Bile-induced 'pili' in Campylobacter jejuni are bacteria-independent artifacts of the culture medium

In 1996, it was reported that the enteric pathogen Campylobacter jejuni pro duces pilus-like appendages in response to bile salts such as deoxycholate (DOC), and that the formation of these appendages requires the putative pep tidase PspA. Pili were known to be important virulence determinants in othe r pathogenic bacteria but had never before been observed for C. jejuni. We report here that these appendages are not pili, but are instead a bacteria- independent morphological artifact of the growth medium. Furthermore, the p spA gene is not required for their formation. Broth cultures containing a t hreshold concentration of DOC inoculated with no bacteria produced identica l abundant, fibrous, pilus-like structures as those cultures that had been inoculated with C. jejuni. These fibres were also found in growth media fro m DOC-containing pspA::Cm-R mutant cultures. These results are consistent w ith the absence of candidate pilin monomers in protein gel analyses as well as the dearth of pilin-like genes and pilus formation gene clusters in the C. jejuni genome.