Plasmids in tributyltin-resistant bacteria from fresh and estuarine waters

Twenty-six tributyltin (TBT)-resistant bacterial strains isolated from sediments were examined for the presence of plasmids. Plasmids of the size reported to carry metal resistance genes were not found in 15 of the strains, indicating that resistance does not have to be plasmid-mediated. Attempts to cure plasmid-containing strains using acridine organge, ethidium bromide, novobiocin or sodium dodecylsulfate, or by growth at elevated temperature were not successful, nor were plasmids transferred from TBT-resistant strains into TBT-sensitive organisms by electroporation. In a broth mating experiment however, plasmid pUM505, a conjugative plasmid known to encode chromium resistance in Pseudomonas aeruginosa PAO1, was introduced into TBT-sensitive Beijerinckia sp. MC-27 isolated from freshwater sediment. The TBT tolerance of the Beijerinckia sp. increased 100-fold, from 8.4 μM TBT in Beijerinckia sp. MC-27 to 840 μM TBT in Beijerinckia sp. MC-27 (pUM505) on solid medium. The plasmid was transferred at a frequency of approximately 6×10^-4. TBT-resistant transconjugants grew faster in media containing TBT and lost their enhanced TBT tolerance and the plasmid upon serial transfer in medium without TBT. Spontaneous mutants of the donor P. aeruginosa lost both TBT resistance and the plasmid. Therefore, TBT resistance in bacteria can be plasmid-mediated. To our knowledge, this is the first report that resistance to a tin compound can be plasmid-mediated.