A Fish-pathogenic Mycobacterium sp. Isolated from an Epizootic of Cultured Yellowtail

Abstract

During the period from September 1985 to February 1986, an epizootic occurred in cultured yellowtail in Sukumo bay, Kochi Prefecture. The typical symptoms of the disease observed were haemorrhagic ascites, hypertrophy of the spleen and kidney with tubercles and visceral adhesions.
In this study, the morphological, biological and biochemical characteristics of the isolated strains were investigated. The strains were Gram-positive, non filamentous, non motile, strongly acid-fast and short rods measuring 0.3-0.4×0.8-1.5μm. Growth occurred from 15 to 33°C with the optimum from 24 to 28°C, at 0 to 2% salt concentration and at optimum pH of 6 to 7. Growth was not observed in 3 days of incubation under optimal conditions and totally negative at 37°C. The isolates produced niacin, catalase, hydrolysis of Tween 80 after 10 days was positive. Acid from mannose, hydrolysis of Tween 80 after 5 days, nitrate reduction, β-galactosidase, iron uptake, nitrate as nitrogen source were negative. The strains of pigment was produced only on exposure to light. On the basis of the above characteristics, the isolates were placed in the genus Mycobacterium Group I photochromogens of Runyon's classification. However, for the biochemical characteristics and particularly the optimal temperature for growth, it is not possible to include the isolates to any previously established species of the genus Mycobacterium.