Vibrio sp. causing Porites ulcerative white spot disease

Abstract

The causative agent of the Indo-Pacific coral disease, Porites ulcerative white spot syndrome (PUWS), that affects Porites spp. and a few other coral genera has so far remained unidentified. Inoculation of thiosulphate citrate bile sucrose (TCBS) agar with tissue material from Porites cylindrica infected with white spot produced colonies of approximately 3 mm diameter consisting of Gram-negative, motile, non-sucrose-fermenting, slightly curved rods with a minimum NaCl requirement of 0.3%. Three of these putative Vibrio sp. isolates were used for infection trials that included different stages of cell growth. Four modes of inoculation and 3 stages of bacterial cell growth were considered for testing Koch's postulates. Stationary phase cells proved more consistently infectious than did exponentially growing or starved cells using a 1-step immersion technique at cell concentrations of 10(4) cells ml(-1). A 1-step immersion technique proved more reliable in producing signs of white spot than did other techniques, such as injection, smearing and 2-step immersion of the inoculum. At inoculum densities >10(4) cells ml(-1) further signs of disease, such as tissue degradation and bleaching, also became evident. At elevated temperatures (>29 degrees C) bleaching remained absent for at least 2 mo from non-inoculated corals serving as controls, but was observed in artificially infected coral fragments. Of the 9 seawater aquaria containing healthy specimens of P. cylindrica, 6 showed signs of white spot 15 d after infection with an isolate tentatively identified as Vibrio sp. Based on 99% similarity of its 16S rRNA gene sequence and selected phenotypical features, this isolate revealed a close relationship to V. natriegens and V. parahaemolyticus.