INTERVENTIONS TO REDUCE EPIZOOTIC PATHOGENIC BACTERIA IN SWINE AND CATTLE
Location: Food and Feed Safety Research
Title: Persistence of resistance plasmids carried by beta-hemolytic E. coli when maintained in a continous-flow fermentation system without antimicrobial selection pressure
Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 16, 2010
Publication Date: May 3, 2011
Citation: Poole, T.L., Harhay, D.M., Callaway, T.R., Beier, R.C., Bischoff, K.M., Loneragon, G.H., Anderson, R.C., Nisbet, D.J. 2011. Persistence of resistance plasmids carried by beta-hemolytic E. coli when maintained in a continous-flow fermentation system without antimicrobial selection pressure. Foodborne Pathogens and Disease. 8:535-540.
Interpretive Summary: Bacteria that cause disease in humans and animals are becoming increasingly resistant to antibiotics. 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. There are many aspects to consider in eliminating or minimizing pathogens in the swine and beef production environments. 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 whether bacteria with differing numbers of plasmids lost plasmids and whether there was an increased metabolic cost from having a greater number of plasmids. The results of this study showed there was very little cost associated with harboring a larger number of plasmids, and the cost that was observed could be compensated for over time.
Multi-drug resistant bacteria are an increasing threat to human and animal health; to combat this threat, a reduction in the use of antimicrobials has been recommended. For reduction in drug usage to reduce the incidence of resistant bacteria, this requires that maintaining antimicrobial resistance incurs a fitness cost to the bacteria. The objective of the present study was to determine: 1) if MDR E. coli field strains with different plasmid profiles show disparate plasmid loss when grown over time without selection pressure; 2) if the number of plasmids present in the cell affects growth rates. Eight ß-hemolytic E. coli strains from swine and one bovine were grown in separate continuous-flow vessels for 36 days without antimicrobial selection. The cultures were enumerated on BHIA and BHIA-T on days 2, 5, 8, 15, 22, 29, and 36. Replicon typing was performed on isolates (n=280) throughout the study. Growth rates, plasmid profiles, and susceptibility profiles of day 36 isolates (n=40) were compared to wildtype strains. Plasmid content of the nine field strains ranged from zero to eight with sizes from 3.2 to 165 kb. Only three of 280 isolates showed the loss of plasmid replicons. Strains with zero to three large plasmids maintained populations between 107-8 cfu/mL; isolates possessing four large plasmids maintained populations between 106-8 cfu/mL. Changes in susceptibility of day 36 isolates were observed among eight of the day 36 isolates. MDR E. coli largely maintained their original plasmid profiles, replicon types, and susceptibility profiles over 36 days of continuous culture. The number of plasmids carried by the host made little difference in log-phase growth rates. This suggests there is little fitness cost to maintenance of multiple plasmids of various sizes under the conditions of this study, which further suggests that other strategies than merely reducing antimicrobial usage are needed.