Abstract
Gliding motility facilitates the movement of bacteria along surfaces in many Bacteroidetes species and results in spreading colonies. The adhesins required for the 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 non-spreading (rough [R]) colony types, of which only the spreading Rz type is virulent. In this study, we explored the spreading behavior 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 by using RT-qPCR (reverse transcription quantitative PCR). The spreading colony types responded to the low resource level with increased colony size. The non-spreading colony type, as well as the cells growing under high nutrient level expressed only moderate cell movements. Yet, a low nutrient level provoked more active gliding motility in individual cells and increased spreading by cooperative gliding. The gene expression survey demonstrated an increased expression level of sprA (a core component of T9SS) and sprF (needed for adhesin secretion) under low nutrient conditions. Surprisingly, the expression of gliding motility genes was not consistently associated with more active spreading behavior. Furthermore, no genetic differences were found between spreading and non-spreading colony types in the studied genes associated with gliding motility. Our study demonstrates that environmental nutrient level is an important regulator of both gliding motility and the expression of some of the associated genes. These results may help to understand the connections between nutrient concentration, gliding motility, and virulence of F. columnare.
Highlights
Gliding motility is a process of bacterial movement on surfaces exploited by several bacterial species in the phylum Bacteroidetes (McBride and Zhu, 2013)
The morphology of bacterial colonies was assayed on 0.5xN, 1x and 2xN Shieh plates, as these nutrient levels have previously been shown to be useful for exemplifying gliding motility in F. columnare (Laanto et al, 2012)
The bacteria originating from the same liquid culture were spread on agar plates and grown for 2 days after which the colony morphology was imaged under a light microscope (Figure 1)
Summary
Gliding motility is a process of bacterial movement on surfaces exploited by several bacterial species in the phylum Bacteroidetes (McBride and Zhu, 2013). Number of studies of flavobacterial gliding motility have led to the identification of several genes involved in motility, including gldA, gldB, gldD, gldF, gldG, gldH, gldI, gldJ, gldK, gldL, gldM, gldN, sprA, sprB, sprE, and sprT (Agarwal et al, 1997; Hunnicutt and McBride, 2000, 2001; Hunnicutt et al, 2002; McBride et al, 2003; McBride and Braun, 2004; Braun and McBride, 2005; Nelson et al, 2007, 2008; Sato et al, 2010; Rhodes et al, 2010, 2011b; Kharade and McBride, 2015). A subset of these genes, gldK, gldL, gldM, gldN, sprA, sprE, and sprT, has been found to compose a protein translocation system, designated the type IX secretion system (T9SS) (Sato et al, 2010; McBride and Zhu, 2013). The periodontal pathogen Porphyromonas gingivalis uses T9SS for secretion of its major virulence factors, which are gingipains and hemagglutinins (Sato et al, 2010; Shoji et al, 2011)
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