High-salt stress conditions increase the pAW63 transfer frequency in Bacillus thuringiensis

Conjugation experiments with Bacillus thuringiensis and transfer kinetics demonstrated that salt stress has a positive impact on plasmid transfer efficiency. Compared to standard osmotic conditions (0.5% NaCl), plasmid transfer occurred more rapidly, and at higher frequencies (>100-fold), when bacteria were exposed to a high-salt stress (5% NaCl) in liquid brain heart infusion (BHI). Under milder salt conditions (2.5% NaCl), only a 10-fold effect was observed in Luria-Bertani broth and no difference was detected in BHI. These observations are particularly relevant in the scope of potential gene exchanges among members of the Bacillus cereus group, which includes food-borne contaminants and pathogens.