HbiF regulates type 1 fimbriation independently of FimB and FimE

Abstract

Type 1 fimbriae have been suggested to play a role in the pathogenesis of extraintestinal Escherichia coli infection. Type 1 fimbriation in E. coli is phase variable and known to be dependent upon FimB and FimE, the two recombinases that invert the molecular switch fimS and control the expression of the downstream fim operon. Here we showed that HbiF, a novel site-specific recombinase, inverted fimS independently of FimB and FimE. HbiF-mediated fimS inversion appeared to be predominantly switching from "off" (termed OFF) to "on" (termed ON) orientation. This is different from the fimS inversion mediated by either FimB (bidirectional ON to OFF and OFF to ON) or FimE (unidirectional ON to OFF). Constitutive expression of the hbiF gene in E. coli resulted in a fimS-locked-ON phenotype, which resulted in the pathogenic E. coli K1 strain being incapable of inducing a high degree of bacteremia in neonatal rats. Discovery of HbiF-mediated OFF-to-ON fimS switching provides a new opportunity to develop a strategy for the prevention and therapy of extraintestinal E. coli infection including bacteremia and meningitis.

FIG. 1.

E. coli YX101 is a type 1 molecular switch fimS-locked-ON mutant. (A) In E. coli YX101 (an RSI 22 deletion derivative of E. coli RS218), genes in the fim operon were significantly induced compared to E. coli RS218 (values are means ± standard errors of the means). The mRNA levels of the fim gene cluster were profiled using an E. coli DNA microarray. (B) The induction of the fim operon in E. coli YX101 was due to OFF-to-ON orientation switching of fimS. The fimS gene was locked in ON orientation in this strain but phase variable in parental strain RS218.

FIG. 2.

The fimS-locked-ON phenotype in E. coli YX101 is due to induction of hbiF. (A) DNA microarray results showed that hbiF was significantly induced in E. coli YX101 compared to RS218 (white bars) but not in the engineered fimS-locked-ON mutant (shaded bars) (values are means ± standard errors of the means). Expression of fimB and fimE genes remained unchanged in both YX101 and fimS-ON. (B) Deletion of fimB and fimE in E. coli YX101 did not affect the locked-ON status of fimS. (C) Overexpression of hbiF in E. coli RS218 resulted in fimS switching from OFF to ON. (D) hbiF was responsible for the fimS-locked-ON phenotype in E. coli YX101.

FIG. 3.

HbiF inverts the molecular switch fimS independently of FimB and FimE. (A) The molecular switch fimS was locked OFF in E. coli YX107, where both fimBE and hbiF genes are deleted from the genome of YX101. (B) HbiF was able to switch fimS from OFF to ON and remains in locked-ON orientation in the absence of FimB and FimE, the two well-known fimS invertases.