Some characteristics of Photobacterium damselae ssp. piscicida isolated in Israel during outbreaks of pasteurellosis in hybrid bass (Morone saxatilis x M. chrysops)

Hybrid bass [Marone saxatilis (Walbaum) striped bass x M. chrysops (Rafinesque) white bass] was introduced into Israel in 1993. Shortly after introduction, a disease outbreak caused widespread mortality. Bacteria from moribund hybrid bass, usually taken from the spleen, were isolated and characterized. They showed white tubercles of 1-2 mm. The pathogen, which was isolated and cultured, was identified as Photobacterium damselae subsp. piscicida (Ph.d.p.). The lethal dose for 50% of the fish population (the LD₅₀) under experimental conditions was determined as 1.3 × 10⁵ bacteria /g fish. The optimum water temperature for experimental infection with pasteurellosis in hybrid bass was 24°C. No mortality occurred at 15-16°C or 30°C. The toxicity of the bacterial extracellular products (ECPs) was assessed, and no mortality was noted in fish weighing 50 g after injection with ECPs, with protein concentration of 18.5 μg /g fish. The minimum inhibitory concentration (MIC) of tetracycline against three isolates taken from the same site over a period of three and a half years increased from 0.016 mg /l (the first isolate, Jan 1995) to the last isolate, three and a half years later (July 1998), when it was resistant to tetracycline at the highest concentration used.