Key Points
- Bacteria actively sense their environment and use chemical cues to navigate their world through a process called chemotaxis.
- Chemotaxis aids in survival by helping bacteria move toward resources and away from danger.
- Chemotaxis shapes far more than individual cell survival-it plays a critical role in health outcomes and global biogeochemical cycles.
Although they are small, bacteria are not helpless and immobile in their environments. Like many other living organisms, bacteria move around (often with the help of cellular protrusions like flagella or pili) and navigate their surroundings to find food, escape predators and other dangers and find others to form communities. But while being able to move is 1 thing-knowing where to go is quite another.
Navigating With Chemical Cues
Although bacteria do not have any of the senses that we use to perceive our world, they can sense chemical gradients in their environment. These cues help them to move toward food sources or other cells of their kin, or to move away from dangers like toxins. This sensing and response by movement is called chemotaxis, combining the prefix chemo (relating to chemistry) and the Greek word táxis (meaning arrangement or order).
Chemotaxis works through receptors embedded in the bacterial cell membrane, which bind molecules in the vicinity and send signals about the environment to proteins that influence the direction of flagellar rotation-allowing the cell to change direction (clockwise rotation in Escherichia coli) or hold its course (counterclockwise rotation in E. coli)-or the extension and retraction of pili-allowing cells to 'twitch' along a solid surface. When moving toward the source of a favorable chemical gradient, also called a chemoattractant, E. coli uses forward flagellar motion to move toward the source and reduces its 'tumbling' behavior to prevent rapid changes in direction.
Scavenging for Food in the Big Blue
Movement based on external cues is important in many environments, and bacteria have evolved a complex array of chemotactic mechanisms to fine-tune this behavior for different settings. In the open ocean, for example, food can be hard to come by and often requires bacterial scavenging. Scavenging bacteria usually involve flagella to actively propel themselves (swim) through dilute liquid environments in search of patches of nutrients.
Swimming requires a lot of energy, which can be costly in dilute environments like the open ocean where starvation is often the norm. Research into bacterial scavenging strategies has shown that cells respond to starvation differently-some take the risk to keep swimming and search for nutrients, while others stop and become immobile to conserve energy until conditions improve. There appears to be a genetic basis for the strategic choice, indicating that factors, such as the ability to handle oxidative stress or form biofilms may be linked to whether a particular microbe decides to swim or stop.
The bacteria that keep swimming navigate using chemotaxis, following chemical signals emanating from resource 'hotspots,' such as phytoplankton, which release nutrients into their environment. These resource hotspots seem to specifically attract bacteria likely to engage in mutually beneficial symbiotic relationships with the phytoplankton. While the phytoplankton release nutrients that the bacteria can consume, the bacteria that swim toward phytoplankton also tend to produce hormones and vitamins that the phytoplankton can use for growth, as well as compounds that help the phytoplankton to take up iron, an essential micronutrient.
In addition to the benefits to individual bacteria navigating the ocean, bacterial chemotaxis toward nutrient hotspots has larger implications on marine ecosystems. In the ocean, bacterial degradation of complex organic matter, often bound together in aggregates known as 'marine snow,' is an important step in the global carbon cycle. Degradation acts as a critical node in the marine carbon cycle, determining whether carbon is returned to the cycle through incorporation into bacterial biomass or sequestered in the deep ocean if it can sink all the way down to the seafloor. By responding to chemical cues with movement, bacteria substantially increase their chances of finding these nutrient hotspots and changing the fate of the carbon that is contained within them.
Forming Biofilms To Enhance Survival
Chemotaxis can also help bacteria to find their friends and band together in thick layers known as biofilms. Biofilms have important roles in human, plant and animal health and disease. For example, they can promote antimicrobial resistance and help pathogens evade the body's immune responses, or help establish healthy plant root microbiomes. In any case biofilm formation is underpinned by chemotaxis.
The process of biofilm formation is particularly well-studied in Pseudomonas aeruginosa, a notorious opportunistic pathogen that can cause pneumonia and other infections by forming biofilms that are difficult to eradicate. The gram-negative, rod-shaped bacterium relies on twitching motility mediated by extending and retracting pili for biofilm formation. Initially, pili help the bacterial cells attach to surfaces and 'climb' over bacterial cells others to the top of the biofilm mound that is forming. Individual P. aeruginosa cells also use chemoattractants to orient themselves during the early stages of biofilm development, when they are still scoping out the best spot where nutrients are likely to be plentiful.
In plant, chemotaxis is used to recruit bacteria from the surrounding soil to the roots. This recruitment process is fundamental for the plant to establish a healthy root microbiome, which helps the plant to sequester nutrients and weather stressful conditions. Plants secrete a complex concoction of molecules through their roots, together termed root exudates-some of these molecules, such as amino acids and certain sugars, can act as chemoattractants to invite nearby soil bacteria into the root environment.
Plants can control the composition of their root microbiota through their exudates, by attracting beneficial bacteria while trying to repel pathogenic ones. This makes root exudates and chemotaxis an interesting avenue for agricultural research. For example, scientists are investigating ways to enhance the construction and maintenance of healthy root microbiomes in crop plants. On the plant side, exudate production can be manipulated to promote the recruitment of beneficial bacteria, while on the bacterial side, the recognition of and chemotactic response to specific root exudates could be enhanced using genetic approaches.
Next Destinations
Far from going with the flow, many bacteria actively navigate through their environments using chemical cues to decide their direction. These processes are important for the survival of individual cells, as well as health outcomes and even global biogeochemical cycles. Continuing to learn about how bacterial locomotion shapes microbial interactions with the world, and human interactions with microbes, can lead to a wealth of new discoveries of immense value across many areas of microbiology.
The ASM Mechanism Discovery Meeting at ASM Microbe 2026 unites researchers across disciplines to uncover the molecular mechanisms that drive microbial life and interactions, Join us in Washington D.C., June 4-7 to learn about the latest research on the fundamental processes underlying microbial function, adaptation and impact across diverse systems.
