Gliding motility represents a distinct form of movement observed in various microorganisms, differing from flagellar or ciliary propulsion. This unique locomotion allows microbes to move smoothly and continuously across solid or semi-solid surfaces without relying on visible external appendages. It is a remarkable adaptation that enables microorganisms to navigate diverse environments.
How Microbes Glide: The Mechanisms
Microorganisms employ several distinct mechanisms to achieve gliding motility, each tailored to their specific cellular structure and environment. One well-studied mechanism involves focal adhesion complexes, particularly in bacteria like Myxococcus xanthus. These bacteria utilize internal motors that move specialized adhesion complexes along their cell surface. These complexes transiently attach to the underlying surface, and as internal motors pull them backward, the bacterium is propelled forward.
Another mechanism observed in some bacteria and cyanobacteria relies on slime secretion. Organisms like Oscillatoria and Phormidium species extrude extracellular polymeric substances (EPS) from pores on their cell surface. The continuous secretion and hydration of this EPS, which adheres to the substrate, generates a propulsive force that pushes the cell forward. This process can lead to visible slime trails left behind the moving organism.
Type IV pili, hair-like protein appendages, facilitate gliding in some bacteria, including Pseudomonas aeruginosa and Myxococcus xanthus. These pili extend from the bacterial surface, attach to the substrate, and then retract, effectively pulling the cell along in a manner akin to a grappling hook. This extension and retraction cycle provides the necessary force for movement across surfaces.
Diatoms, a type of algae, exhibit a unique gliding mechanism involving a specialized structure called a raphe system. These pennate diatoms secrete extracellular polymeric substances (EPS) through a slit-like channel in their silica cell wall. The forceful discharge and interaction of this adhesive EPS with the substrate, potentially driven by internal actomyosin motors, generate the propulsion needed for their smooth, continuous movement.
Who Glides? Diverse Microorganisms
Gliding motility is observed across a wide range of microorganisms, reflecting its diverse utility in various ecological niches. Among bacteria, Myxococcus xanthus, a soil-dwelling Gram-negative bacterium, is a prominent example. It exhibits both “adventurous” (A-motility) for individual cell movement and “social” (S-motility) for coordinated group movement. This social gliding is important for its predatory behavior and the formation of multicellular fruiting bodies.
Another group of bacteria known for gliding are Flavobacterium species, commonly found in aquatic environments and soil. Flavobacterium johnsoniae, for instance, glides rapidly over surfaces and utilizes mobile cell-surface adhesins propelled by rotary motors. Some Flavobacterium species, like Flavobacterium columnare, are also fish pathogens, where gliding plays a role in their virulence and colony spreading.
Cyanobacteria also display gliding motility. Filamentous species such as Oscillatoria can glide slowly and smoothly across surfaces or through water.
Diatoms, single-celled algae, are also notable gliders. This motility is particularly relevant for benthic diatoms, which inhabit sedimentary environments and use gliding to adjust their position in response to light changes.
Why Gliding Matters: Ecological and Health Roles
Gliding motility plays significant roles in both microbial ecology and human health. One of its most recognized implications is in biofilm formation. Gliding microbes can effectively colonize solid surfaces and form complex, structured communities known as biofilms. For instance, Flavobacterium johnsoniae uses gliding to spread and mature biofilms.
Gliding also assists microbes in nutrient acquisition. By moving along surfaces, these organisms can explore their environment to locate and access nutrient sources or find optimal conditions for growth. This surface-associated movement is particularly advantageous in environments with low water content, such as soil or microbial mats, where swimming might be less effective. The production of extracellular enzymes, often associated with gliding motility, further aids in breaking down complex organic matter, making nutrients available to the gliding cells.
In environmental contexts, gliding motility contributes to various ecological processes in soil, aquatic environments, and on plant surfaces. In soil, for example, cyanobacteria that glide can help stabilize soil crusts, preventing erosion and retaining water. In aquatic systems, gliding diatoms can migrate vertically within sediments to optimize photosynthesis and avoid damaging light levels. The ability to colonize surfaces also influences nutrient cycling and the decomposition of organic matter in diverse habitats.
Beyond environmental roles, gliding motility has medical relevance, particularly in the pathogenesis of certain diseases where surface colonization is a factor. For example, the gliding motility of some oral bacteria contributes to their ability to form dental plaque and initiate periodontal diseases. In fish, certain Flavobacterium species, known pathogens, utilize gliding to spread and establish infections. Understanding these mechanisms can inform strategies to manage or prevent biofilm-related infections and environmental issues.