin) and are typically poorly inhibited by clavulanic acid. Many AmpC enzymes are not efficiently neutralized by common beta-lactamase inhibitors, though newer inhibitors such as avibactam and relebactam can inhibit a broad subset of AmpC enzymes in combination therapies.

Genetically, chromosomal ampC expression can be derepressed through mutations, particularly loss of repressors like ampD, leading to hyperproduction and high-level resistance. Plasmid-mediated AmpC genes belong to families including CMY, DHA, FOX, MOX, ACT, and EBC, and they can disseminate rapidly among Enterobacterales and other Gram-negative bacteria.

Clinical relevance and detection are important because AmpC producers may appear resistant to multiple beta-lactams, complicating empirical therapy and sometimes mimicking extended-spectrum beta-lactamase producers. Laboratory detection strategies include cefoxitin-based screening, AmpC disk tests, and molecular methods to identify specific ampC genes; phenotypic methods may distinguish AmpC producers from ESBL producers by inhibition profiles.

Treatment considerations emphasize carbapenems as a reliable option for many AmpC-producing infections, with other agents chosen based on susceptibility testing. Beta-lactam/beta-lactamase inhibitor combinations that include avibactam or other inhibitors may be effective against many pAmpC enzymes, but susceptibility should guide therapy. Infection control and antimicrobial stewardship remain key to limiting spread.