EFFECT OF STORAGE AT 5 C ON SURVIVAL OF VIBRIO PARAHAEMOLYTICUS IN PROCESSED MARYLAND OYSTERS (CRASSOSTREA VIRGINICA)1

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Abstract
Freshly processed Maryland oysters (Crassostrea virginica) were inoculated with various levels of Vibrio parahaemolyticus Strain 3525 (03:K30), Kanagawa negative, and Strain 8700 (04:K11), Kanagawa positive. Inoculated oysters were stored at 5 C for up to 13 days and numbers of viable cells determined at regular intervals by both a direct plating, method and a most probable number (MPN) method. The number of cells detected was dependent on strain, inoculum level, and method of enumeration. In general, the direct plating method was unreliable and results varied according to plating medium used. At an inoculum level of 106 cells/g, viable cells of Strain 3525 and Strain 8700 were not detected by the direct plating method after 3 and 5 days of storage, respectively, while by the MPN method low numbers of Strain 3525 and Strain 8700 were still detected after 7 and 13 days of storage, respectively. At inoculum levels of 104 and 102 cells/g, the direct plating method did not accurately enumerate viable cells. Neither strain was detected by the MPN method following 5 days of storage at these inoculum levels. Loss of viability of both strains occurred most rapidly within the first 24 h and in some instances was as great as 3 log cycles. In general, higher levels of survivors of Strain 8700 than Strain 3525 were noted throughout the study. The pH change of oysters during storage was slight and could not account for the loss of viability of either strain.

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