How bacteria get organised

An international team of researchers has been investigating how bacterial colonies spread through the water-filled cracks and crevices they inhabit in natural settings like rock, soil or body tissue. Now, the groups believe they’ve figured out how these complex dynamics emerge.

Writing in the New Journal of Physics, the study authors stated that dense suspensions of swimming bacteria are known to exhibit collective behaviour, by which they follow seemingly coordinated patterns. As noted by co-author Enkeleida Lushi, from Brown University, “It’s not at all obvious how they do this.”

“Bacteria don’t have leaders to follow, they have limited sensory systems, and they’re not very smart,” Lushi explained. “They can’t really make decisions about where to go, so it is all down to mechanical interactions between themselves and their surroundings.”

Looking to understand how collective patterns in bacterial colonies form, Lushi and her colleagues sought to design an experiment that would mimic the tiny channels in which bacteria thrive in nature. Their solution was to make tiny racetracks from clear plastic, through which they observed the behaviour of Bacillus subtilis bacteria under a microscope.

The experiments showed that the chaotic motion of all the individual swimmers quickly organises itself into a collective motion, with bacteria in the middle of the track going in the opposite direction from those on the outer edges. In order to understand what was driving that pattern, Lushi developed a computer model that captured two critical parameters that affect how the bacteria move in a densely packed colony.

The first parameter involves the collisions that occur as bacteria try to swim in close quarters. The model showed that, as bacteria bang into each other, the collisions tend to orient individuals in the same direction. As for the bidirectional motion — with the middle swimmers moving in opposite direction from the ones at the edges — that is a function of fluid flow.

Bacteria propel by pushing against the tiny fluid corkscrew-like appendages called flagella. That pushing creates a fluid flow moving opposite the direction of the swimming. The bacteria along the edges of the track are closely aligned by the track’s outer surface. They swim at an angle against that outer surface, which causes the backflow they create to be aimed towards the middle of the track. The swimmers in the middle have to fight the currents generated on either side of them. Eventually, the flow becomes more than they can handle.

“Even though they’re trying to go in the same direction as the ones on the boundaries, they’re being carried backward by the fluid,” Lushi said, claiming that the results underscore the importance of fluid flows in explaining collective dynamics. Despite the fact that most bacteria thrive and have evolved in fluid environments, the effects of fluid flows are often overlooked in explaining their behaviour.

As well as deepening our understanding of how bacteria spread in their natural environment, Lushi and her colleagues are hopeful that their research could also aid in the development of medical devices with surfaces and architectures that can manipulate bacterial movements and spread.

Image caption: Bacteria clustered together in tiny racetracks exhibit complex collective dynamics. Image credit: H Wioland/E Lushi/RE Goldstein.