Virulence of Vibrio isolated from a mussel hatchery

Samples were collected from SBS mussel hatchery from a range of sources, water samples, biological samples, and hatchery surfaces and equipment to estimate the number of presumptive Vibrio. The concentration of Vibrio from seawater decreased significantly by filtration ( 5 µm pore size) in header tank and no Vibrio (<0.1 CFU ml -1 ) were detected in the water treated with UV irradiation (F= 332.274, df = 3.8, P<0.001). Settlement ropes possesed the highest density of Vibrio, 3.0 ± 0.8 x 10 6  CFU cm -1 . Larvae and spat had similar population densities of presumptive Vibrio . The biological samples typically had one log unit higher presumptive Vibrio numbers than water samples from the same source. A significant increase in Vibrio population density was observed during the first week of conditioning and settlement. Characterisation of Vibrio isolates by sequencing of 16S rRNA was attempted for 23 strains from a range of hatchery sources, however only 12 strains were successfully sequenced in one or both directions. Phylogenetic analysis showed these 12 strains clustered within the genus Vibrio and primarily within the large, poorly resolved cluster of Vibrio splendidus -like strains. Three strains were selected for pathogenicity assays. Strain 86 from a sperm/seawater sample allied with a cluster containing V. splendidus, V. crassostreae, V. chagasi, V. pomeroyi and V. gigantis, strain 125 from spat clustered with V. cyclitrophicus and strain 169 from spat clustered distantly with a group containing V. orientalis, V. tubiashi, V. nereis, V. corallilyticus and V. mediterranei . The LC 50 values in 24 h post-challenge ranged from around 1 ± 0.04 x 10 5 CFU ml -1 (mixture of strain 89 and 169) to 4 ± 0.05 x 10 6 CFU ml -1 (mixture of strain 125 and 169).  Strain 86 was more virulent than the other two other strains (F = 40.721, df = 6.14, P<0.001). Combinations of strains generally showed lower LC 50 values (increased virulence) compared to challenges using single strains, indicating synergistic/additive mechanism in pathogenicity. Meanwhile, the combination of strain 125 and 169 was less virulent (higher LC 50 ), suggesting that bacteria act in an antagonistic manner.