Motility in microbiology describes the ability of a microorganism to move itself purposefully through its environment. This self-propulsion is a fundamental characteristic of many bacteria, archaea, and protozoa, distinguishing them from non-living particles. Observing this movement is a key trait for identifying different species. The capacity for movement is inherently linked to a microbe’s survival and its ability to interact with its surroundings.
Defining Motility in Microorganisms
True microbial motility is defined as the active, self-directed movement of a cell that requires the expenditure of metabolic energy. This results in a noticeable change in the organism’s position, often over long distances relative to its size. A cell exhibiting true motility will move in a clear, directional path before changing course.
This true movement must be carefully distinguished from two types of passive motion that can be observed under a microscope. Brownian motion is the random, non-directional jiggling of particles suspended in a fluid, caused by constant collision with water molecules. In contrast, streaming is the passive movement of cells in the same direction due to currents or flow within the liquid medium.
Distinguishing these passive movements is important because only true motility indicates the presence of specialized cellular machinery and the expenditure of energy. This is a significant factor in species identification and biological function. Observing a microbe move across the field of view, rather than simply vibrating in place, confirms its motile status.
Structures Enabling Movement
The most common structure enabling movement in bacteria is the flagellum, a long, whip-like appendage that acts like a microscopic propeller. The flagellar motor is a complex rotary machine embedded in the cell membrane and cell wall. This motor is powered not by ATP, but by the proton motive force (PMF), where the flow of hydrogen ions across the membrane acts like a turbine, spinning the flagellar filament at speeds up to 60 body lengths per second.
This rapid rotation generates thrust, allowing the bacterium to “swim” through liquid environments. Other microbes, specifically spirochetes, utilize a unique structure called an axial filament, which is essentially a flagellum located between the cell wall and the outer membrane. The rotation of this internal filament causes the entire corkscrew-shaped cell body to flex and rotate, resulting in a distinctive boring or twisting motion.
Some bacteria move across solid or moist surfaces using mechanisms that do not involve flagella. Twitching motility utilizes hair-like appendages called Type IV pili, which function like a grappling hook. The pilus is extended, attaches to a surface, and then retracts, pulling the cell forward in a jerky, stop-start manner. Gliding motility is a smooth, surface-dependent movement that employs various complex mechanisms to push the cell along a solid substrate.
Observing Motility in the Laboratory
Microbiologists use two primary techniques to determine if a species is motile in a laboratory setting. Direct observation is achieved using a wet mount or hanging drop slide, where a sample is placed on a slide and immediately viewed under a microscope. Motile cells are seen actively traversing the field of view, while non-motile cells only exhibit random Brownian jiggling.
The second method involves the use of a semi-solid agar deep, which is a test tube containing a growth medium with a low concentration of agar. This soft medium allows motile bacteria to swim freely, but is firm enough to contain non-motile growth. The sample is inoculated by stabbing a sterile needle straight down the center of the medium.
After incubation, a non-motile organism will show growth confined strictly to the straight line of the stab. A motile organism, however, will swim away from the initial inoculation line and colonize the surrounding soft agar, resulting in diffuse, cloudy growth throughout the tube. This visual difference provides an easy and reliable way to differentiate between motile and non-motile species for diagnostic purposes.
The Biological Role of Microbial Movement
Motility is a survival mechanism that allows microorganisms to sense and respond to their environments. The primary function is chemotaxis, the ability to direct movement based on chemical gradients. Microbes move toward chemical attractants, such as nutrients, or away from chemical repellents, such as toxic waste products.
This directed movement is fundamental to finding optimal living conditions and acquiring resources necessary for growth. Motility also plays a significant role in the initial stages of colonization and dispersal. For instance, motile bacteria can quickly spread across a moist surface or through a liquid medium to find a suitable location to form a biofilm.
In the context of host interaction, motility is often a requirement for pathogenesis, or the ability to cause disease. Many pathogens must swim through mucus layers or other host barriers to reach their specific site of infection. For symbiotic microbes, chemotaxis allows them to actively locate and home in on chemical signals produced by their host, ensuring the successful establishment of the partnership.