Streptococcus iniae, a bacterial infection in barramundi Lates calcarifer

The cause of ongoing mortality in barramundi Lates calcarifer (Bloch) in se awater culture was identified as Streptococcus iniae by biochemical and phy siological tests. This is the first published record of this bacterial spec ies in Australia and the first confirmed report of S. iniae causing mortali ty in barramundi. The bacterium was highly pathogenic for barramundi when c hallenged by bath exposure. The pathogen was found to have a LD50 Of 2.5 x 10(5) and 3.2 x 10(4) colony-forming units at 48 h and 10 d respectively. E xperimental challenge of barramundi resulted in high levels of mortality (> 40%) within a 48 h period. Ten days after the challenge, S, iniae could not be isolated from kidney, spleen, liver or eye of surviving fish. However, the organism was easily isolated from the brain of both moribund and health y fish, indicating that barramundi can carry the bacterium asymptomatically .