Are bacterial virulence factors involved in initial host colonization processes responsible for host specificity?
Résumé
Bacteria belonging to the genus Xanthomonas are grouped in pathovars defined on the basis of their host range and type of symptoms. Within a pathovar strains are highly specialized on a restricted host range. Currently, the genetic basis of host specificity for pathogenic bacteria remains poorly understood. We hypothesize that virulence factors which are involved in the initial stages of host colonization play a role in host specificity. Candidate host specificity factors we selected are methyl accepting chemotaxis proteins (MCPs) and adhesins. Actually, nothing is known about the repertoires of MCPs and adhesins in strains of the genus Xanthomonas. We have determined the distribution of 30 MCPs and 13 adhesins among 180 strains belonging to 18 pathovars of X. axonopodis and 3 pathovars of X. campestris, as well as phylogenetic relationships of the strains based on polymorphism analyses of housekeeping genes. The selected strains were isolated from different host plants and various geographical origins. Some pathovars were polyphyletic while others were monophyletic. Repertoires of MCPs and adhesins were polymorphic among strains and displayed both ubiquitous and variable genes. Among the pathovars and genetic lineages tested, thirteen were distinguished by their distinct suites of MCPs and adhesins. The other pathovars and genetic lineages remained difficult to differentiate: five groups with unique repertoires were constituted. Polyphyletic pathovars displayed essentially homogeneous repertoires. These results show that events leading to host specificity occur as early as initial steps of chemotactic attraction and adhesion on host tissues.