Intra- and Interspecies Conjugal Transfer of Tn916-Like Elements from Lactococcus lactis In Vitro and In Vivo

Tetracycline-resistant Lactococcus lactis strains originally isolated from Polish raw milk were analyzed for the ability to transfer their antibiotic resistance genes in vitro, using filter mating experiments, and in vivo, using germfree rats. Four of six analyzed L. lactis isolates were able to transfer tetracycline resistance determinants in vitro to L. lactis Bu2-60, at frequencies ranging from 10⁻⁵ to 10⁻⁷ transconjugants per recipient. Three of these four strains could also transfer resistance in vitro to Enterococcus faecalis JH2-2, whereas no transfer to Bacillus subtilis YBE01, Pseudomonas putida KT2442, Agrobacterium tumefaciens UBAPF2, or Escherichia coli JE2571 was observed. Rats were initially inoculated with the recipient E. faecalis strain JH2-2, and after a week, the L. lactis IBB477 and IBB487 donor strains were introduced. The first transconjugants were detected in fecal samples 3 days after introduction of the donors. A subtherapeutic concentration of tetracycline did not have any significant effect on the number of transconjugants, but transconjugants were observed earlier in animals dosed with this antibiotic. Molecular analysis of in vivo transconjugants containing the tet(M) gene showed that this gene was identical to tet(M) localized on the conjugative transposon Tn916. Primer-specific PCR confirmed that the Tn916 transposon was complete in all analyzed transconjugants and donors. This is the first study showing in vivo transfer of a Tn916-like antibiotic resistance transposon from L. lactis to E. faecalis. These data suggest that in certain cases food lactococci might be involved in the spread of antibiotic resistance genes to other lactic acid bacteria.The abuse of antibiotic use is regarded as the major cause of the accumulation and dissemination of antibiotic resistance genes in the environment (33). For several decades, studies on selection and spread of antibiotic resistance genes have focused mainly on clinically relevant microbial species. Nevertheless, many investigators have recently speculated that commensal bacteria, including lactic acid bacteria (LAB), may act as reservoirs of antibiotic resistance determinants (40). Genes conferring acquired resistance to tetracycline, erythromycin, and vancomycin have been detected and characterized for Lactococcus, Enterococcus, and Lactobacillus species isolated from fermented meat and milk products (13, 18, 23, 49, 50, 56). Introduction of such bacteria into humans through ingestion of commercial food products may have negative consequences by dissemination of antibiotic resistance genes via the food chain to the resident microbiota of the human gastrointestinal tract and, in the worst case, to pathogenic bacteria (4, 17, 55). Therefore, it seems important to assess the risk of antibiotic resistance gene transmission in the environment and in the guts of animals and humans and to establish the genetic basis of the detected resistance and transmission mechanisms.Dissemination of genetic information by horizontal gene transfer is common in the microbial world and is accomplished mainly by the following three mechanisms: natural transformation, conjugation, and transduction (14). Many antibiotic resistance genes have been detected on mobile genetic elements, such as plasmids and conjugative transposons, and it is believed that conjugation is the main mode of horizontal dissemination of antibiotic resistance determinants between bacterial species.Conjugative transposons mediate their own transfer from a donor DNA molecule in one bacterial cell to a target molecule in another cell. Tn916, which spans about 18 kb and confers resistance to tetracycline via tet(M), belongs to the Tn916-Tn1545 family of conjugative transposons and was first identified in Enterococcus faecalis DS16 (20). It is able to be maintained in a wide range of clinically important gram-positive and gram-negative species (12, 44).Excision of Tn916 from the donor molecule is required for conjugative transposition and results in a covalently closed circular transposon molecule that is an intermediate in conjugal transfer (10). A single strand of the covalently closed circular transposon is transferred to the recipient cell, where the complementary strand is synthesized to recreate a double-stranded circular transposon, which inserts into a target site (48).Lactococcus lactis strains are used worldwide as starter organisms in the dairy industry and for the manufacturing of many fermented products. Conjugation has been described widely for lactococci, although mainly for exploitation of this process for development of improved starter strains (22, 38, 39, 51, 53).The objective of the present study was to establish the ability of wild-type L. lactis isolates to transfer tetracycline resistance determinants to gram-positive bacteria, namely, L. lactis Bu2-60, E. faecalis JH2-2, and Bacillus subtilis YBE01, and to gram-negative bacteria, namely, Pseudomonas putida KT2442, Agrobacterium tumefaciens UBAPF2, and Escherichia coli JE2571, by using the filter mating approach. In order to confirm whether these donor strains were able to transfer the tetracycline resistance genes to E. faecalis JH2-2 in vivo in the gastrointestinal tract, we also used germfree rats.