Watch out bacterial pathogens!

A review of: Selvin, J., Manilal, A., Sujith, S., Kiran. G.S., Lipton, A.P. (2011). Efficacy of marine green alga Ulva fasciata extract on the management of. Latin American Journal of Aquatic Research. 39 (2), 197-204

11 species of Vibrio are commonly recognised as members of the normal bacterial flora of shrimp culture systems. However some of these members such as V. alginolyticus and V. harveyi pose serious threats as opportunistic pathogens and have been known to have a serious impact on the shrimp production industry. Marine secondary metabolites, from a number of different sources, have been recognised in previous studies as promising agents in shrimp disease management. The marine algae Ulva fasciata is found on the southwest coast of India amongst diverse variety of seaweed species. This highly competitive environment would insure that this alga’s secondary metabolites are highly reactive. The aim of this study was to establish the effect and the efficacy of U. fasciata based feed on the survival of shrimps experimentally infected with a range of pathogens.
They initially found the found the Lethal dose (LD) value of the pathogens: V. fischeri, V. alginolyticus, V. harveyi and Aeromonas sp. They intramuscularly injected samples of shrimp with the pathogens and then observed the effects. They increased the dose until total mortality was observed. A control group was injected with saline solution. They then top-coated commercially formulated shrimp feed with varying quantities of U. fasciata extracts 500mg, 1000mg and 1500mg/ kg of shrimp. Different groups shrimp were fed with U. fasciata diet for 15 days prior to being injected with the LD of the different pathogens. Again they used a control group which was not given U. fasciata in their diet to compare results. The diets were continued for 15 days after they were inoculated with the pathogens and a number of physiological and behaviour features were observed.
Shrimp fed with 500mg had the lowest protection against bacterial infection. Shrimp fed with 1000mg showed the highest level of protection in comparison to the other doses. They found that the survival of treated shrimp against the bacterial infection was significant at P<0.01. They found 100% mortality in the control shrimps even before the lethal dose, found in the preliminary experiments, was met. They believed this could be due to enhanced virulence obtained by the pathogenic isolates passed through the host, prior to in captivity control experiments. The median lethal dose of algal extract was determined to be 1120mg which is most likely the reason why there is a higher percentage of infection in the group of shrimps treated with 1500mg of U. fasciata. In an earlier study it was observed that pathogens were cleared off from the shrimp’s haemolymph within an hour in groups treated with U. fasciata. This suggests that the secondary metabolites of the algae stimulate the rapid response of bactericidins inside the shrimps. This study presents a promising development in bacterial pathogen management using natural marine products. The extracts can be stored in a dry form for long periods and this coupled with how easy it is to extract the bioactive ingredients makes it a low cost solution. Algae can be cultured in large quantities making its secondary metabolites not only eco-friendly but sustainable too. The large number of similar studies recently published is not surprising considering that this multimillion pound industry can be so easily threaten by things that are so hard to see.