Abstract
Vibrio spp. form a part of the microbiome of the highly abundant marine mesozooplankton termed copepods. The tendency of Vibrio to associate with copepods has been linked to transmission of cholera, and several Vibrio species cause disease through associations with marine macroinvertebrates and finfish which may use copepods as a food source. The biological mechanisms of the Vibrio-copepod association are largely unknown. The aims of this thesis were to: 1) compare biofilm formation of Vibrio isolated from copepods and closely related Vibrio originating from seawater, 2) visualize the attachment of Vibrio on live copepods and dead particles, and 3) investigate growth of Vibrio on live copepods. Two strain types of V. cyclitrophicus were studied that are separated by small genomic differences in biofilm formation loci. Two ‘L-strains’ were isolated from copepods and two ‘S-strains’ were included that originated from seawater. It was expected that the L-strains would have a superior biofilm formation capacity, but in biofilm assays developed in this study, L- and S-strains formed similar biofilms in the presence of all sea salts, irrespective of carbon source, suggesting previously undescribed biofilm mechanisms are present in the S-strains. Calcium promoted biofilm formation and resulted in substantially denser biofilms in the L-strains, suggesting regulation of the pilus or polysaccharide pathways present in the L- but not in the S-strains. Calcium regulation may be important for colonization of shellfish by V. cyclitrophicus and other Vibrio spp. Colonization of Vibrio on Acartia tonsa copepods was observed by generating Green Fluorescent Protein -expressing strains and visualizing and quantifying attachment with fluorescence microscopy. Cells tended to localize on the antennae of live copepods, suggesting that vortices generated by animal movement and feeding promote bacterial attachment. L-strains attached to copepods at significantly higher numbers than the S-strains, suggesting that biofilms of L-strains may be better at withstanding shear. The data also demonstrate that V. cyclitrophicus cells grow epibiotically on live copepods after attachment, indicating that copepods can both selectively recruit and sustain Vibrio growth. This study provides information about the complex biological mechanisms underlying the Vibrio association with marine zooplankton and particles and elucidates the V. cyclitrophicusecology. The V. cyclitrophicus – A. tonsa model used here could be useful in future studies on Vibrio-invertebrate associations.