Tetracycline resistance transfer among obligate anaerobes from the ruminant gut
The main aim of this work was to investigate the nature, distribution and transmissibility of tetracycline resistance (TcR) genes among ruminant anaerobic bacteria. Two TcR rumen strains of Butyrivibrio fibrisolvens, 1.230 and 1.23, were able to transfer the resistance phenotype to the type strain, 2221R although a third TcR strain, 1.210, could not. PCR amplification of 16S rDNA sequences showed that the three isolates were phylogenetically distinct from the recipient strain, but related to each other. Hybridisation work suggested the presence of two chromosomal TcR determinants among the B. fibrisolvens isolates. All three strains contained a non-transferable tet(O) gene, 100% identical at the nucleotide level with tet(O) from S. pneumoniae. The mobile TcR determinant present in strains 1.230 and 1.23, proved to be a novel ribosome protection tet gene, tet(V), whose gene product shares only 68% amino acid identity with its closest relatives, TetO/TetM and has G+C content considerably higher than that of other B. fibrisolvens genes. tet(V) was also identified in two Australian rumen B. fibrisolvens strains, in the rumen anaerobes Selenomonas ruminantium and Mitsuokella multiacidus, and in a pig isolate of M. multiacidus. These results provide evidence for gene transfer between obligate and facultative anaerobes from different gut ecosystems and different geographical locations. PFGE demonstrated that mobile chromosomal elements 40-50 kb in size, TnB1230 and TnB123, with preferred insertion sites in the recipient genome mediated the transfer of tet(V) in B. fibrisolvens. No homology was found between TnB1230 and regions from Tn916 and Tn5253. TnB1230 is not associated with tet(V) in the other bacterial strains, suggesting that a diverse range of elements carry the gene in different bacteria. Although tet(V) is chromosomally encoded in the majority of the strains examined, there is some evidence that the gene may be located in a plasmid in S. ruminantium FB32 and FB34.