Gliding motility and expression of motility-related genes in spreading and nonspreading colonies of Flavobacterium columnare

Gliding motility machinery enables moving on surfaces in many species among Bacteroidetes, resulting in bacterial colonies with spreading appearance. The adhesins required for gliding are secreted through a gliding motility –associated protein secretion system known as the type IX secretion system (T9SS). The fish pathogen Flavobacterium columnare produces spreading (Rhizoid, Rz; Soft, S) and nonspreading (Rough, R) colony types, of which only the spreading Rz type is virulent. In this study, we explored the spreading behaviour of these colony types by microscopic imaging and measured the expression of genes associated with gliding motility and T9SS (gldG, gldH, gldL, sprA, sprB, sprE, sprF, sprT and porV) under high and low resource levels. The spreading colony types responded to low resource level by increased colony size. The nonspreading colony type as well as the cells subjected to high nutrient level expressed only moderate cell movements. Yet, low nutrient level provoked more active gliding motility by individual cells and increased biofilm spreading by cooperative gliding. The gene expression survey demonstrated an increased expression level of sprA and sprF under low nutrient conditions. Surprisingly, the expression of gliding motility genes was not consistently associated with more active spreading behaviour. Our study demonstrates that environmental nutrient level is an important regulator of gliding motility and also the expression of some of the associated genes. Furthermore, our results may help to understand the connections between nutrient concentration, gliding motility and virulence of F. columnare.