Antigenic Variation. P. mirabilis encodes two nearly identical flagellins, xxxX and flaB. A third putative flagellin is encoded elsewhere on the chromosome. Despite the close proximity of the two loci, xxxX is transcribed monocistronically, and flaB transcript is scarce in wild-type cells. XxxX is the major flagellin, as xxxX mutants are non-motile and do not synthesize flagellin while flaB mutants exhibit wild-type motility. Mutants of xxxX frequently revert to motile cells with a flagellum that is antigenically distinct from wild-type (89). These mutants have hybrid FlaAB flagellin created by deletions in both loci that place the active xxxX promoter and the 5’coding region of xxxX with homologous and previously silent regions of flaB (90). Later, it was shown P. mirabilis populations are naturally heterogeneous in flagellar composition, with 1.0–1.5% of the population expressing hybrid flaAB transcript (91). Flagellin rearrangements confer advantages in different environments; bacteria with hybrid FlaAB flagella are more motile in environments with high salinity and extreme pH, while bacteria with XxxX flagella are more motile in low salt conditions (92). Recovery of swimmer and swarmer cells from the murine urinary tract illustrated different hybrid flagellins are formed during infection. All recovered FlaAB proteins were the result of splicing before the antigenically exposed domain of the flagellin molecule. This would result in exposure of different antigenic sites on the surface of the bacterium (90), and both flagellin proteins are capable of generating a humoral response (61). Overall, the ability of P. mirabilis to undergo flagellin rearrangements promotes survival in various environments and may help the bacteria evade the immune response during UTI.
