Dairy farm age and resistance to antimicrobial agents in Escherichia coli isolated from dairy topsoil

Abstract

Antimicrobial agent usage is common in animal agriculture for therapeutic and prophylactic purposes. Selective pressure exerted by these antimicrobials on soil bacteria could result in the selection of strains that are resistant due to chromosomal- or plasmid-derived genetic components. Multiple antimicrobial resistances in Escherichia coli and the direct relationship between antimicrobial agent use over time has been extensively studied, yet the relationship between the age of an animal agriculture environment such as a dairy farm and antibiotic resistance remains unclear. Therefore, we tested the hypothesis that antimicrobial-resistance profiles of E. coli isolated from dairy farm topsoil correlate with dairy farm age. E. coli isolated from eleven dairy farms of varying ages within Roosevelt County, NM were used for MIC determinations to chloramphenicol, nalidixic acid, penicillin, tetracycline, ampicillin, amoxicillin/clavulanic acid, gentamicin, trimethoprim/sulfamethoxazole, cefotaxime, and ciprofloxacin. The minimum inhibitory concentration values of four antibiotics ranged 0.75 to >256 μg/ml, 1 to >256 μg/ml, 12 to >256 μg/ml, and 0.75 to >256 μg/ml for chloramphenicol, nalidixic acid, penicillin, and tetracycline, respectively. The study did not show a direct relationship between antibiotic resistance and the age of dairy farms.

Figure 1

Susceptibilities of dairy soil isolates to clinically- and veterinary- relevant antibiotics (n/R = number tested/number resistant). A. Dairy farm age and E. coli resistance to tetracycline. With the exception of five of 33 E. coli isolates, all isolates conferred low-level resistance to tetracycline. A positive but not significant correlation between dairy farm age and E. coli resistance to tetracycline was observed (r_s=0.125, n=33, p>0.05). B. Dairy farm age and E. coli resistance to chloramphenicol. With the exception of one of 38 E. coli isolates, all isolates conferred low-level resistance to chloramphenicol. A positive but not significant correlation between dairy farm age and E. coli resistance to chloramphenicol was observed (r_s=0.238, n=38, p>0.05). C. Dairy farm age and E. coli resistance to penicillin. With the exception of five of 34 strains that conferred high-level resistance, all isolates conferred low-level resistance to penicillin. A positive and but not significant correlation between dairy farm age and E. coli resistance to penicillin was observed (r_s=0.495, n=34, p<0.0025). D. Dairy farm age and E. coli resistance to nalidixic acid. With the exception of one of 37 E. coli isolates, all isolates conferred low-level resistance to nalidixic acid. A positive but not significant correlation between dairy farm age and E. coli resistance to nalidixic acid was observed (r_s=0.107, n=37, p>0.05).

Figure 2

Susceptibilities of dairy soil isolates to clinically- and veterinary- relevant antibiotics. A. Dairy farm age and E. coli resistance to ciprofloxacin. A positive but not significant correlation between dairy farm age and E. coli resistance to ciprofloxacin was observed (r_s=0.0740, n=41, P>0.05). B. Dairy farm age and E. coli resistance to ampicillin. A positive but not significant correlation between dairy farm age and E. coli resistance to ampicillin was observed (r_s=0.130, n=43, P>0.05). C. Dairy farm age and E. coli resistance to amoxicillin/clavulanic acid. A negative and not significant correlation between dairy farm age and E. coli resistance to amoxicillin/clavulanic acid was observed (r_s=−0.115, n=43, P>0.05). D. Dairy farm age and E. coli resistance to gentamicin. A positive but not significant correlation between dairy farm age and E. coli resistance to gentamicin was observed (r_s=0.204, n=42, p>0.05).

Figure 3

Susceptibilities of dairy soil isolates to clinically- and veterinary- relevant antibiotics. A. Dairy farm Age and E. coli resistance to trimethoprim/sulfamethoxazole. A positive but not significant correlation between dairy farm age and E. coli resistance to trimethoprim/sulfamethoxazole was observed (r_s=0.352, n=23, p>0.05). B. Dairy farm age and E. coli resistance to cefotaxime. A negative but not significant correlation between dairy farm age and E. coli resistance to cefotaxime was observed (r_s=−0.0211, n=42, P>0.05).