Surfactin restores and enhances swarming motility under heavy metal stress

The present work reports the importance of lipopeptide biosurfactant on swarming motility of multi-metal resistant (MMR) bacterium under heavy metal stress. The MMR bacteria strain CM100B, identified as Bacillus cereus, was isolated from the coal mine sample. The strain was able to grow and reduce several metals namely Cd2+, Co2+, Cu2+, Ni2+, Mn2+ and Pb2+ ions which are common environmental pollutants. Presence of toxic heavy metal ions in the swarming medium significantly altered the motility of CM100B. Presence of Cd2+ and Pb2+ ions inhibited development of peritrichous flagella, thus inhibiting swarming motility. However, the addition of anionic biosurfactant surfactin restored (in case of Cd2+ and Pb2+ ions) or enhanced (in case of Co2+, Cu2+, Ni2+ and Mn2+) the swarming ability of CM100B. Zeta potential studies for determining bacterial cell surface charge indicated that surfactin provided a suitable swarming environment to bacteria even under metal stress by chelating to cationic metal ions. Non-ionic surfactant Triton X-100 was unable to restore swarming under Cd2+ and Pb2+ ion stress. Thus, suggesting that surfactin can aid in motility not only by reducing the surface tension of swarming medium but also by binding to metal ions in the presence of metal ions stress.