Submitted to: Safepork
Publication Type: Proceedings
Publication Acceptance Date: 7/8/2005
Publication Date: 9/6/2005
Citation: Ramlachan, N., Poole, T.L., Kim, W.K., Callaway, T.R., Ricke, S.C., Anderson, R.C., Harvey, R.B., Nisbet, D.J. 2005. Effects of antibiotic-supplemented media on recovery of enterobacteria. Proceedings of SafePork 2005. p. 296-298.
Interpretive Summary: The use of antibiotics in food production is thought to be associated with drug resistance in foodborne pathogens such as Salmonella and Escherichia coli. In order to understand the nature of antibiotic resistance among different bacterial species and to propose new strategies to reduce emergence of resistant strains, it is necessary to study effective means for measuring the development antibiotic resistance in different types of cultured bacteria. There are an incredible number of methods for measuring total numbers, both resistant and sensitive, of Salmonella and E. coli. This study uses two species, Salmonella Typhimurium (ST) and E. coli to examine differences in growth when using various media with different levels of added antibiotics. Several types of antibiotics were used at high and low levels representing the major classes of antimicrobial drugs used in treatment and prevention of bacterial infection in food animals such as swine. In this study, we tested three common methods and media types for their effectiveness in quantifying growth of Salmonella and E. coli with different levels of added antibiotics. It was found that while all three gave accurate measurements of total numbers of bacteria, none were able to solely measure accurate numbers of bacteria that were susceptible to the antibiotics tested. This research provides important information to other researchers to reduce improper measurements of antibiotic resistance in the field. Ultimately, this research will help scientists develop new and better management tools for swine farmers so that they can continue to produce safe and affordable products for the American consumer.
Technical Abstract: The frequency at which Escherichia coli and Salmonella Typhimurium (ST) were recovered from tryptic soy agar (TSA), brilliant green agar (BGA) and MacConkey agar (MAC) alone or supplemented with 2 and 16 ug oxytetracycline, 4 and 16 ug tetracycline, 2 and 64 ug kanamycin and 0.25 and 2 ug enrofloxacin was investigated. Recovery of E. coli and ST from unsupplemented TSA, BGA and MAC was > 1 X 10**9 CFU for all strains. Recovery of ST from TSA and MAC, and two strains of E. coli from BGA supplemented with the high antibiotic levels were less than 1 in 10**8 for all antibiotics tested. Differences observed in frequencies of E. coli and ST at both low and high antibiotic levels on TSA, BGA and MAC media has implications for in vitro and in vivo assessments of antibiotic resistance.