Abstract
Enterococcus faecalis has the ability to conjugate with the aid of aggregation substance (AS) and inducible sex pheromones to exchange genetic elements in food matrix. To evaluate the food safety condition and the transferable factor, 250 tetracycline-resistant food-borne E. faecalis were collected in Korea. Among the isolates, a majority of tetracycline-resistant isolates (49.6 %) harbored both the tet(M) and tet(L) genes together, followed by tet(M) (19.6 %), and tet(L) (6.8 %) alone. Also, we found the combination of tet(L)/tet(M)/tet(O) or tet(M)/tet(O). We identified two tet(S) genes including the isolate carrying tet(M) + tet(S) genes. Additionally, most E. faecalis were positive for cpd and ccf (both 96.8 %) followed by cob (57.2 %). Through mating experiments, we confirmed E. faecalis possessing the Int-Tn gene and/or any AS gene successfully transferred tet genes to JH2-2 E. faecalis, whereas neither E. faecalis carrying AS genes nor the Int-Tn gene showed the conjugation. Pulsed-field gel electrophoresis results supported a distinct pattern, implying transfer of genetic information. Our study revealed a high occurrence of tetracycline resistance genes in E. faecalis from various foods. The widespread dissemination of tetracycline resistance genes would be promoted to transfer tetracycline resistance genes by pheromone-mediated conjugation systems.
Highlights
Enterococci are found in a diversity of foods as normal inhabitants of the gastrointestinal tracts of food-producing animals and humans [41]
Enterococcus faecalis has the ability to conjugate with the aid of aggregation substance (AS) and inducible sex pheromones to exchange genetic elements in food matrix
The prevalence of antimicrobial-resistant enterococci in food-producing animal is becoming a matter of concern, as these resistant bacteria may be transmitted to humans via the food chain [29]
Summary
Enterococci are found in a diversity of foods as normal inhabitants of the gastrointestinal tracts of food-producing animals and humans [41]. Despite their omnipresence, enterococci have become important nosocomial pathogens and appear to have increasing resistance to antimicrobials [31]. Tetracycline resistance commonly appears as acquired antimicrobial resistance (AMR) in Enterococcus [36]. Because tetracycline has been widely used to promote livestock growth and to treat human diseases, the widespread use of this antimicrobial has caused selective pressure and led to an increase in the number of acquired resistance genes among bacteria [30, 38]. Many studies have reported that AMR has persisted due to horizontal transfer of antibiotic resistance [21, 32] and linkage to other classes of antibiotics [23, 34]
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