Characterisation of the marine bacterium Phaeobacter inhibens: diversification, competition and surface colonisation

In the marine environment microbial communities establish close association with living surfaces forming biofilms analogous to a “second skin” believed to protect the host from biofouling and disease. Complex interactions occur on marine surfaces including competition between microorganisms for space and nutrients. Biofilm development occurs in a cycle and the cycle is complete when dispersal cells leave the biofilm community. It has been observed that biofilm dispersal cells display stable and heritable phenotypic variation distinct from the parental biofilm. Phaeobacter inhibens is an effective coloniser of marine surfaces, in which a subpopulation of its biofilm dispersal cells displays a non-competitive phenotype. The overall aim of this thesis was to characterise non-competitive biofilm dispersal variants of P. inhibens, to elucidate the genetic basis of these phenotypic changes and to determine impact of such phenotypic variation on the ability of P. inhibens to influence microbial community establishment on a natural host surface. Despite the progress made to date in characterising the dispersal variants in P. inhibens and other organisms, little is understood about the underlying genetic changes that result in the development of the specific P. inhibens phenotypic variants. Here, P. inhibens phenotypic variation was linked to single nucleotide polymorphisms (SNPs), in particular in genes affecting the competitive ability of P. inhibens, including genes related to the production of the antibiotic tropodithietic acid (TDA) and bacterial cell-cell communication (e.g. quorum sensing). We hypothesise that P. inhibens plays an important role in establishing and maintaining algal microbiomes. 16S rRNA gene sequencing from of T. rotula-attached and the surrounding microbial communities demonstrated they are distinct and host-attached communities were comprised predominately of members of the Proteobacteria, Bacteroidetes and Cyanobacteria. Communities that established in the presence of P. inhibens were distinctly different from the control community and this difference was consistent over time. Interestingly, while being overall less competitive in laboratory experiments the P. inhibens non-competitive variant (NCV12a1), the TDA deficient mutant (ΔtdaA) and the quorum sensing mutant (ΔpgaR) were not totally out-competed by other members of the seawater community. Sequencing of the microbial communities in the presence of P. inhibens WT or either of these mutants revealed minor differences between the communities that established in the presence of either one of these strains. This study highlights that specific microbial communities develop on the host with time and these communities may be influenced by the presence of specific bacterial strains (i.e. P. inhibens). Also, this study suggests that factors other than direct antagonism might be contributing to the success of P. inhibens in the natural environment