Flagella are molecular organelles of motility found in many species of bacteria, both Gram positive and Gram negative. They may be distributed around the cell (an arrangement called peritrichous from the Greek trichos for “hair”), at one pole (polar or monotrichous), or at both ends of the cell (lophotrichous).
In all cases, they are individually helical in shape and propel the cell by rotating at the point of insertion in the cell envelope. The presence or ab-sence of flagella and their position are important taxonomic characteristics.
The flagellar apparatus is complex, but consists entirely of proteins, encoded in genes called fla (for flagella). They are attached to the cell by a basal body consisting of several proteins organized as rings on a central rod (see Fig 2 – 5). In Gram-negative cells, there are four rings: an outer pair that serve as bushings through the outer membrane and an in-ner pair located in the peptidoglycan gel and the cell membrane. In Gram-positive cells, only the inner pair is present. The hook consists of other proteins organized as a bent structure that may function as a universal joint. Finally, the long filament consists of polymerized molecules of a single protein species called flagellin (Fig 2 – 11). Flagellin varies in amino acid sequence from strain to strain. This makes flagella useful surface antigens for strain differentiation, particularly among the Enterobacteriaceae.
Motility and chemotaxis, both important properties contributing to colonization.