Bacteria Change Behavior to Swim to Freedom

Researchers from the University of Hawai‘i at Mānoa (UHM) have discovered that bacteria change their swimming patterns when caught in tight places to facilitate escape.

Nearly all organisms host bacteria that live symbiotically on or within their bodies. The UH Mānoa study specifically studied marine bacterium Vibrio fischeri, which forms such a relationship with Hawaiian bobtail squid, Euprymna scolopes, and possesses a whip-like tail that it uses to swim to specific places in the squid’s body.

Led by Jonathan Lynch, formerly a postdoctoral fellow at the Pacific Biosciences Research Center (PBRC) at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), the team designed controlled chambers in which they could observe the bacteria swimming.  Using microscopy, the team discovered that as the bacteria moved between open areas and tight spaces, they appeared to swim differently. In open spaces, without chemicals to be attracted to or repelled from, the bacteria appeared to meander with no discernible pattern—changing direction randomly and at different points in time. In confined spaces, they changed their swimming behavior to avoid getting stuck.

“This finding was quite surprising,” said Lynch. “At first, we were looking for how bacterial cells changed the shape of their tails when they moved into tight spaces, but discovered that we were having trouble actually finding cells in the tight spaces. After looking more closely, we figured out that it was because the bacteria were actively swimming out of the tight spaces, which we did not expect.”

Using a computational model, the team was able to attribute this escape response to two factors: reduced directional fluctuation and a refractory period between reversals. “Additional experiments in asymmetric capillary tubes confirmed that V. fischeri quickly escape from confined ends, even when drawn into the ends by chemoattraction,” according to the paper, published recently in Biophysical Journal.  “This avoidance was apparent down to a limit of confinement approaching the diameter of the cell itself, resulting in a balance between chemoattraction and evasion of physical confinement. Our findings demonstrate that nontrivial distributions of swimming bacteria can emerge from simple physical gradients in the level of confinement.”

The relationship between the squid and V. fischeri is a useful model of how bacteria live with other animals, such as the human microbiome. Microbes often traverse complicated routes, sometimes squeezing through tight spaces in tissues, before colonizing preferred sites in their host organism. A variety of chemicals and nutrients within hosts are known to guide bacteria toward their eventual destination. However, less is known about how physical features like walls, corners, and tight spaces affect bacterial swimming, despite the fact that these physical features are found across many bacteria-animal relationships.

“Our findings demonstrate that tight spaces may serve as an additional, crucial cue for bacteria while they navigate complex environments to enter specific habitats,” said Lynch. “Changing swimming patterns in tight spaces may allow some bacteria to quickly swim through the tight spaces to get to the other side, but for the others, they turn around before the get stuck—kind of like choosing whether to run across a rickety bridge or turn around before you go too far.”

In the future, the researchers hope to figure out how these bacteria are changing their swimming activity, as well as determining if other bacteria show the same behaviors.