Q-317. The Effects of Dairy Age on Antimicrobial Agent Susceptibility in E. coli from Dairy Farm Topsoil
The widespread use of antibiotics in animal agriculture for growth and prophylactic purposes has raised concerns regarding transmission of antibiotic-resistant bacterial strains to humans. Some antimicrobial agents are absorbed poorly by the treated animal and may persist in the soil after being excreted. Exposure to sub-therapeutic levels of antimicrobial agents in the soil may select for antibiotic-resistant strains, creating a source of transmission to humans. However, it is not known whether there is a direct relationship between bacterial antibiotic-resistance and the age of an animal environment. In this study, a non-parametric Spearman rank correlation test was used to assess the effects of dairy farm age on E. coli resistance to antimicrobial agents. E. coli isolated from farms aged 6 months to 24 years were tested for susceptibility to penicillin, chloramphenicol, nalidixic acid and tetracycline. MICs were determined according to the CLSI guidelines using the E-test strip method. All isolates conferred low-level resistance to chloramphenicol (n=38, rs0.05(1) =0.271; P > 0.05), nalidixic acid (n=37; rs0.05(1) =0.275; P > 0.25) and tetracycline (n=33; rs0.05(1) =0.291; P > 0.1), suggesting no correlation between dairy farm age and E. coli resistance to these antimicrobial agents. With the exception of five strains showing high-level resistance, all isolates conferred low-level resistance to penicillin (n=34; rs0.05(1) =0.287; P < 0.0025). The r value of -0.495 indicates a negative correlation between dairy farm age and E. coli susceptibility to penicillin. The significant negative correlation between dairy farm age and E. coli resistance to penicillin and the lack of correlation between dairy farm age and E. coli resistance to chloramphenicol, nalidixic acid, and tetracycline indicate that dairy farm topsoil is not a hotspot for production of high-level antibiotic resistance over time.