Effects of Management and Environmental Conditions on Antibiotic Resistance in Bacteria Associated with Swine

Weaned pigs (n=58) were challenged with nalidixic resistant Salmonella enterica serotype Typhimurium and separated into eight treatments to determine the effects of various environmental and management conditions on the development of antibiotic resistance among pathogenic and commensal bacteria. Apramycin sulfate was administered in the feed (150g/ton) two days postchallenge for a period of 14 days with the exception of one control group. Treatments included: control without apramycin (control-1); control with apramycin (control-2); and apramycin plus either cold stress, heat stress, overcrowding, intermingling, poor sanitation, and intervention with oxytetracycline (100 ug / ton). Treatments were applied 5 days post initial antibiotic administration and maintained throughout the study. Fecal swabs were obtained prior to antibiotic treatment (day 0) and on days 2, 7, 14, 28, 64, 148, and 149 post-treatment. Salmonella Typhimurium, Escherichia coli, and Enterococcus faecalis were isolated and tested for resistance to apramycin sulfate, ceftiofur sodium, oxytetracycline, and sulfamethazine via broth microdilution. Increased (P < .0001) resistance to apramycin was noted in E. coli in all groups administered apramycin by day 14. Control-2 minimum inhibitory concentrations (MICs) returned to baseline following removal of the antibiotic, whereas cold stress, overcrowding, and oxytetracycline groups expressed significantly (P < .05) greater MICs through day 64 before returning to baseline. S. Typhimurium generally displayed lower MICs for all test antibiotics compared to E. coli, while E. faecalis demonstrated elevated resistance throughout the study to all antibiotics. These data indicate that E. coli resistance to apramycin is significantly increased upon exposure to various stressors, whereas S. Typhimurium and E. faecalis may be less affected.

Key Words: antibiotic resistance, E. coli, swine