PHENOTYPIC AND PATHOBIOLOGICAL PROPERTIES OF CORYNEBACTERIUM-AQUATICUM ISOLATED FROM DISEASED STRIPED BASS

A Corynebacterium strain was isolated from the brain tissues of 3 yr old striped bass Morone saxatilis with pronounced bilateral exophthalmia that were being held in experimental aquaculture facilities. Taxonomic analysis, using conventional tests and the API-Coryne system, allowed us to identify the striped bass strain (RB 968 BA) as Corynebacterium aquaticum. An agglutination assay demonstrated that the fish isolate was serologically related to the reference strain of C. aquaticum (ATCC 14665) originally isolated from water. Although both strains exhibited different surface protein patterns, immunoblot analysis revealed they shared a protein antigen of 68 kDa. Only the C aquaticum from striped bass proved to be pathogenic for fish (LD50 Values of 1.0 x 10(5) and 5.8 x 10(4) for striped bass and rainbow trout Oncorhynchus mykiss respectively) and mice (LD50 = 8.3 x 10(4)). Similarly, the extracellular products of this strain (but not of several other corynebacteria species also tested) possessed proteolytic activity, elicited a cytotoxic response in both fish and homoiothermic cell lines, showed dermonecrotic activity in rabbits, and contained exotoxins lethal for fish (LD50 = 1.70 mug protein g-1 fish). All these biological activities, exhibited in vitro and in vivo, were concomitantly lost after heating (100-degrees-C for 10 min). Histopathological examination showed lesions in the internal organs of experimentally infected fish. The brain and eyes were the most affected organs and showed acute hemorrhages. Personnel involved in culturing fish should be made aware of the pathogenic capability exhibited by C aquaticum for mammals.