Transfer of plasmid pFT30 between bacilli in soil as influenced by bacterial population dynamics and soil conditions

Abstract

The 2.8 Md plasmid pFT30 carrying a tetracycline resistance determinant was transferred in filter matings from Bacillus cereus FoTc-30 to B. subtilis SEm-2, at an average frequency of 1 × 10 −6. The insensitivity of this process to deoxyribonuclease, the absence of infectious phage particles and the lack of plasmid transfer upon separation of the strains by a bacterial filter suggested that the transfer mechanism was probably conjugational. Plasmids were also transferred, at a frequency of 0.7 × 10 −7, upon mixed incubation in a sterile loamy sand at 27°C. Both donor and recipient strains survived well under these conditions in this soil. Donor populations rapidly formed spores, while recipients sporulated at a lower rate. Donor and recipient inoculum levels above 5 × 10 7 cfu g −1of dry soil were essential for the detection of transfers. The presence of nutrients in the soil stimulated plasmid transfer. Bentonite clay further significantly enhanced plasmid transfers, resulting in a maximal frequency of 1.6 × 10 −6. Plasmids were also transferred in soil at 15°C and at 8% moisture content, however, at a lower rate. Essentially no transconjugants were obtained in non-sterile soil, which might be due to the rapid decline of the recipient population. However, in the presence of bentonite clay plasmids were transferred at a frequency of 0.9 × 10 −7 and survival of the recipient population was concomitantly enhanced.