High-frequency transfer of a naturally occurring chromosomal tetracycline resistance element in the ruminal anaerobe Butyrivibrio fibrisolvens

Butyrivibrio fibrisolvens strains resistant to tetracycline were isolated from the bovine rumen. Two of three Tc-r B. fibrisolvens tested were able to donate tetracycline resistance at frequencies ranging from 10(-7) to 10(-1) per donor cell in anaerobic filter matings to a rifampin-resistant mutant of the type strain of B. fibrisolvens, 2221(R). The recipient strain 2221(R) exhibited rapid autoaggregation, which might be a factor in the high transfer rates observed. Tc-r transconjugants of B. fibrisolvens 2221(R) were also capable of further transferring tetracycline resistance to a fusidic acid-resistant mutant, 2221(F). Comparison of genomic DNAs by pulsed-field gel electrophoresis demonstrated altered band profiles in transconjugants, consistent with the acquisition of a large mobile chromosomal element. The transferable elements from the two B. fibrisolvens donors 1.23 and 1.230 (TnB123 and TnB1230, respectively) showed the same preferred insertion site in the B. fibrisolvens 2221(R) chromosome and are likely to be similar, or identical, elements. Hybridization experiments showed no close relationship between TnB1230 and int-xis regions from Tn916 of Tn5253. Although DNA from the B. fibrisolvens donor stains hybridized with probes carrying tet(M) of tet(O) sequences, transconjugants were found to have acquired a distinct band that hybridized only weakly with these probes, suggesting that a second, distantly related Tc-r determinant had been transferred.