Diversity’s Impact On E. Coli Mobility

Escherichia coli is a rod-shaped bacterium; certain strains are associated with food-borne illness. Image courtesy of Wikimedia Commons.

Movement is essential for life. We all must move to gather and consume the nutrients needed to fuel our bodies. Without the ability to orient ourselves in the best positions for nutrient consumption, we would have to be spoon-fed to survive, and biologists know that mother nature does not spoon-feed her children.


Some species, such as E. coli, a kind of bacteria commonly found in animal intestines, travel in groups in their quest for food. They travel in wide swathes, consuming all the nutrients found in their path.


“They are like a herd of sheep moving and eating grass,” said Thierry Emonet, Associate Professor of Molecular, Cellular, & Developmental Biology and Physics at Yale. However, just as humans do, E. coli express many different traits and ability levels when it comes to mobility. In a recent study, Emonet and his team explored just how a species with such diversity could function as a cohesive group.


To further investigate this behavior, the team had to decide on a mobility-related trait that has significant variance within a colony of E. coli. The phenotype, or trait, chosen was the tumble bias phenotype. To search for chemical gradients that indicate a food source, E. coli alternate straight “runs,” or spurts of swimming in a straight direction, with “tumbles,” where the cells change direction randomly. These tumbles are suppressed by individual E. coli when a chemical gradient is detected, thus allowing the E. coli to perform an influenced random walk that allows them to follow high concentrations of food.


With the phenotype chosen, the group proceeded to investigate how the tumble bias impacted what individual E. coli travelled within the group and if only a specific subset of bacteria travelled within the group. It should be noted that previous experiments by the team determined that a higher tumble bias, or the probability a cell is tumbling, is associated with decreased mobility. The experiment involved placing a group of bacteria at one end of a tube and aspartate, a food source that attracts bacteria, throughout the tube. When the bacteria had travelled to the opposite end, consuming all the attractant, they were trapped in a chamber, which allowed the team to assess the variability of the tumble bias within the group. They discovered that although high tumble bias was selected against, a wide range of tumble biases were represented within the travelling group.


The group then set out to discover how E. coli with such a diverse range of tumble biases could travel as a coherent group. They discovered that bacteria spontaneously assorted themselves into the position within the group that would best fit their own specific ability to travel, which can be mathematically represented as a moving curve. As Emonet put it, “It is hardest to cycle in the front of a group of cyclers.”


This behavior exhibited by E. coli is similar to the behavior of human societies, in which people occupy positions according to their personal abilities. Future studies in this field could branch into the behavioral sciences and give us insight into our own species’ habits.