Halobacteriovorax (HBx) are Gram-negative predatory bacteria that invade and kill other Gram-negative marine-associated bacteria. We showed broad specificity of four HBx strains toward 23 well-characterized strains of human pathogenic V. parahaemolyticus (Vp). Host specificity was not affected by Vp sequence type or serotype, the presence of Vp hemolysins (tdh and/or trh), or the geographic locations from which the vibrios or the HBx were obtained. Two of the more rapidly growing HBx were selected for use as potential treatments to reduce Vp in naturally contaminated market oysters obtained from a commercial harvesting site in the Delaware Bay.
The 23 strains of clinically-derived Vp were kindly provided by the US FDA and represented strains obtained from Atlantic, Pacific, Gulf, and Hawaiian coasts of the US. Two of the HBx strains were isolated from the Atlantic coast. The others were from the Gulf of Mexico and the Hawaiian coast. Host specificity assays of the four strains were performed in each of the 23 Vp strains using a double agar plaque assay technique. Plates were incubated at 26°C for up to 7 days and plaque presence or absence was evaluated as well as plaque sizes. The Vp strains consisted of 6 different sequence types (ST) including pandemic strains ST3 and ST36, as well as 12 Vp serotypes. Strains were also selected based on the presence or absence of the well-known Vp hemolysins tdh and/or trh.
Results showed that all four HBx predated upon at least 21 of the 23 strains of Vp with a mixture of HBx strains H4 and G3 capable of infecting and killing all 23 Vp strains. HBx strains originally isolated from the Atlantic, Gulf, and Hawaiian coasts were capable of killing Vp obtained from distant locations. HBx also attacked most of the Vp without regard to hemolysin presence or
absence. HBx strains H4 and G3 produced the largest plaques, suggesting faster growth or infection rates, therefore, they were selected as potential biocontrol agents to reduce Vp levels in market oysters.
Over the summer, tank studies on naturally contaminated oysters were performed to compare the levels of Vp present in the gills and digestive tissues of HBx-treated and untreated (negative control) oysters over a 3-day period. Results were inconclusive due to the presence of other Vp predators present at higher-than-expected levels in the oysters. These included bacteriophages, high levels of suspected Pseudoalteromonas spp., and some background levels of HBx. 16S rRNA sequencing is underway to identify these predators. Studies on oysters are continuing.