||Interspecies interaction stimulates gliding motility of filamentous photosynthetic bacteria through the action of protease
The thermophilic photosynthetic bacterium Chloroflexus aggregans has been widely found in microbial mats of hot springs all over Japan. C. aggregans has been reported as a major component in the microbial mats developed at the temperature of 50 to 70℃. C. aggregans shows gliding motility to form dense cell aggregation in a liquid medium, and the ability would contribute to the formation of microbial mats in natural hot springs. Interspecies interaction may occur in the microbial mats where bacterial cells of diverse species are densely packed. It has been reported that interspecies interaction causes various phenotypical changes including biofilm formation and motility. I studied the effect of coexisting bacteria on the cell aggregation of C. aggregans. Heterotrophic bacteria were isolated from the microbial mats to evaluate effect on the cell aggregation of C. aggregans. Cell aggregation was promoted by the addition of culture supernatants of 5 isolates. Strain BL55a showed the most remarkable promoting effect on the cell aggregation and was revealed to be closely related to Bacillus licheniformis based on 16S rRNA gene sequences analysis. The promoting effect was completely suppressed after heating the culture supernatants at 105℃ for 10 min. Size fractionation of the culture supernatant indicated that molecular weight of the promoting factor was above 10,000. From these results, a possible promoting factor was extracellular enzyme. Protease activity was detected in all of the isolates that showed the promoting effect on cell aggregation. A purified protease obtained from B.licheniformis was also showed the promoting effect. These results indicate that protease in the culture supernatant promoted cell aggregation of C. aggregans. To examine the possible digestion of C. aggregans cells by the heterotrophic bacteria, Strain BL55a was spread on agar medium that contains C. aggregans without any other carbon source. After 2 days of cultivation, colonies of BL55a were detected and cell lysis of C. aggregans around the colonies was observed. These results indicate that of C. aggregans were digested and prayed cells by BL55a. Escape behavior of C. aggregans from protease was evaluated. C. aggregans cell-containing agar was placed between protease containing agar and fresh agar in a glass cuvette. During the incubation at 55℃ in the light, cells of C. aggregans were moved to the fresh agar layer as if it had escaped from the attack of protease. This is the first report that protease produced by other bacterium stimulates bacterial cell aggregation. Strain BL55a shows predatory behavior through protease, and C. aggregans moves away from the protease. Such escape behavior of prey from predator observed in this study has never been reported in bacteria.