Mating system for transfer of plasmids among Bacillus anthracis, Bacillus cereus, and Bacillus thuringiensis

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Abstract
To facilitate the analysis of genetic determinants carried by large resident plasmids of B. anthracis, a mating system was developed which promotes plasmid transfer among strains of B. anthracis, B. cereus and B. thuringiensis. Transfer of the selectable tetracycline resistance plasmid pBC16 and other plasmids from B. thuringiensis to B. anthracis and B. cereus recipients occurred during mixed incubation in broth. Two plasmids, pXO11 and pX012, found in B. thuringiensis were responsible for plasmid mobilization. B. anthracis and B. cereus transcipients inheriting either pX011 or pX012 were, in turn, effective donors. Transcipients harboring pXO12 were more efficient donors than those harboring pXO11; transfer frequencies ranged from 10-4-10-1 and from 10-8-10-5, respectively. Cell-to-cell contact was necessary for plasmid transfer, and the addition of DNase had no effect. The high frequencies of transfer, along with the fact that cell-free filtrates of donor cultures were ineffective, suggested that transfer was not phage mediated. B. anthracis and B. cereus transcipients which inherited pXO12 also acquired the ability to produce parasporal crystals (Cry+) resembling those produced by B. thuringiensis, indicating that pXO12 carries a gene(s) involved in crystal formation. Transcipients which inherited pXO11 were Cry-. This mating system provides an efficient method for interspecies transfer of a large range of Bacillus plasmids by a conjugation-like process.