Finding the target: bacterial motility and events that lead to bacterial contact with and attachment to a surface
Prior to adherence, bacteria must locate and physically make contact with a surface. Motility plays a key role in this initial step and is influenced by Brownian motion and the production of appendages that directly facilitate movement through fluids. Flagella are flexible, motor-based, filamentous appendages that utilize the flux of ions to rotate and propel bacteria through fluids (known as swimming motility) or enable gliding across a surface (known as swarming motility). Besides flagella, bacteria may harbor specific types of flexible adhesive fibers (pili) that can facilitate motility across a surface via a combination of adhesive properties and dynamic pilus movement. The different types of motility are described in greater detail below.
Brownian motion is the random, uncontrolled movement of particles in a fluid as they constantly collide with other molecules (Mitchell and Kogure, 2006). Brownian motion is in part responsible for facilitating movement in bacteria that do not encode or express motility appendages, such as Streptococcus and Klebsiella species. Brownian motion can also affect “deliberate” movement exhibited by inherently motile bacteria that harbor pili or flagella. For example, an Escherichia coli cell that is swimming toward an area of higher oxygen concentration may fall “off-track” if it physically encounters a particle moving by Brownian motion or if such a particle(s) obstructs the bacterial cell’s path of motion. This form of “interference” adds to the stochasticity with which bacterial direction can change.