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Seven bacteriophages specific to Vibrio harveyi, the causative agent of luminous vibriosis in shrimp, were isolated from coastal aquaculture systems like shrimp farms, hatcheries and tidal creeks along the east and west coast of India. All the seven phages were found to have the typical head and tail morphology with double-stranded DNA as genetic material. Morphologically, six phages were grouped under family Siphoviridae and one under Myoviridae. These phages were further characterized with respect to host range, morphology and structural proteins. Genomic fingerprinting was carried out using restriction fragment length polymorphism (RFLP) and randomly amplified polymorphic DNA (RAPD). Major capsid proteins of all the phages detected by SDS-PAGE were distinct from one another. All the phages were found to be highly lytic for V. harveyi and had different lytic spectrum for the large number of isolates tested. Six of the seven phages isolated had a broad lytic spectrum and could be potential candidates for biocontrol of V. harveyi in aquaculture systems.

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Aeromonas hydrophila is a Gram-negative bacterium that causes high mortality in different fish species and at different growth stages. Although vaccination has significantly contributed to the decline of disease outbreaks in aquaculture, the use of oral vaccines has lagged behind the injectable vaccines due to lack of proven efficacy, that being from primary immunization or by use of boost protocols. In this study, the outer membrane protein W (OmpW) of A. hydrophila was cloned, purified, and encapsulated in poly d,l-lactide-co-glycolic acid (PLGA) nanoparticles (NPs) for oral vaccination of rohu (Labeo rohita Hamilton). The physical properties of PLGA NPs encapsulating the recombinant OmpW (rOmpW) was characterized as having a diameter of 370–375 nm, encapsulation efficiency of 53% and −19.3 mV zeta potential. In vitro release of rOmpW was estimated at 34% within 48 h of incubation in phosphate-buffered saline. To evaluate the efficacy of the NP-rOmpW oral vaccine, two antigen doses were orally administered in rohu with a high antigen (HiAg) dose that had twice the amount of antigens compared to the low antigen (LoAg) dose. Antibody levels obtained after vaccination showed an antigen dose dependency in which fish from the HiAg group had higher antibody levels than those from the LoAg group. The antibody levels corresponded with post challenge survival proportions (PCSPs) and relative percent survival (RPS) in which the HiAg group had a higher PCSP and RPS than the LoAg group. Likewise, the ability to inhibit A. hydrophila growth on trypticase soy agar (TSA) by sera obtained from the HiAg group was higher than that from the LoAg group. Overall, data presented here shows that OmpW orally administered using PLGA NPs is protective against A. hydrophila infection with the level of protective immunity induced by oral vaccination being antigen dose-dependent. Future studies should seek to optimize the antigen dose and duration of oral immunization in rohu in order to induce the highest protection in vaccinated fish.

There is growing evidence that climate change will increase the prevalence of toxic algae and harmful bacteria, which can accumulate in marine bivalves. However, we know little about any possible interactions between exposure to these microorganisms and the effects of climate change on bivalve health, or about how this may affect the bivalve toxin-pathogen load. In mesocosm experiments, mussels, Perna viridis, were subjected to simulated climate change (warming and/or hyposalinity) and exposed to harmful bacteria and/or toxin-producing dinoflagellates. We found significant interactions between climate change and these microbes on metabolic and/or immunobiological function and toxin-pathogen load in mussels. Surprisingly, however, these effects were virtually eliminated when mussels were exposed to both harmful microorganisms simultaneously. This study is the first to examine the effects of climate change on determining mussel toxin-pathogen load in an ecologically relevant, multi-trophic context. The results may have considerable implications for seafood safety.

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