Molecular characterization of antibiotic resistant and potentially virulent enterococci isolated from swine farms and feed mills

A total of 108 swine feed samples were collected from six feed mills and two farms and tested for enterococcal contamination. Nearly 43% of these samples were positive for enterococci. The mean ± SE concentration of enterococci in feed samples ranged from 2.0 × 101 to 7.3 × 103 CFU/g of feed. About 38% of processed feed mill samples were contaminated with enterococci compared to 59% of swine farm samples. A total of 208 enterococcal isolates were represented by Enterococcus casseliflavus (54.8% of total isolates), E. gallinarum (17.8%), E. faecium (17.8%), E. hirae (5.8%), and E. faecalis (3.8%). These isolates were phenotypically resistant to tetracycline (48.5%), erythromycin (14.4%), streptomycin (13.4%), kanamycin (11.5%), ciprofloxacin (10.0%), ampicillin (2.8%), and chloramphenicol (1.4%). All isolates were susceptible to vancomycin and gentamicin. Tetracycline resistance was encoded by tetM gene (52.8%), tetO (14.4%), tetK (1.0%), and tetS (0.5%), whereas ermB conferred erythromycin resistance in 10.6% of all isolates. Several isolates carried genes coding for virulence factors, including gelatinase (gelE; 18.2%), an enterococcal surface protein (esp; 2.4%), and cytolysin (cylA; 2.4%). Only E. faecalis was β-hemolytic (2.9%) and gelatinolytic (3.4%). The aggregation substance gene asa1 was detected in 5 out of 8 E. faecalis isolates, of which four were phenotypically positive. The transposon Tn916/1545 was detected in 11.5% of all isolates. Mating assays revealed that 7 out of 8 E. faecalis could transfer tetM gene, and rate of transfer ranged from 2.0 × 10−3 to 1.6 × 10−5. The presence of antibiotic resistant and potentially virulent enterococci in swine farm samples and feed mill samples, though in low prevalence, raises concern and emphasizes the need for improved hygiene and quality standards on farms and in feed mills.