Reverse and flick: Hybrid locomotion in bacteria

Abstract

Many bacteria are motile. They use one or more helical flagella as propellers, rotating them like the corkscrew on a wine bottle opener. Despite the limited morphological repertoire of the propulsive system, radically different movement strategies have evolved, likely reflecting the diversity of physicochemical conditions among bacterial habitats. In PNAS, Xie et al. (1) report on a newly discovered mechanism for turning used by Vibrio alginolyticus , an inhabitant of the coastal ocean: These monotrichous (“single-haired”) bacteria change direction with a “flick” of their flagellum. Intriguingly, Xie et al. (1) show that less can be more when it comes to bacterial flagella: With its single flagellum, V. alginolyticus outperforms the multiflagellated Escherichia coli in climbing nutrient gradients (“chemotaxis”), suggesting that the flick is part of an advanced chemotaxis system. Our understanding of bacterial locomotion has long been driven and biased by the wealth of knowledge on E. coli , commonly found in animal intestines. E. coli is peritrichous, having four to eight flagella emerging from random points on its 2 × 1 μm hotdog-shaped body (2). Each flagellum is powered by a reversible rotary motor. When all motors spin counterclockwise (as seen from behind), hydrodynamic interactions cause the flagella to form a bundle that propels E. coli forward in a nearly straight “run” at ∼30 μm/s. When one or more motors switch direction, the bundle comes apart, causing a change in direction (“tumble”) before a new run begins. The angle of reorientation during a tumble is nearly random, with the new run only slightly biased in the direction of the old one. This “run-and-tumble” movement pattern is common among peritrichous bacteria, including the pathogen Salmonella typhimurium and the soil-dwelling Bacillus subtilis . Other bacteria, like V. alginolyticus , have a single flagellum and thus lack E. coli ’s tumbling mechanism. … 1E-mail: romans{at}mit.edu.