Secreted protease mediates interspecies interaction and promotes cell aggregation of the photosynthetic bacterium Chloroflexus aggregans

Interspecies interactions were studied in hot spring microbial mats where diverse species of bacterial cells are densely packed. The anoxygenic photosynthetic bacterium, Chloroflexus aggregans, has been widely found in the microbial mats as a major component in terrestrial hot springs in Japan at the temperature from 50 to 70°C. C. aggregans shows cellular motility to form a microbial mat-like dense cell aggregate. The aggregating ability of C. aggregans was affected by another bacterial species, strain BL55a (related to Bacillus licheniformis) isolated from the microbial mats containing C. aggregans. Cell aggregation rate of C. aggregans was promoted by the addition of culture supernatants of strain BL55a. Similar effects were also detected from other bacterial isolates, specifically Geobacillus sp. and Aeribacillus sp. Protease activity was detected from the culture supernatants from all of these isolates. The promoting effect of strain BL55a was suppressed by a serine protease inhibitor, phenylmethylsulfonyl fluoride. A purified serine protease, subtilisin obtained from B. licheniformis, showed a promoting effect on the cell aggregation. These results suggest that an extracellular protease, secreted from co-existing bacterial species promoted the aggregating motility of C. aggregans. This is the first report that exogenous protease affects bacterial cellular motility. Cellular motility of a thermophilic photosynthetic bacterium was promoted by a protease secreted from other bacteria. It is a new mode of interspecies interaction in bacteria.