Type 1 fimbriation and phase switching in a natural Escherichia coli fimB null strain, Nissle 1917

Escherichia coli Nissle 1917 has been used as a probiotic against intestina l disorders for many decades. It is a good colonizer of the human gut and h as been reported to be able to express type 1 fimbriae, Type I fimbriae are surface organelles which mediate ol-D-mannose-sensitive binding to various host cell surfaces. The expression is phase variable, and two tyrosine rec ombinases, FimB and FimE, mediate the inversion of the fimbrial phase switc h. Current evidence suggests that FimB Fan carry out recombination in both directions, whereas FimE-catalyzed switching is on to off only, We show her e that under liquid shaking growth conditions, Nissle 1917 did not express type 1 fimbriae, due to a truncation of the fimB gene by an 1,885-bp insert ion element, Despite its fimB null status, Nissle 1917 was still capable of off-to-on switching of the phase switch and expressing type I fimbriae whe n grown under static conditions. This phase switching aas not catalyzed by FimE, by truncated FimB, or by information residing within the insertion el ement. No further copies of fimB seemed to be present on the chromosome of Nissle 1917, suggesting that another tyrosine recombinase in Nissle 1917 is responsible for the low-frequency off-to-on inversion of the phase switch that is strongly favored under static growth conditions. This is the first report documenting the non-FimB- or non-FimE-catalyzed inversion of the fim switch.