Abstract
There is increasing evidence to suggest that macroalgae (seaweeds) are susceptible to infectious disease. However, to date, little is known about the mechanisms that facilitate the colonization and virulence of microbial seaweed pathogens. One well-described example of a seaweed disease is the bleaching of the red alga Delisea pulchra, which can be caused by the bacterium Nautella italica R11, a member of the Roseobacter clade. This pathogen contains a unique luxR-type gene, varR, which we hypothesize controls its colonization and virulence. We show here that a varR knock-out strain is deficient in its ability to cause disease in D. pulchra and is defective in biofilm formation and attachment to a common algal polysaccharide. Moreover complementation of the varR gene in trans can restore these functions to the wild type levels. Proteomic analysis of bacterial cells in planktonic and biofilm growth highlight the potential importance of nitrogen scavenging, mobilization of energy reserves, and stress resistance in the biofilm lifestyle of N. italica R11. Moreover, we show that VarR regulates the expression of a specific subset of biofilm-associated proteins. Taken together these data suggest that VarR controls colonization and persistence of N. italica R11 on the surface of a macroalgal host and that it is an important regulator of virulence.
Highlights
Macroalgae are major habitat formers and contribute to the primary production in temperate marine ecosystems (Jones et al, 1994; Phillips, 2001; Smale et al, 2011)
In order to investigate the role of varR in the virulence of the macroalgal pathogen N. italica R11, an allelic replacement mutant was generated and subsequently tested in an in vivo infection assay for D. pulchra
Previous reports have proposed that colonization is an important virulence factor for N. italica R11 (Case et al, 2011), and given the impaired pathogenicity exhibited by N. italica R11 varR, we investigated surface attachment and biofilm formation in this strain
Summary
Macroalgae are major habitat formers and contribute to the primary production in temperate marine ecosystems (Jones et al, 1994; Phillips, 2001; Smale et al, 2011). One of the best-studied models for disease in macroalgae is the bacterial-induced bleaching of the red alga Delisea pulchra (Campbell et al, 2011; Case et al, 2011; Fernandes et al, 2011; Gardiner et al, 2015), which has significant negative consequences for the health and fecundity of the algal population (Campbell et al, 2014). LSS9, have been identified to induce the bleaching disease in vivo and in vitro (Campbell et al, 2011; Fernandes et al, 2011), and for N. italica R11 the infection process was shown to be temperature dependent (Case et al, 2011). The protein encoded by N. italica R11 varR (EEB72782) possesses the autoinducer-binding (pfam03472) and transcriptional-activator (pfam00196) domains characteristic of LuxR-type response regulators (Fernandes et al, 2011)
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