Affiliation:
1. Department of Veterinary and Biomedical Sciences
2. Department of Food Science, Pennsylvania State University, University Park, Pennsylvania
Abstract
ABSTRACT
A study was conducted to understand the descriptive and molecular epidemiology of antimicrobial-resistant gram-negative enteric bacteria in the feces of healthy lactating dairy cattle. Gram-negative enteric bacteria resistant to ampicillin, florfenicol, spectinomycin, and tetracycline were isolated from the feces of 35, 8, 5, and 42% of 213 lactating cattle on 74, 39, 9, 26, and 82% of 23 farms surveyed, respectively. Antimicrobial-resistant gram-negative bacteria accounted for 5 (florfenicol) to 14% (tetracycline) of total gram-negative enteric microflora. Nine bacterial species were isolated, of which
Escherichia coli
(87%) was the most predominant species. MICs showing reduced susceptibility to ampicillin, ceftiofur, chloramphenicol, florfenicol, spectinomycin, streptomycin, and tetracycline were observed in
E. coli
isolates. Isolates exhibited resistance to ampicillin (48%), ceftiofur (11%), chloramphenicol (20%), florfenicol (78%), spectinomycin (18%), and tetracycline (93%). Multidrug resistance (≥3 to 6 antimicrobials) was seen in 40% of
E. coli
isolates from healthy lactating cattle. Of 113 tetracycline-resistant
E. coli
isolates,
tet
(B) was the predominant resistance determinant and was detected in 93% of isolates, while the remaining 7% isolates carried the
tet
(A) determinant. DNA-DNA hybridization assays revealed that
tet
determinants were located on the chromosome. Pulsed-field gel electrophoresis revealed that tetracycline-resistant
E. coli
isolates (
n
= 99 isolates) belonged to 60 subtypes, which is suggestive of a highly diverse population of tetracycline-resistant organisms. On most occasions,
E. coli
subtypes, although shared between cows within the herd, were confined mostly to a dairy herd. The findings of this study suggest that commensal enteric
E. coli
from healthy lactating cattle can be an important reservoir for tetracycline and perhaps other antimicrobial resistance determinants.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology