AmpC beta-lactamases

Abstract

AmpC beta-lactamases are clinically important cephalosporinases encoded on the chromosomes of many of the Enterobacteriaceae and a few other organisms, where they mediate resistance to cephalothin, cefazolin, cefoxitin, most penicillins, and beta-lactamase inhibitor-beta-lactam combinations. In many bacteria, AmpC enzymes are inducible and can be expressed at high levels by mutation. Overexpression confers resistance to broad-spectrum cephalosporins including cefotaxime, ceftazidime, and ceftriaxone and is a problem especially in infections due to Enterobacter aerogenes and Enterobacter cloacae, where an isolate initially susceptible to these agents may become resistant upon therapy. Transmissible plasmids have acquired genes for AmpC enzymes, which consequently can now appear in bacteria lacking or poorly expressing a chromosomal bla(AmpC) gene, such as Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Resistance due to plasmid-mediated AmpC enzymes is less common than extended-spectrum beta-lactamase production in most parts of the world but may be both harder to detect and broader in spectrum. AmpC enzymes encoded by both chromosomal and plasmid genes are also evolving to hydrolyze broad-spectrum cephalosporins more efficiently. Techniques to identify AmpC beta-lactamase-producing isolates are available but are still evolving and are not yet optimized for the clinical laboratory, which probably now underestimates this resistance mechanism. Carbapenems can usually be used to treat infections due to AmpC-producing bacteria, but carbapenem resistance can arise in some organisms by mutations that reduce influx (outer membrane porin loss) or enhance efflux (efflux pump activation).

FIG. 1.

Diagram of AmpC from E. coli complexed with acylated ceftazidime (PDB accession number 1IEL) (265) created with Cn3CD, version 4.1 (available at http://www.ncbi.nlm.nih.gov). The R2 loop at the top of the molecule and conserved residues S64, K67, Y150, N152, K315, and A318 are shown in yellow. β-Strands are gold, and α-helixes are green.

FIG. 2.

Schematic representation of ceftazidime with the R1 side chain at C7 and the R2 side chain at C3. (Adapted from reference with permission from Blackwell Publishing Ltd.)

FIG. 3.

Phylogram of AmpC enzymes listed in Table 1 constructed with ClustalX (available at http://bips.u-strasbg.fr/fr/Documentation/ClustalX/).

FIG. 4.

Genetic environment of representative AmpC genes: CMY-3 (GenBank accession number DQ164214), CMY-9 (accession number AB061794), CMY-13 (accession number AY339625), and DHA-1 (accession number SEN237702).