Graduate Thesis Or Dissertation
 

Characterization of a new Lactobacillus species infectious for salmonid fish

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  • A new Lactobacillus species is proposed to include 17 strains of a bacterium from rainbow (Salmo gairdneri) and cutthroat trout (Salmo clarki), and chinook salmon (Oncorhynchus tshawytscha). The bacterium was isolated most frequently from fish stressed by handling or spawn ing. Phenotypically, the strains belong to the family Lactobacillaceae and can be distinguished from other species of Lactobacil1 us by their morphology and physiological characteristics. Growth occurred between 6 and 40°C. DL-lactic acid was produced homofermentatively from glucose. Diaminopimelic acid was present in the cell wall peptidoglycan. The 17 isolates were closely related gene tically, as demonstrated by similar percent guanine plus cytosine content (35 mol%) and high DNA reassociation values, both characteristics of a single species. Isolates exhibited less than 10% DNA reassoci ati on to other reference Lactobacillus strains with similar mol% G+C. The bacterium was pathogenic for rainbow trout maintained at 18 or 21°C. A 73 % mortality after 18 days was achieved at each water temperature after the fish were injected intraperitoneally with 3.3 x 10⁸ cells. The mean time from infection to death was 4 days for fish held at 18°C and 3 days for fish held at 21°C. The 50% lethal dose (LD50) for rainbow trout injected intraperitoneally and maintained in 18°C water was 5.4 x 10⁷. Histopathological examination of experimentally in fected fish showed that the bacterium was localized in the visceral adipose tissue, the peritoneal membrane, the loose connective tissue surrounding the liver, esophagus and spleen, and in the cardiac muscle and muscularis of the esophagus. Necrosis of the spleen, liver, or kidney was not observed. Gross pathology included ascites fluid accumulation and hemorrhaging of the visceral adipose tissue and lower intestine. The isolates from fish best match the description of the genus Lactobacillus but differ from other members of the genus genetically, in their fermentation patterns, and by their association with disease in fish and should be recognized as a new species for which the name Lactobacil! us piscicol us is proposed. Strain B270, isolated from diseased cutthroat trout at Bandon Hatchery, Coos County, Oregon, U.S.A., is designated the type strain.
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