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Colonization of Oysters by Two Distinct Genotypes of Vibro Vulnificus


Of all deaths resulting from seafood consumption, 95% are caused by V. vulnificus. We developed a PCR-based assay that separates this species into two groups that correlate with isolation source (clinical vs environmental). We typed 646 V. vulnificus isolates from 75 oysters and found only 13.3% were the C-genotype, whereas 86.7% were the E-genotype. In contrast, 50.4% of isolates from surrounding waters were the C-genotype and 49.6% were the E-genotype. Our studies indicate that 1) the C-genotype is in relatively low levels in oysters compared to the non-pathogenic E-genotype, helping to explain the lower than expected disease frequency, 2) an occasional oyster may have a disproportionate number of C strains, also relevant to disease production, 3) the incidence of C strains in oysters increases in the summer months, and 4) the relative levels of the two genotypes in oysters is not reflected by their levels in surrounding waters. We found that two genotypes differ significantly physiologically, which likely accounts for why the C-genotype is the form seen in human infections whereas the E-genotype is found primarily in the environment. Clearly, the C- and E-genotypes of V. vulnificus differ significantly in their ecology, physiology, and molecular genetic responses to stress. Within such differences lie also the reasons why the C-genotype is the form primarily seen in human infections, whereas the E-genotype is found primarily in the environment. We intend to investigate these differences in an attempt to understand the physiological and molecular basis for the selective advantage of the E-genotype in oysters.

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NON-TECHNICAL SUMMARY: Vibrio vulnificus causes 95% of all seafood-borne food infections. Two genetic types of this pathogen exist, a C-genotype which is the form which causes human disease, and an E-genotype which appears relatively unable to cause disease. We have found the E-genotype to predominate in oysters, even though the numbers of the C-genotype equal that of the E-genotype in surrounding waters. This project will investigate a variety of the physiological and molecular aspects of the two genetic types of this pathogen, in an attempt to understand why the infective (C-genotype) is selected against in oysters. If the mechanisms can be elucidated, it might be possible to manipulate the oyster's environment to dramatically reduce the levels of the disease-causing form in oysters.


APPROACH: The proposed studies will be conducted by two oyster physiologists/molecular geneticists and a microbiologist with 30 years experience studying V. vulnificus. Thus, we bring together a unique collaboration of three experienced researchers to conduct studies not otherwise possible. Specifically, we wish to pursue the observations described above to determine what genetic and physiological factors of V. vulnificus determine the distributional differences observed with this pathogen, which genes might be expected to contribute to these virulence and survival differences, and what aspects of the host oysters may be involved in regulating the uptake and/or survival of V. vulnificus. Our specific objectives are: 1. Why is the C-genotype in relatively low levels in oysters compared to the E-genotype? 2. What causes an occasional oyster to have a high number of C-genotype type strains? 3. Why are the relative levels of the two genotypes in oysters not reflected in the levels in surrounding waters? 4. What genes differ, in existence or expression, between the two genotypes? We are especially interested in those genes that would be expected to contribute to virulence or to differential survival within oysters.

Oliver, James
University of North Carolina
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