Jokes that open with a bacterium walking into a bar just got a little less far-fetched.
Some bacteria can just stand up and toddle away on hairlike legs, a new study shows. The finding, reported October 8 in Science, could help scientists better understand how bacteria form dense antibiotic-resistant communities called biofilms and may lead to better ways to combat troublesome and sometimes deadly microbes.
Researchers had already documented bacteria swimming through liquids or crawling on their bellies across a surface, but no one had ever seen bacteria getting up and walking. No one, that is, until a group of undergraduate students at the University of Illinois at Urbana-Champaign made movies of Pseudomonas aeruginosa bacteria moving on a microscope slide. Working under the supervision of Gerard Wong, a biophysicist now at UCLA, the students adapted a technique used by physicists to track microscopic particles. Computer programs allowed the researchers to quickly sort through video footage of teeming bacteria to find out what individual cells were up to.
“My students started seeing all this neat stuff,” Wong says. “They’d tell me, ‘Yeah, sometimes they just pop wheelies and stand up.’”
What the students saw were rod-shaped P. aeruginosa bacteria standing up on end and then staggering around the slide. The unsteady walks required the use of hairlike appendages called Type IV pili, the scientists found. Without pili, bacteria just lie there. But with pili, P. aeruginosa bacteria “have the ability to both be a sprinter and a long distance runner,” says George O’Toole, a microbiologist at Dartmouth Medical School in Hanover, N.H.
The stringy appendages were already known to be needed for twitching motility, a type of locomotion in which pili at one end of a bacterium pull the cell across the surface. “When the bacteria are lying down flat it’s almost like front wheel drive,” Wong says. Crawling bacteria move in relatively straight lines over fairly long distances — an average of six micrometers by Wong’s measurements — possibly enabling the microbes to move toward chemical attractants.
Walking bacteria stand on splayed pili. Tugging on one of the pili sends the cell lurching in that direction. As each pilus gets tugged the bacterium staggers and stumbles, moving randomly across the surface. Walking bacteria covered more ground and moved faster than their crawling counterparts, the researchers found. Such behavior could enable microbes to explore the local environment quickly.
As it turns out, walking is a common activity for bacteria. After a cell divided in two, about 67 percent of the time one of the newborn cells would get up and move away from its sibling, often by walking, the researchers observed.
Interactions with the surface are important for forming biofilms. Bacteria need to attach to the surface and release if conditions aren’t favorable. “And it really seems like standing upright is a key transitional step,” O’Toole says.
In the new study, Wong and his colleagues watched as P. aeruginosa bacteria used their pili as launch platforms. A bacterium first rises up on its end and then spins itself around, powered by a molecular motor that drives a whiplike swimming apparatus called a flagellum. Pili adjust the angle at which the cell is tilted. Finally, the microbe builds up momentum and shoots off the surface.
“They don’t just fly off a surface,” Wong says. “There’s a whole coordinated series of pirouettes.”
Blocking bacteria’s ability to stand up may prevent biofilms from forming on medical implants and other surfaces, O’Toole says.
Standing up means bacteria can move in three dimensions, not just on flat surfaces, says John Kirby, a microbiologist at the University of Iowa in Iowa City. “That’s a real eye-opener,” he says. “It’s like the Earth was flat, but now it’s not flat anymore.”
A newborn bacterial cell stands up and walks away from its sister cell.
Credit: Courtesy of Gerard Wong, UCLA Bioengineering, CNSI