This study was designed to determine the survival of Vibrio parahaemolyticus in salted oysters as prepared in Korea. Three levels of salt concentration were included; 3.0%, 6.8%, and 10.6% of the weight of the raw oysters. Two methods were used to inoculate the products, surface-inoculation and injection into the oysters, which were then held at 18°C. Multiplication of V. parahaemolyticus (Japanese strain T-3765-1) did not occur in the salted oysters, except for a slight increase in numbers after 24 hours from surface-inoculated samples with 3.0% salt and all samples of injected oysters. The samples with the higher salt concentrations showed longer survival of the organisms. Numbers of survivors in surface-inoculated oysters were markedly reduced within three days; the samples of injected oysters did not show reduced numbers in this period. There were more survivors from the injected samples over the storage period than from the surface-inoculated samples. No viable cells were observed after seven days from the surface-inoculated samples with 3.0% salt and the injected samples with 3.0% salt, after eight days from the surface-inoculated samples with 6.8% salt, after nine days from the injected samples with 6.8% salt, and after 11 days from the injected samples with 10.6% salt. A small number of V. parahaemolyticus were recovered from the surface-inoculated samples with 10.6% salt after eight days, the last testing period for these samples. The results of pH measurement suggested that the samples with higher salt concentration may have prolonged survival of V. parahaemolyticus through the effect of the higher pH value. An analysis of variance revealed a significant difference (5% level) in recovery between a modified isolation medium (Twedt and Novelli) and thiosulfate-citrate-bile salts-sucrose agar. Salt concentrations of 3.0%, 6.8%, and 10.6% slightly reduced the total bacterial counts after three days from the two lots of oysters used for the surfaceinoculated samples and further reduction was observed after seven days; reduction in numbers was not detected within 8 or 11 days from the two lots used for the injected samples. The latter two lots had much lower plate counts at the beginning of the experiment. No relationship was found between pH and total bacterial counts on the salted oysters. / Graduation date: 1973

Aerobic heterotrophic bacteria associated with oyster larvae feed algae Isochrysis galbana, Monochrysis lutheri and Pseudoisochrysis paradoxa were isolated and enumerated. The bacterial numbers ranged from 7.8x10³ to 3.9x10⁶ CFU per ml. The bacteria associated with Pseudoisochrysis sp. were identified and the majority of isolates belonged to genera Leucothrix (51%). Also present were members of Pseudomonas III sp. (19.1%), atypical Moraxella sp. (16.8%), Moraxella sp. (7.2%) and Flavobacterium sp. (5.9%). The growth of bacteria on marine agar was fastidious and took four days to form visible colonies. None of the bacterial isolates grew in buffered salt broth in which the algae had been grown. Marine broth supplemented with 0.1% beef extract best supported the growth of the isolates, while ferric citrate (3x10⁻⁴ M) supported their growth in buffered salt broth. The role of algae as a solid support for bacteria was investigated by studying the attachment of bacteria on glass slides suspended in the growth medium. The percent of Leucothrix sp. attached to the slide was 2.1-3.0%. This was four to six times greater than that of Staphylococcus (0.5%), an organism well known for its commensal growth on skin and mucus membranes of man and animal. Hydrophobic attraction, which is thought to play an important role in the orientation of bacteria to solid surfaces, was studied by measuring the adsorption of bacteria on hydrocarbons introduced to a bacterial suspension. Leucothrix sp. exhibited the strongest affinity, whereas Flavobacterium sp. and Pseudomonas sp. adsorbed the least. An attempt to obtain an axenic culture of algae by antibiotic treatment was unsuccessful due to the detrimental effect of antibiotics to both algae and bacteria. / Graduation date: 1982

The microbial composition of Pacific oysters (Crassostrea gigas) subjected to UV-treated seawater was determined by quantitative and qualitative means. A total of 2,779 microorganisms were identified from seawater and oysters during a 72 hour sampling period employing a computer assisted replica-plating technique. UV treatment effectively eliminated coliforms and Pseudomonas Type I from seawater but other gram-negative asporogenous rods were more resistant. The microogranisms commonly found in oysters, whether subjected to UV-treated seawater or not, were, in the order of predominance, Pseudomonas Type III or IV, Vibrio/Pseudomonas Type II, Flavobacterium/ Cytophaga and Acinetobacter /Moraxella. The composition of microbial flora in oysters remained relatively stable irrespective of the microorganisms present in the seawater. A total of 18 presumptive hemolytic vibrios were found in oysters but further confirmation revealed two isolates to be Vibrio parahaemolyticas and the remainder Aeromonas species. Approximately 10 percent of the microorganisms isolated from seawater and oysters were gram-positive cocci and 14 to 23 percent of these were coagulase positive, DNase positive, and (β-hemolytic on human blood agar. / Graduation date: 1971