Comparative analysis of the development of swarming communities of Bacillus subtilis 168 and a natural wild type: Critical effects of surfactin and the composition of the medium

The natural wild-type Bacillus subtilis strain 3610 swarms rapidly on the synthetic B medium in symmetrical concentric waves of branched dendritic patterns. In a comparison of the behavior of the laboratory strain 168 (trp) on different media with that of 3610, strain 168 (trp), which does not produce surfactin, displayed less swarming activity, both qualitatively (pattern formation) and in speed of colonization. On E and B media, 168 failed to swarm; however, with the latter, swarming was arrested at an early stage of development, with filamentous cells and rafts of cells (characteristic of dendrites of 3610) associated with bud-like structures surrounding the central inoculum. In contrast, strain 168 apparently swarmed efficiently on Luria-Bertani (LB) agar, colonizing the entire plate in 24 h. However, analysis of the intermediate stages of development of swarms on LB medium demonstrated that, in comparison with strain 3610, initiation of swarming of 168 (trp) was delayed and the greatly reduced rate of expansion of the swarm was uncoordinated, with some regions advancing faster than others. Moreover, while early stages of swarming in 3610 are accompanied by the formation of large numbers of dendrites whose rapid advance involves packs of cells at the tips, strain 168 advanced more slowly as a continuous front. When sfp(+) was inserted into the chromosome of 168 (trp) to reestablish surfactin production, many features observed with 3610 on LB medium were now visible with 168. However, swarming of 168 (sfp(+)) still showed some reduced speed and a distinctive pattern compared to swarming of 3610. The results are discussed in terms of the possible role of surfactin in the swarming process and the different modes of swarming on LB medium.