ARS | Publication request: Competitive effect of commensal faecal bacteria from growing swine fed chlortetracycline-supplemented feed on beta-haemolytic Escherichia coli strains with multiple antimicrobial resistance plasmids

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 13, 2012
Publication Date: September 16, 2012
Citation: Poole, T.L., Callaway, T.R., Bischoff, K.M., Loneragan, G.H., Anderson, R.C., Nisbet, D.J. 2012. Competitive effect of commensal faecal bacteria from growing swine fed chlortetracycline-supplemented feed on beta-haemolytic Escherichia coli strains with multiple antimicrobial resistance plasmids. Applied and Environmental Microbiology. 113:659-668.

Interpretive Summary: Bacteria that cause disease in humans and animals are becoming increasingly resistant to antibiotics. This has caused a public health crisis because diseases once thought eradicated are reappearing. Bacteria are not only becoming resistant to one or two antibiotics, they are becoming resistant to many antibiotics; such bacteria are called multi-drug-resistant bacteria. Many public health officials blame the food animal industry for the emergence of multi-drug-resistant bacteria and downplay the role of human medicine. Swine and cattle producers have long been under pressure to limit the presence of disease-causing bacteria (pathogens) that are often present on retail meat products. However, with the emergence of multi-drug-resistant pathogens, there are new pressures to limit the use of antimicrobial agents. However, there is evidence that the reduction in antibiotic use alone is not sufficient to reduce antimicrobial-resistant bacterial populations. The limitation of antimicrobial use poses an additional dilemma for producers with regard to the maintenance of healthy herd as well as preventing the presence of pathogens on retail meat. Many of the bacteria that are resistant to multiple drugs carry resistance genes on genetic elements called plasmids. Bacteria can carry multiple plasmids, but it may have a significant metabolic cost. Because of this cost, plasmids may be easily lost from the cells when antibiotics are not present. This study investigated the effect of Aureomycin, an antimicrobial given to swine, on Escherichia coli with differing numbers of plasmids when they were grown in mixed bacterial cultures swine fecal fluid. The results of this study showed that some plasmids were lost in some E. coli isolates, but plasmid loss did not correlate to Aureomycin treatment. Two strains had very stable plasmid content while two strains lost some but not all of their plasmids.

Technical Abstract: Multidrug-resistant (MDR) bacteria are an increasing threat to human and animal health. The objectives of the present study were to determine: (1) the effect of Aureomycin® on MDR E. coli field strains in growing swine fecal fluid; (2) the competitive fitness of each of these strains in long-term continuous-flow (RPCF) fecal cultures without antimicrobial selection pressure. E. coli were inoculated into fecal fluid from swine fed unmedicated (group 1) or medicated feed with 50g/ton Aureomycin® (group 2) and enumerated at 0, 6, and 24 hr (study 1). The same strains were inoculated into continuous-flow swine fecal cultures that were maintained for up to 22 days without antimicrobial treatment (study 2). Plasmid isolation, PCR-based replicon typing, and susceptibility testing was done on all isolates selected for characterization from both studies. The plasmid bearing E. coli strains in the Aureomycin-treated group of study 1 had a numerical growth advantage over the 24-hr time period compared to those in the control group, maintaining a population of between 105–106 CFU/ml at 24 hr. The 834 and 843 isolates were largely unchanged from the field strain in both studies, and both were able to survive long-term in RPCF. Isolates of strains 155 and 1568 displayed more pheno- genotype differences than 834 or 843. These results suggest that MDR E. coli may be quite competitive in mixed microbial environments in the absence of selection pressure; thus reducing the use of antimicrobials use may not curtail the emergence of MDR bacteria.