Tn916-mediated genetic exchange in soil

In sterile soil, Tn916 was observed to transfer between Bacillus subtilis and Bacillus thuringiensis subsp. israelensis at an average frequency of 2.1 × 10−5 conjugants per donor. Moreover, transfer of Tn916 between Enterococcus faecalis, from which Tn916 was originally isolated, and B. thuringiensis was demonstrated at an average frequency of 2.3 × 10−6. The ability of Tn916 to transfer between B. subtilis and B. thuringiensis was evaluated over varying conditions of soil moisture (2–13%), temperature (− 20–45°C), pH (3.8–9.2) and nutrient (0–2.5 mg g−1 brain heart infusion). Conjugal transfer of Tn916 was observed over the entire range of moisture conditions with maximum rates in the range of 2–5% whereas transfer was restricted to temperatures in the range 18–37°C (optimum of 30°C). Transfer of the transposon was restricted to pH values ranging from 6 to 8.9 but within this range, pH had minimal effect on transfer frequency. Increasing nutrients enhanced conjugal transfer of Tn916 throughout the range tested. Finally, besides conjugal transfer, mobilization of the non-conjugal plasmid pC194 by Tn916 was observed in a soil environment at an average frequency of 2.0 × 10−6. These results suggest that transposon-mediated genetic exchange between bacteria may facilitate transfer of antibiotic-resistance determinants as well as other non-conjugal DNA in soil.