Excreted antibiotics may be key to emergence of increasingly efficient antibiotic resistance in food-animal production

At a time when antimicrobial resistance is seemingly ubiquitous worldwide, understanding the mechanisms responsible for successful emergence of new resistance genes may provide insights into the persistence and pathways of dissemination for antimicrobial resistant organisms in general. For example, E. coli strains harboring a Class A beta-lactamase encoding gene (blaCTX-M-15) appear to be displacing strains that harbor a Class C beta-lactamase gene (blaCMY-2) in Washington State dairy cattle. We cloned these genes with native promoters into low-copy-number plasmids that were transformed into isogenic strains of E. coli, and growth curves were generated for the two commonly administered antibiotics (ampicillin and ceftiofur). Both strains met the definition of resistance for ampicillin (=32 µg/ml) and ceftiofur (=16 µg/ml). Growth of the CMY-2-producing strain was compromised at 1 mg/ml ampicillin whereas the CTX-M-15-producing strain was not inhibited in the presence of 3 mg/ml ampicillin and for most concentrations of ceftiofur except for mixed outcomes when exposed to ceftiofur metabolites. Consequently, in the absence of competing genes, E. coli harboring either gene would experience a selective advantage if exposed to these antibiotics. Successful emergence of CTX-M-15-producing strains where CMY-2-producing strains are already established, however, requires high concentrations of antibiotics that can only be found in the urine of treated animals (e.g., known to achieve >2 mg/ml for ampicillin). This ex vivo selection pressure may be important for the emergence of new and more efficient antibiotic resistance genes, and likely for persistence of antimicrobial resistant bacteria in food-animal populations.