Relationship between biofilm forming ability and biological activity of Bacillus subtilis strains

Ivo Ganchev

Abstract: Relatively little is known about the exact mechanisms used by Bacillus subtilis in its behavior as a biocontrol agent in the rhizosphare. Using this strain model, this study demonstrated the biocontrol ability of a wild-type B. subtilis 170 and 168 strain against E. coli K-12. The formed biofilms by co-cultivation of B. subtilis and E. coli K-12 strains were analyzed by using of confocal scanning laser microscopy. It is known that formation of biofilms by B. subtilis is a complex process that includes secretion of surfactin, a lipopeptide antimicrobial agent or suplancin or the mechanism of cannibalism. To determine the role of soy protein hydrolyzate as inductor of suplancine production in biocontrol by B. subtilis, in this study was tested the concentration of soy protein hydrolyzate on the biofilm formatuion by and, thus, deficient concentration of protein in the culure medium. B. subtilis 170 and 168 were ineffective as a biocontrol agent against E. coli K-12 in vitro and also failed to form robust biofilms on polystyrene surface. The antibacterial activity of B. subtilis 170 and 168 strains against E. coli K-12 was determined using of static cultivation of biofilms in 96-well plate and also by live-dead staining methods. Although the minimum inhibitory concentrations determined were relatively high (20 μg mL-1), the levels of the suplancine in polystyrene surface colonized by B. subtilis are likely to be sufficient to kill E. coli K-12 strain. The results in this study collectively indicate that upon polystyrene surface colonization, B. subtilis 170 and 168 form a stable, extensive biofilm during their co-cultivation with E. coli K-12 and secretes suplancine, which is the proposed mechanism for a plant protection against attack by pathogenic bacteria.

Keywords: B. subtilis; biocontrol agent; biofilm; E. coli K-12

Date published: 2020-09-04