Mercury resistance determined by a self-transmissible plasmid in Bacillus cereus 5

Inducible mercuric reductase activity in Bacillus cereus 5 was plasmid-encoded. Plasmid analysis revealed three plasmids with molecular masses of 2.6, 5.2 and 130 MDa. A mating system permitted transfer of the resistance determinant among strains of B. cereus and B. thuringiensis. Transfer of mercury resistance from B. cereus 5 to B. cereus 569 and B. thuringiensis occurred during mixed culture incubation on agar surfaces. The 130-MDa plasmid (pGB130) was responsible for transfer; frequencies ranged from 10(-5) to 10(-4). B. cereus 569 transconjugants inheriting pGB130 were also effective donors. High transfer frequencies and the finding that cell-free filtrates of donor cultures were ineffective in mediating transfer suggested mercury-resistance transfer was not phage-mediated. Transfer was also insensitive to DNase activity. Further evidence that pGB130 DNA carried the mercury-resistance determinant was transformation of B. cereus 569 by electroporation with pGB130 DNA isolated from B. cereus 5 and a mercury-resistant B. cereus 569 transconjugant. Mercury-resistant transconjugants and transformants exhibited mercuric reductase activity. Plasmid pGB130 also conferred resistance to phenylmercuric acetate.