Predatory Bacteria as Natural Modulators of Vibrio parahaemolyticus and Vibrio vulnificus in Seawater and Oysters

Abstract

dThis study shows that naturally occurring Vibrio predatory bacteria (VPB) exert a major role in controlling pathogenic vibrios in seawater and shellfish. The growth and persistence of Vibrio parahaemolyticus and Vibrio vulnificus were assessed in natural seawater and in the Eastern oyster, Crassostrea virginica. The pathogens examined were V. vulnificus strain VV1003, V. parahaemolyticus O1:KUT (KUT stands for K untypeable), and V. parahaemolyticus O3:K6 and corresponding O3:K6 mutants deficient in the toxRS virulence regulatory gene or the rpoS alternative stress response sigma factor gene. Vibrios were selected for streptomycin resistance, which facilitated their enumeration. In natural seawater, oysters bioconcentrated each Vibrio strain for 24 h at 22°C; however, counts rapidly declined to near negligible levels by 72 h. In natural seawater with or without oysters, vibrios decreased more than 3 log units to near negligible levels within 72 h. Neither toxRS nor rpoS had a significant effect on Vibrio levels. In autoclaved seawater, V. parahaemolyticus O3:K6 counts increased 1,000-fold over 72 h. Failure of the vibrios to persist in natural seawater and oysters led to screening of the water samples for VPB on lawns of V. parahaemolyticus O3:K6 host cells. Many VPB, including Bdellovibrio and like organisms (BALOs; Bdellovibrio bacteriovorus and Bacteriovorax stolpii) and Micavibrio aeruginosavorus-like predators, were detected by plaque assay and electron microscopic analysis of plaque-purified isolates from Atlantic, Gulf Coast, and Hawaiian seawater. When V. parahaemolyticus O3:K6 was added to natural seawater containing trace amounts of VPB, Vibrio counts diminished 3 log units to nondetectable levels, while VPB increased 3 log units within 48 h. We propose a new paradigm that VPB are important modulators of pathogenic vibrios in seawater and oysters.

Halobacteriovorax, an underestimated predator on bacteria: potential impact relative to viruses on bacterial mortality

“…The fact that the experimental results concerned native predators in NWs adds relevance to the findings. Other investigators using different experimental models have also reported a reduction of prey bacteria by Halobacteriovorax and the potential of the predators to affect the mortality of susceptible bacterial populations in aquatic systems (Richards et al, 2012;Li et al, 2014;Wen et al, 2014). Richards et al (2012) used several strains of V. parahaemolyticus and a strain of V. vulnificus as amendments to a natural seawater sample and observed a decline in their numbers along with an increase in Halobacteriovorax as we observed in the experiments described in this report.…”

“…The findings of this and other studies demonstrating the impact of BALO predation in aquatic systems (Li et al, 2011;Richards et al, 2012;Wen et al, 2014) argues for recognition of their potential role as biological agents of bacterial mortality along with viruses and protists. Johnke et al (2014) have made a strong case for assessing the potential of multiple micro-predators rather than focusing on a single predator, as many previous studies have done.…”

“…To establish the microcosms, the filtered water samples were amended with a high concentration (10 8 ml -1 ) of V. parahaemolyticus prey cells to efficiently elicit and amplify the predators' responses. Others have used this same approach in Halobacteriovorax predation experiments (Richards et al, 2012;Wen et al, 2014) and achieved the same pattern of reduction of the prey and increase in predator numbers. Some studies using viral microcosms have also involved high numbers of total bacteria (up to 3 × 10 7 ml -1 ) (Guixa-Boixereu et al, 1999).…”

“…The fact that Halobacteriovorax are also abundant in the surface microlayer (Williams, 1987) suggests that they prey on the large numbers of bacteria present. Although high concentrations of prey were used in this study, it should be noted that BALOs are typically capable of reducing prey concentrations via population dynamics similar to those shown in Figures 2 and 3 at prey concentrations as low as of 1 × 10 4 ml -1 , albeit over a longer time period (Ahmed, 2007;Richards et al, 2012;Li et al, 2014;Wen et al, 2014).…”

Bacterial predation in a marine host-associated microbiome

“…Predation by BALOs also releases nutrients (Martínez et al, 2013), affecting biogeochemical cycling and production in nutrient-limited environments, similar to the effects of phage (Fuhrman, 1999;Brussaard et al, 2008). In contrast to viruses, which are the most abundant biological entity in the marine environment and maintain abundances approximately in an order of magnitude greater than their hosts, BALOs do not need to be in high initial concentrations in the environment to drive significant bacterial mortality of their prey (Richards et al, 2012;Williams et al, 2016). One group of Bdellovibrionales, the marine Halobacteriovorax, are broadly distributed in marine waters across temperature, salinity and pH gradients (Pineiro et al, 2007).…”