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A cell capsule was revealed by transmission electron microscopy on the surface of non-agglutinating (KG-), but not on agglutinating (KG+), cells of the bacterial fish pathogen Enterococcus serioliada. The capsule of KGcells may be involved in resistance to opsonophagocytosis by head kidney phagocytes of yellowtall Seriola quinqueradiata.

Enterococcus s e n o h c~d a is the causative agent of an enterococcal Infection in yellowtail Seriola q~llnqueradlata and IS antigen~cally classified into 2 phenotypes KG+ and KG-Phenotypic variation from K G to KG+ occurs readily on an artihcial medium The surface morphoIogies of the K G and KG+ phenotypes were differentiated by scanning electron microscopy KG-cells were more hydroph~lic than KG+ cells and were resistant to phagocytos~s by yellowtail head kidney phagocytes The c h e m~l u m~n e s c e n t response of these phagocytic cells was lower with the KG-phenotype than with the KG+ phenotype The immune responses of yello.ruta11 f o l l o~~i n g injection of the 2 phenotypes differed h~g h e r agglutinating titers were o b t a~n e d with the KG+ phenotype compared to the KG-phenotype Thus, the cell surface may function as an anti-phagocytic factor in E seriolic~da and apparently affects ~t s immunogeniclty KEY WORDS. Enterococcus senol~cida . Seriola quinqueradlata . Phagocytosis . Chemiluminescence . lmmune response

Abstract. The efficacy of florfenicol against laboratory‐induced infection with Aeromonas salmonicida was tested in Atlantic salmon, Salmo salar L., parr. Medication at three dose levels in the feed was started 24 h after bath challenge with A. salmonicida. The specific mortality rate in the unprotected infected control group was 75% compared with 5, 13 and 17% when florfenicol was given at dose levels of 20, 10 and 5 mg per kg body weight per day, respectively. Florfenicol was palatable to the fish at doses in excess of effective therapeutic levels and feeding for 10 days at 100 mg per kg, or for prolonged periods at 50 mg per kg and 10 mg per kg, resulted in no feeding problems or histopathological abnormalities. Florfenicol appears to have a good therapeutic index and considerable potential in the control of furunculosis in salmon.