Pilus-mediated adhesion of Neisseria meningitidis is negatively controlled by the pilus retraction machinery

The type IV pili (Tfp) of Neisseria meningitidis play an essential role in meningococcal virulence by mediating the initial interaction of bacteria with host cells. Tfp are also subject to retraction, which relies on the PilT protein. Among the other components of the Tfp machinery, PilC1, a pilus-associated protein, is important for Tfp biogenesis and adhesion. Adhesion of N. meningitidis to living epithelial cells was previously shown to rely on the upregulation of the pilC1 gene. On the other hand the lack of induction of pilC1 is believed to be responsible for the low adhesion of N. meningitidis onto fixed dead cells. Surprisingly, a pilT mutant, unable to retract its pili, has been shown to adhere very efficiently onto both living and fixed epithelial cells. To elucidate the mechanisms by which the pilus retraction machinery mediates meningococcal adhesion onto fixed cells, an analysis of gene expression levels in wild-type and pilT meningococci was performed using DNA microarrays. One of the upregulated genes in the pilT strain was pilC1. This result was confirmed using quantitative real-time reverse-transcription polymerase chain reaction (RT-PCR) and immunoblot analysis. The transcription starting point responsible for the upregulation of pilC1 in a pilT background was shown to be different from those controlling the induction of pilC1 upon contact with living host cells. Subsequent work using a strain hyperproducing PilT confirmed that PilT downregulates the production of PilC1. Furthermore using a pilC1 allele under the control of IPTG, we demonstrated that the upregulation of pilC1 in a pilT background was responsible for the adhesive phenotype onto fixed dead cells. Taken together our results demonstrate that the pilus retraction machinery negatively controlled the adhesiveness of the Tfp via the expression of pilC1