Though they are single-celled organisms, many bacteria are capable of movement. This mobility is a regulated activity that allows bacteria to navigate their surroundings to seek out favorable conditions, such as areas rich in nutrients, and escape from hazardous ones. This ability to relocate is fundamental to their survival, enabling them to colonize new environments and find the resources necessary to thrive.
How Bacteria Propel Themselves
Bacteria have developed several methods to move through their environments. One of the most common forms of locomotion is swimming, powered by a structure called the flagellum. The flagellum is a long, helical filament that extends from the cell surface and acts like a propeller. It is connected to a rotary motor embedded in the bacterial cell wall, which can spin to push or pull the cell through liquids. The arrangement of these flagella can vary, with some bacteria having a single flagellum (monotrichous) and others having multiple flagella distributed over their surface (peritrichous).
This variation in flagellar arrangement influences how bacteria navigate. A well-studied example is the “run and tumble” method used by E. coli. During a “run,” the flagella rotate counter-clockwise, forming a cohesive bundle that propels the bacterium in a straight line. To change direction, one or more flagella will briefly reverse their rotation to clockwise, causing the bundle to fly apart and the bacterium to “tumble” randomly, reorienting it for the next run.
Beyond swimming, bacteria employ other strategies for movement on solid surfaces. Twitching motility is one such method, facilitated by appendages called Type IV pili. These pili are shorter, hair-like structures that can extend from the cell, anchor to a surface, and then retract, pulling the bacterium forward in a jerky motion similar to a grappling hook.
Another form of surface-based locomotion is gliding motility, which is a smoother movement that does not rely on flagella or pili. The mechanisms behind gliding often involve the secretion of a polysaccharide slime that reduces friction and allows the cell to slide across a surface. In other cases, internal protein complexes move along the cell’s length, generating a force that propels the bacterium forward over a solid substrate.
Why Bacteria Move: Responding to Signals
Bacterial movement is rarely random; it is a direct response to environmental cues. This directed movement, known as taxis, allows bacteria to seek beneficial conditions and avoid harmful ones. The most understood form is chemotaxis, which is movement in response to chemical gradients. Bacteria can sense increasing concentrations of attractants, such as sugars and amino acids, and move towards them.
Specialized receptor proteins on the bacterial surface, called methyl-accepting chemotaxis proteins (MCPs), detect these chemical signals. When an attractant binds to a receptor, it initiates a signaling cascade inside the cell that controls the flagellar motor. This system allows the bacterium to compare the concentration of a chemical over time, suppressing tumbles when moving up a favorable gradient to extend its runs. Conversely, they move away from repellents like toxins or metabolic byproducts.
Light is another environmental signal for some bacteria, guiding their movement through a process called phototaxis. This is useful for photosynthetic bacteria, such as cyanobacteria, which depend on light for energy. These organisms can move towards light sources to maximize their photosynthetic activity (positive phototaxis). They are also capable of moving away from light that is too intense or harmful, such as UV radiation (negative phototaxis).
Bacteria also respond to the concentration of gases, most notably oxygen, in a behavior called aerotaxis. Aerobic bacteria, which require oxygen for their metabolism, will move towards areas where it is plentiful. In contrast, anaerobic bacteria may be harmed by oxygen and will actively move away from it. This allows different types of bacteria to find the oxygen levels best suited for their survival.
The Impact of Bacterial Mobility
The movement of individual bacteria has significant consequences, particularly when they act together. One example is swarming motility, a rapid and coordinated movement of a bacterial population across a surface. This collective behavior allows bacteria to expand their territory quickly, enhancing their ability to colonize new areas and access nutrients. Swarming bacteria often exhibit increased resistance to antibiotics, making this a concern in clinical settings.
Motility contributes to the formation of biofilms, which are structured communities of bacteria encased in a self-produced protective matrix. The initial stage of biofilm development requires motile bacteria to reach and attach to a surface. Motility also aids in dispersal, as some bacteria may detach from a mature biofilm to colonize new locations.
For pathogenic bacteria, movement can be a virulence factor. Motility enables pathogens to travel to specific sites within a host, such as Helicobacter pylori swimming through the thick mucus lining of the stomach to reach the stomach wall. This movement helps bacteria invade tissues, evade the host’s immune defenses, and spread within the body, making it part of the infection process.
Bacterial motility is also fundamental to their ecological roles. In soil and water, the movement of bacteria is part of nutrient cycling. Motile bacteria travel to find and decompose organic matter, releasing nutrients back into the ecosystem and participating in the biogeochemical cycles that sustain life.