Temperature, pH and bile dependent in vitro cultivation of Hexamita salmonis from rainbow trout Oncorhynchus mykiss intestine

The intestinal diplomonadid flagellate Hexamita salmonis was isolated from the intestinal contents of freshwater reared rainbow trout Oncorhynchus mykiss in Denmark and maintained in axenic cultures in the laboratory. A cell culture medium (MEM) or MEM supplemented with a bile solution (Keister's modified TYI-S-33) and penicillin, gentamicin and nystatin (mycostatin) was used as culture substrate. Temperature, pH and bile requirements for laboratory maintenance were investigated. Optimum temperature for H. salmonis population increase was determined to be 10 degrees C. Satisfactory although decreased growth was found at 5 degrees C. At 15 degrees C and 20 degrees C flagellate cells survived but did not exhibit significant population growth. In the pH range from 4 to 10 parasite survival was seen from 5.5 to 9.0, although optimum pH conditions for population increase were pH 7.5 to 8.0. Bile in low concentrations (30 to 960 mg l(-1)) enhanced flagellate propagation slightly. This study on the basic requirements explains the occurrence of the flagellate in rainbow trout in less warm seasons and biotopes as well as particular microhabitats in the host.