peritrichous

Peritrichous describes a flagellar arrangement in which a bacterial cell bears flagella distributed over its entire surface, rather than at a single pole. The term comes from the Greek peri- meaning around and trichos meaning hair. In peritrichous species, there are many flagella, often dozens or more, which can form a loose or tight bundle during swimming.

Motility in peritrichous bacteria is achieved by the rotation of the flagella, driven by the proton motive force. During a run, flagella rotate in a coordinated counterclockwise direction to form a propulsive bundle; a switch to clockwise rotation disrupts the bundle, causing a tumble that reorients the cell. This run-and-tumble behavior enables chemotaxis and navigation through liquid environments, allowing the cells to respond to chemical gradients.

Common examples of peritrichously flagellated bacteria include Escherichia coli and Salmonella enterica, among others in diverse ecological niches. By contrast, monotrichous bacteria have a single flagellum at one pole, lophotrichous bacteria possess a tuft of flagella at one end, and amphitrichous bacteria have flagella at both ends.

The peritrichous arrangement has implications for social behaviors such as swarming and biofilm formation. It also influences interactions with hosts and the immune system, since flagellin, the protein that makes up the flagellar filament, is a strong antigen recognized by pattern-recognition receptors. Flagellar number and activity are regulated in response to environmental conditions and developmental stage, reflecting adaptation to different habitats and lifestyles.