Inhibition of biofilm formation by Cd on Bacillus subtilis 1JN2 depressed its biocontrol efficiency against Ralstonia wilt on tomato.

Bacillus subtilis 1JN2 can serve as an effective biocontrol agent against Ralstonia wilt on tomato, but the efficiency of control depends on the levels of heavy metals in the rhizosphere soil. Here, we investigated how the heavy metal Cd affects the biocontrol efficacy of B.subtilis 1JN2 on Ralstonia wilt. We found that low Cd content of 2 mM or lower had no effects on the biofilm formation of 1JN2, while media containing 3 mM or higher Cd levels inhibited biofilm formation. Interestingly, high concentration of Cd (5 mM) showed inhibition of B.subtilis 1JN2 cell growth. We next tested the effects of Cd on the colonization of 1JN2 by supplementing artificial Cd in the tomato rhizosphere in a greenhouse setting. We found that 3 mM Cd in the tomato rhizosphere inhibited the colonization of B.subtilis 1JN2, Only 10 CFU/mL 1JN2 was detected one week post treated with 10 CFU/mL but 10 CFU/mL could be detected without Cd in the soil. The presence of Cd had no effect on the colonization of Ralstonia solanacearum on tomato, but the biocontrol efficacy against Ralstonia wilt by 1JN2 decreased 54.2% when the soil contained 3 mM Cd compared to the control without Cd. Taken together, we found that the failure of biofilm formation of Bacillus subtilis 1JN2 that affected by Cd lead to the decrease of its biocontrol efficacy against Ralstonia wilt on tomato.