Biosurfactant production maintains viability in anoxic conditions by depolarizing the membrane in Bacillus subtilis

Abstract

The presence or absence of oxygen in the environment is a strong effector of cellular metabolism and physiology. Like many eukaryotes and some bacteria, Bacillus subtilis is an obligate aerobe that primarily utilizes oxygen during respiration to generate ATP. Despite the importance of oxygen for B. subtilis survival, we know little about how oxygen is consumed during growth and how populations respond to shifts in oxygen availability. Here, we find that when oxygen was depleted from stationary phase cultures [~]90% of B. subtilis 3610 cells died and lysed due to autolysin activity; the remaining cells maintained colony-forming ability. Interestingly, the domesticated 168 strain maintained a higher optical density than 3610 during oxygen depletion due to the formation of cell-wall-less protoplasts, but the remaining, rod-shaped cells were >100-fold less viable than 3610. We discovered that the higher viability in 3610 was due to its ability to produce the antibacterial compound surfactin, as surfactin addition rescued 168 viability and also increased yield in aerobic growth. We further demonstrate that surfactin strongly depolarizes the B. subtilis membrane, and that other known membrane-potential disruptors restore viability to 168. These findings highlight the importance of surfactin for survival during oxygen-depleted conditions and demonstrate that antimicrobials normally considered harmful can instead benefit cells in stressful conditions when the terminal electron acceptor in respiration is limiting.