How Bacteria Use Their Flagella
Do an imaginary mind-meld with a bacterium for a moment. Visualize yourself encased in a membrane, surrounded by fluid. You have no eyes, ears, or hands. You need to find where food is, and avoid danger, so you have organelles that can take in molecules that provide information about what is going on outside, where other bacteria can also communicate information to you. To get around, you have a powerful outboard motor, called a flagellum. Lacking eyes, how do you know where to go? How do you steer and make progress toward food or away from danger? These are the questions of chemotaxis – the ability to move toward or away from chemicals. Two recent papers discuss how bacteria use their rotary motors to succeed in life.
Some bacteria have only one flagellum (monotrichous, or “one-haired,” since the flagella look like hairs at low resolution). One such critter is Vibrio alginolyticus, an inhabitant of the coastal ocean. In a PNAS Commentary,1 Roman Stocker discussed how this microbe uses its single flagellum in a “reverse and flick” movement to explore its environment. This “newly discovered mechanism for turning,” he said, “….is part of an advanced chemotaxis system.” The bacterium can actually make better progress toward or against a concentration gradient with this semi-random search method. “How can a simple back-and-forth movement result in high-performance chemotaxis, rather than causing the bacterium to endlessly retrace its steps?” Stocker asked. The answer is that the flick action, which involves a sudden kinking of the U-joint of the flagellum, combined with reversal of flagellar rotation, provides three times the chemotaxis efficiency of E. coli. He showed this with mathematical models.
Stocker attributed this to evolution: “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.” But what he was really talking about was adaptation of different microbes to different habitats and conditions. He ended with praise, not for evolution, but for the cleverness of microbe transportation: the study he cited “makes monotrichous marine bacteria an appealing model system to expand our knowledge of motility among the smallest life forms on our planet.”
Other bacteria have 2, 4, or 8 flagella (“peritrichous”), like Escherichia coli. When all 8 flagella begin turning in the same direction, they bundle into a kind of V8 engine that can propel the germ at around 30 micrometers per second (�m/s). To change direction, they reverse one or more flagella, causing the bundle to fall apart, stopping forward movement in a strategy called tumbling, after which unified motion begins in another direction. While not as efficient at chemotaxis as V. alginolyticus, it should be remembered that E. coli live in different environments – and they have other tricks up their sleeve.
Flagellum specialist Howard Berg and colleagues figured out how to watch fluid movement around swarms of bacteria. Reporting in PNAS,2 they discovered that bacteria, by rotating their flagella counterclockwise in swarms, create small “rivers” of fluid moving clockwise ahead of the swarm that help them move faster as a group than they could be swimming alone. They wrote,
we discovered an extensive stream (or river) of swarm fluid flowing clockwise along the leading edge of an Escherichia coli swarm, at speeds of order 10 �m/s, about three times faster than the swarm expansion. The flow is generated by the action of counterclockwise rotating flagella of cells stuck to the substratum, which drives fluid clockwise around isolated cells (when viewed from above), counterclockwise between cells in dilute arrays, and clockwise in front of cells at the swarm edge. The river provides an avenue for long-range communication in the swarming colony, ideally suited for secretory vesicles that diffuse poorly.
The observations may have practical applications: “These findings,” they wrote, “broaden our understanding of swarming dynamics and have implications for the engineering of bacterial-driven microfluidic devices.”
1. Roman Stocker, “Reverse and flick: Hybrid locomotion in bacteria,” Proceedings of the National Academy of Sciences, published online before print February 2, 2011, doi: 10.1073/pnas.1019199108 PNAS February 2, 2011.
2. Wu, Hosu, and Berg, “Microbubbles reveal chiral fluid flows in bacterial swarms,” Proceedings of the National Academy of Sciences, published online before print February 7, 2011, doi: 10.1073/pnas.1016693108 PNAS February 7, 2011.
And these are “simple” or “primitive” organisms that were the first to evolve, they tell us. The outboard motors alone are phenomenally complex, but when they work together with signal transduction mechanisms and group search strategies, it’s overkill for Darwin, who was dead anyway.