The pathogenicity of bacteria associated with transport-stressed Chanos chanos fingerlings

Abstract

Two isolates of Aeromonas hydrophila biovar hydrophila and two isolates of Pseudomonas-like bacteria cultured from milkfish, Chanos chanos, fingerlings stocked in Laguna Lake(Philippines) pens after transport were tested for virulence against healthy milkfish fingerlings. Various combination of bacterial and different routes of inoculation (intraperitoneal injection, bath treatment of scaled fish and bath treatment of unscaled fish) were tested. Results show that bacterial entry in the pathogenesis of the test bacteria to challenged fish is more effective in fishes with scales removed than in uninjured fish or though intraperitoneal injection. The minimum lethal dose of A. hydrophila to scaled fish appeared less than 10⁵ cells/ml of water medium. For Pseudomons-like isolates, the minimum lethal dose was at the level 10⁵ cells/ml of water medium. Fish mortalities significantly increased on day 2 in all bacteria but A. hydrophila was significantly more virulent than the Pseudomonas-like inocula. When A. hydrophilawas injected intraperitoneally into undamaged fish, the maximum dose of 10⁷cells per 2-g fish was needed to cause significant moralities. In general, virulence of the isolates was directly proportional to dose as well as to the length of exposure. A cute signs of bacterial infected scaled milkfish. Lethargic fish displayed spastic and unbalanced swimming movement before death. The virulence of the bacteria as evaluated in this study should lead to further investigation on the role of extracellular factors in bacterial pathogenesis to milkfish fingerling.