Information processing constrains how E. coli bacteria move along chemical gradients

To climb the chemical gradients of nutrients, E. coli bacteria need to constantly decide whether to continue swimming upright or change the direction of swimming. Researchers at Yale University recently showed that although E. coli makes this decision under a great deal of uncertainty, it climbs gradients as fast as it is theoretically possible, given this uncertainty. Credit: Yale University.

Organisms adapt their behavior and movements based on the information they obtain from the environment around them. But often this information is incomplete, and the organism needs to act under uncertainty. So, does incomplete information limit an organism’s performance on specific tasks?

Researchers at Yale University recently investigated this possibility, specifically examining the behavior of the Escherichia coli bacteria (E. coli). Their findings, published in Nature Physicscoli bacteria gather information from their environment that limits their performance in chemotaxis, the process by which they direct their movements in response to chemical cues.

“The work of cells in the face of noise and uncertainty has long been appreciated,” Henry Mattingly, Kita Camino, Benjamin Matchta and Terry Emonette, the researchers who conducted the study, told Phys.org by email. “Since information theory was first invented around 1950, researchers have recognized that it can be a powerful tool for understanding how organisms deal with noise.”

Most previous research has focused on the amount of information cells can obtain from their environment. Measurements collected so far indicate that evolution selects biological systems to effectively transmit information, accessing fundamental limitations imposed by physics.

“However, obtaining information is not necessarily the end of the biology story: the information must be handled appropriately,” the researchers explained. “We wanted to test a large-scale biological hypothesis: that organisms make best use of the information they acquire to perform behaviors and other functions. To verify this, we needed a simple enough behavior that we could determine how much information they need and the chemical focus through the E. coli bacteria being an example Pretty cool for such behaviour.”

coli bacteria are constantly gaining information about chemical signals in their environment using biosensors. They then use this information to adapt their behavior and navigate chemical gradients. If the E. coli bacteria do not gather information from their surroundings, they cannot determine whether they are moving towards nutrients or away from toxins.

“Experimental tools are now available to measure this behavior and to investigate the dynamics of the chemical pathway that connects cell sensors to the actuators that use them for locomotion,” the team said. “By combining these measurements with simple models of their behavior, we realized we could measure the amount of information the bacteria were able to gather (bits per second), while also understanding how much information they needed to navigate at the observed speeds.”

The authors’ primary goal was to gain a better understanding of whether the limits of biosensors influence chemotaxis behavior in Escherichia coli. To achieve this, they first set out to compute a theoretical performance limit, which is the maximum speed at which bacteria can navigate a chemical gradient, based on a constant rate at which they acquire information about chemical signals.

“To do this, Henry modeled how the swimming behavior of bacteria responds to the cues they are exposed to,” the researchers explained. “Different response strategies have different ‘information costs’ to be implemented, but a strategy that requires a high information cost does not necessarily cause a cell to climb gradients quickly. Finding a response strategy that maximizes gradient climbing speed has a fairly constant information cost the performance “.

To determine the rate at which Escherichia coli cells gain information during chemotaxis, the team had to understand how they translated the signals they saw, in the form of changing nutrient concentrations, into the activity of their intrinsic signaling network. They achieved this using a microfluidic approach developed by Kamino. This approach works by delivering calibrated pulses of nutrients to the bacteria, while simultaneously measuring activity in the chemical response network.

“Using the same experimental system, Keita also measured the magnitude of spontaneous noise in the response network in the absence of signals,” the researchers said. “By these measurements, as well as measurements of the typical signals that a swimming cell goes through in a gradient, we estimated how much information Escherichia coli can extract from its chemical environment while navigating a normal gradient, for the first time.”

As a final experimental step, the team had to determine how quickly E. coli climbed chemical gradients using the information they had collected. To do this, Mattingly created shallow chemical gradients between two tanks and traced the swimming paths of the bacteria as they navigated the gradients. After observing hours of these trails at different gradients, the researchers could determine the speeds at which they ascended the gradients.

“Combining measurements of information acquisition and gradient climb velocity allowed us to determine how well Escherichia coli uses information for navigation relative to theoretical limits,” the researchers said. “We found that while climbing shallow gradients, E. coli gets very little information from its environment, about 0.01 bits/sec. With an Internet connection at this rate, it would take several thousand years to download a typical 4K feature film.”

Mattingly and colleagues found that bacteria use the little information they acquire to climb gradients at speeds approaching as fast as theoretically possible, given the limitations of their sensory system. This indicates that in order to survive, organisms must efficiently use the information they gain to perform specific tasks.

“The 0.01 bits/sec that Escherichia coli needs, and gets, are orders of magnitude less than the ~1 bits per second that a cell needs to determine whether or not it is swimming in the gradient before choosing a new direction at random,” the researchers said. The first that uses information theory to set limits on an organism’s performance on a behavioral task, and to relate the accuracy of a signal transduction pathway to its ability to perform functions critical to its survival.”

This work provides interesting new insights into the effects of information acquisition on E. coli chemotaxis. In the future, new studies aimed at understanding the relationship between an organism’s ability to process signals in its environment and perform survival tasks could benefit.

In their next studies, Mattingly, Camino, Mashta, and Emunet plan to measure the accuracy of E. coli sensing and compare it to theoretical predictions. This will allow them to test the hypothesis that information processing for biological systems is close to the limits imposed by physics.

“So far, we’ve measured how much information Escherichia coli cells acquire during chemotaxis and how efficiently they use that information, but we haven’t addressed how much information they could get with an ideal sensor,” the team said. “E. coli senses the chemicals in their environments by counting molecules that randomly collide with their surface, and this imposes limitations on their sensing accuracy. This is something others have thought a lot about, but so far without comparing it with actual E. coli sensing loyalty.”

coli bacteria multiply very quickly, doubling the amount in less than 30 minutes. Their speed of reproduction makes them ideal candidates for studying evolution in laboratory settings. In their future work, the researchers also want to take advantage of this useful quality to test hypotheses about how evolutionary selection works on information processing.

“Finally, models and measurements of both behaviors and signal processing in more complex organisms may be at the point where we can apply the same insights to study their behaviors as well,” the researchers added. “The simplicity of chemotaxis for E. coli made it an ideal system to start with, but similar ideas may apply to animal behaviors.”


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more information:
HH Mattingly et al, The chemotaxis of Escherichia coli is of limited information, Nature Physics (2021). DOI: 10.1038 / s41567-021-01380-3

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