Flagella are dynamic organelles that provide bacteria with a means for motility and, as will be discussed further below, flagella also constitute a pathway for the export of proteins to the environment. From a historical perspective, the ability to produce flagella has been a key factor that distinguishes the gastrointestinal pathogens, Y. enterocolitica and Y. pseudotuberculosis, from the plague pathogen, Y. pestis. Essentially all pathogenic isolates of Y. enterocolitica and Y. pseudotuberculosis exhibit the ability to produce flagella whereas the production of flagella has not been observed for any isolate of Y. pestis. The lack of flagella production by Y. pestis may reflect the adaptation of this species to a specialized lifestyle in which the organism is directly spread from one animal host to another by flea-mediated transmission or aerosol exposure. The recently completed annotation of the Y. pestis genome provides clues that support this widely held inference (Parkhill et al., 2001; Deng et al., 2002). However, this does not mean that flagella are not important for Yersinia. Recent studies of Y. enterocolitica and Y. pseudotuberculosis suggest flagella play a role in virulence (Young et al., 1999; Young et al., 2000; Young and Young, 2002). At this time, we have the most complete picture of
how flagella contribute to the lifestyle of Y. enterocolitica. Therefore, this chapter primarily focuses on this species with additional information about Y. pseudotuberculosis and Y. pestis included to provide a complete perspective.