Not so simple, not so subtle: the interspecies competition between Bacillus simplex and Bacillus subtilis and its impact on the evolution of biofilms

Bacillus subtilis biofilms have a fundamental role in shaping the soil ecosystem. During this process, they unavoidably interact with neighbour bacterial species. We studied the interspecies interactions between biofilms of the soil-residing bacteria B. subtilis and related Bacillus species. We found that proximity between the biofilms triggered recruitment of motile B. subtilis cells, which engulfed the competing Bacillus simplex colony. Upon interaction, B. subtilis secreted surfactin and cannibalism toxins, at concentrations that were inert to B. subtilis itself, which eliminated the B. simplex colony, as well as colonies of Bacillus toyonensis . Surfactin toxicity was correlated with the presence of short carbon-tail length isomers, and synergistic with the cannibalism toxins. Importantly, during biofilm development and interspecies interactions a subpopulation in B. subtilis biofilm lost its native plasmid, leading to increased virulence against the competing Bacillus species. Overall, these findings indicate that genetic programs and traits that have little effect on biofilm development when each species is grown in isolation have a dramatic impact when different bacterial species interact. Interspecies competition: bacterial 'turf wars' Soil bacterial colonies employ sophisticated tactics to wage war against rival species, say researchers. Ilana Kolodkin-Gal of Israel's Weizmann Institute of Science and colleagues examined interactions between two species commonly found in soil, Bacillus subtilis and Bacillus simplex . They found that B. subtilis communities dispatch highly mobile cells that surround and ultimately eradicate B. simplex biofilms. Subsequent analysis enabled Kolodkin-Gal and colleagues to identify three molecules produced by B. subtilis as part of its attack, including two 'cannibal toxins'. These molecules, which were previously presumed to help this bacterium police its own population growth, actually appear to be far more toxic to other Bacillus species. Interestingly, B. subtilis cells on the front lines discard genetic material that could inhibit their attack on B. simplex , presumably reacquiring these DNA fragments when their rivals are eliminated.