β-Lactam resistance and β-lactamases in bacteria of animal origin

β-Lactams are among the most clinically important antimicrobials in both human and veterinary medicine. Bacterial resistance to β-lactams has been increasingly observed in bacteria, including those of animal origin. The mechanisms of β-lactam resistance include inaccessibility of the drugs to their target, target alterations and/or inactivation of the drugs by β-lactamases. The latter contributes predominantly to β-lactam resistance in Gram-negative bacteria. A variety of β-lactamases have been identified in bacteria derived from food-producing and companion animals and may further serve as a reservoir for β-lactamase-producing bacteria in humans. While this review mainly describes β-lactamases from animal-derived Escherichia coli and Salmonella spp., β-lactamases from animal-derived Campylobacter spp., Enterococcus spp., Staphylococcus spp. and other pathogens are also discussed. Of particular concern are the increasingly-isolated plasmid-encoded AmpC-type CMY and extended-spectrum CTX-M β-lactamases, which mediate acquired resistance to extended-spectrum β-lactams. The genes encoding these enzymes often coexist with other antimicrobial resistance determinants and can also be associated with transposons/integrons, increasing the potential enrichment of multidrug resistant bacteria by multiple antimicrobial agents as well as dissemination of the resistance determinants among bacterial species. Characterization of β-lactam-resistant animal-derived bacteria warrants further investigation of the type and distribution of β-lactamases in bacteria of animal origin and their potential impact on human medicine.