Characterization of self-aggregation in Bacillus licheniformis strain RK14

Abstract

Abstract Purpose Some Bacillus licheniformis strain isolated from the environment has been reported to form aggregates during the suspension culture under carbon limitation. The aim of this study is to characterize the aggregation process of B. licheniformis RK14 strain isolated from the soil. Methods B. licheniformis RK14 was cultured in a glucose-free lysogeny broth (LB) to confirm the aggregate formation under glucose limitation conditions and compared to model B. licheniformis strains. To characterize the aggregation process of RK14, microscopic analyses and time-lapse observations were conducted. Thioflavin T (ThT) was used to assess the involvement of transient cell elongation in aggregate formation. Results Although RK14 did not form aggregate when cultured in a glucose-rich γ-PGA production medium, it was found to self-aggregate when grown in a glucose-free LB. The optimal temperature for aggregation was approximately 40℃. Microscopic analysis showed that the aggregates were composed of viable cells with cell–cell cluster-like structures. Time-lapse observations clarified elongation of individual cells after 1 h of culture, followed by a return to rod-shape while maintaining the aggregation state. Addition of ThT at micromolar concentration inhibits aggregate formation, probably due to the suppression of cell elongation. Conclusions The aggregation process of RK14 was similar to the pellicle and submerged biofilm formation process of B. subtilis in terms of morphological changes. It was concluded that transient cell elongation is critical for aggregate formation of RK14 strain.