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Strains of the fish pathogen Enterococcus seriolicida were identified as agglutinating and non-agglutinating, according to their reaction with anti-serum raised against type strain YT-3 (ATCC49156). The non-agglutinating strains are highly pathogenic in contrast to agglutinating strains. A 96 kDa immunoprotective glycoprotein G1 antigen from non-agglutinating Ent. seriolicida strain SS91-014 (N) was purified and characterized. The purification procedure entailed extraction of antigen by glass bead agitation, 80% (NH4)(2)SO4 precipitation, gel filtration and electroelution. An immunofluorescence microscopy study using monoclonal antibody M3A5 raised against G1 antigen revealed that G1 antigen is present only on the cell surface of non-agglutinating strains. Therefore, the G1 antigen of virulent Ent. seriolicida could be a potential candidate for protective vaccine against enterococcosis in fish.

The monoclonal antibody YeT-2, generated in mice hyper-immunized with thymic lymphocytes of the yellowtail, Seriola quinqueradiata, reacts with the major population of peripheral blood lymphocytes, which might be putative T cells. In this study, we examined the cross-reactivity of YeT-2 with lymphocytes from various fish species. Flow cytometric analysis showed that YeT-2 reacts with 69.8% lymphocytes in the thymus, 89.7% in the peripheral blood, 87.5% in the spleen, and 59.7% in the head-kidney. Among the six fish species examined, only the red sea bream, Pagrus major, which is included in the same suborder Percoidei with the yellowtail, showed the presence of YeT-2 positive cells. Electron microscopic studies revealed that YeT-2 positive cells in the peripheral blood of the red sea bream were lymphocytes or unidentified leucocytes. Thymic lymphocytes of the red sea bream were also immunocytochemically stained with YeT-2. The molecular weight of the YeT-2 cross-reacting antigen on blood cells from the red sea bream was identical with that from the yellowtail, which was identified at approximately 115 kDa. These results suggest that the monoclonal antibody YeT-2 recognizes a conserved antigen on lymphocytes common to the red sea bream and yellowtail.

The properties and taxonomic position of bacterial strains isolated from diseased specimens of cultured yellowtail and eels were examined. The isolates were gram-positive, short-chain-forming, catalase-negative, facultatively anaerobic cocci. Growth at 10 and 45 degrees C in 6.5% NaCl (pH 9.6) with 40% bile and in 0.1% methylene blue-milk were both positive. The isolates could be distinguished from other species of the genus Enterococcus by several biochemical characteristics and by Lancefield's group antigen. Guanine-plus-cytosine content of DNA was 44 mol% as determined by the thermal melting temperature. The value for DNA-DNA hybridization was sufficiently low to warrant distinguishing this species from reported. Enterococcus species. The name Enterococcus seriolicida is proposed. The type strain is YT-3 (=ATCC 49156).

A previous paper has revealed that experimental streptococcal infection was associated with a rapid growth of the intestinal bacteria, suggesting association of the condition with an exotoxin. The present study was undertaken to see the effect of the exotoxin on the streptococcal infection by administering cell free culture before the bacterial challenge, Cell free culture of Streptococcus sp. strain YT-3 was inoculated intramuscularly at a dosage of 0.5 ml per 100 g body weight 1 h prior to bacterial challenge. Three fish were killed at intervals after challenge for viable counting of bacteria in the viscera and blood. Intramuscular inoculation with either low virulent or virulent bacteria alone at dosages of 106 to 107 cells did not produce the disease in the fish, with almost complete clearance of the bacteria from the viscera and blood within 120 h. When the exotoxin was inoculated intramuscularly prior to the bacterial challenge, however, either low virulent or virulent bacteria at 106 to 107 cells could produce fatal infection with prominent streptococcal clinical signs. [ABSTRACT FROM AUTHOR]