Pseudomonas aeruginosa is a bacterium found in environments like soil and water. As an opportunistic pathogen, it primarily causes illness in individuals with weakened immune systems or underlying health conditions. This bacterium can colonize surfaces from medical equipment to human tissues and cause a range of infections. A defining feature of Pseudomonas is its motility, the ability to move, which is a significant factor in how it establishes infections and survives within a host.
Types of Pseudomonas Movement
Pseudomonas aeruginosa uses three different methods to move, each adapted to specific conditions. The first, swimming motility, involves individual bacteria moving through liquid. This directed movement allows the bacterium to travel toward nutrients or away from harmful substances, enabling it to reach and explore new areas.
A second form of movement is swarming, a coordinated and rapid migration of bacterial populations across semi-solid surfaces. Unlike the individual action of swimming, swarming is a collective behavior where communication allows the group to expand its territory quickly. This type of motility is also influenced by the availability of specific nutrients.
The third mode is twitching motility, which occurs on solid surfaces and is characterized by jerky, intermittent movements. This movement is often compared to a grappling hook, where the bacterium extends a filament, attaches to a surface, and retracts it to pull itself forward. Twitching is a slower process than swimming or swarming and allows for movement where fluid is not present.
The Cellular Machinery for Movement
The different forms of movement are made possible by distinct cellular structures. Swimming and swarming are powered by a flagellum, a long, whip-like appendage extending from the cell’s surface. A motor at its base rotates the flagellum, propelling the bacterium. While swimming involves a single flagellum, during swarming, some cells produce multiple flagella to generate the force needed for collective movement.
Twitching motility relies on an entirely different apparatus known as type IV pili (T4P). These are thin, flexible filaments that are shorter than the flagellum and located at the poles of the cell. The mechanism involves extending these pili, attaching them to a nearby surface, and retracting them. This process pulls the bacterium along the surface in short steps.
The actions of these structures are highly regulated. The bacterium can control the rotation of its flagellum to change direction while swimming, and the extension and retraction of its pili are also controlled. This allows the cell to explore and navigate solid surfaces.
Influence on Biofilm Formation
Motility is a preparatory step for forming biofilms, which are structured communities of bacteria encased in a self-produced slimy matrix. Individual bacteria must first travel to and contact a suitable surface. Swimming motility allows bacteria to approach a surface from a liquid environment and explore it for favorable conditions before attaching.
Once bacteria have reached a surface, they transition from a motile (moving) state to a sessile (stationary) one. This change involves shutting down movement machinery and initiating attachment. Following initial attachment, twitching motility plays a part in the next phase of biofilm development. This movement allows bacteria to arrange themselves across the surface, forming small clusters called microcolonies.
These microcolonies are the foundational structures of a mature biofilm. The ability to move and organize allows the bacteria to create a complex, three-dimensional community. Within this protective structure, the bacteria are more resistant to antibiotics and the host’s immune defenses.
Role in Infection and Pathogenesis
The ability of Pseudomonas aeruginosa to move is directly linked to its capacity to cause disease, a measure known as virulence. Motility allows the bacterium to spread throughout the body and establish infections in various tissues. For example, in the lungs of a patient with cystic fibrosis, swimming motility enables bacteria to move through thick mucus, spreading the infection.
Movement also serves as a mechanism for evading the host’s immune system. Bacteria can physically distance themselves from immune cells attempting to engulf and destroy them. This evasive action gives the pathogen more time to multiply and establish a more persistent infection.
Motility facilitates the colonization of both host tissues and medical devices. Bacteria use their movement to reach and attach to surfaces like urinary catheters, ventilator tubes, or burn wounds. Once attached, they can initiate an infection or form a biofilm, leading to chronic or device-associated infections.