Date of Award

2015

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Specialization

Cell and Molecular Biology (CMB)

Department

Cell & Molecular Biology

First Advisor

David R. Nelson

Abstract

Aliiroseovarius crassostreae, the causative agent of Roseovarius Oyster Disease, is a marine α-Proteobacterium and a member of the Roseobacter clade. Roseovarius Oyster Disease, formerly known as Juvenile Oyster Disease, has been the reason for high mortality rates in hatchery-raised eastern oysters (Crassostreae virginica) since the late 1980s. Juvenile oysters less than 25 mm in shell length are more heavily impacted by the disease than large adult oysters. Because mortality rates can exceed 90%, the disease is responsible for large economic losses to the New England aquaculture industry. The probiotic organism, Phaeobacter gallaeciensis is a marine α-Proteobacterium and like A. crassostreae a member of the Roseobacter clade. Oyster larvae that are pretreated with P. gallaeciensis show significantly reduced mortality rates in challenge experiments with A. crassostreae. The goal of this study was to elucidate the physiological responses of A. crassostreae and P. gallaeciensis to oyster pallial fluid and identify putative virulence factors via genome analysis of the pathogen. A. crassostreae and P. gallaeciensis grew rapidly in oyster pallial fluid. When growth medium was supplemented with oyster pallial fluid biofilm formation by A. crassostreae was significantly increased. Both organisms were chemoattracted to pallial fluid and a molecule >10 kDa seems to be responsible for this positive chemotactic response. These results suggest that oyster pallial fluid is likely to contribute to the initial colonization of A. crassostreae in the oyster in three ways – by promoting positive chemotaxis, growth, and biofilm formation. Genome analysis of A. crassostreae revealed multiple putative virulence genes including cytolysins, RTX toxin and related Ca2+-binding proteins, and a serralysin peptidase. In addition, the genome encodes pilus/fimbriae biogenesis machinery and other proteins that appear to facilitate surface attachment.

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