Disease Control in Shrimp Aquaculture with Probiotic Bacteria
Shrimp aquaculture production in much of the world is depressed by disease, particularly caused by luminous Vibrio and/or viruses. Antibiotics, which have been used in large quantities, are in many cases ineffective, or result in increases in virulence of pathogens and, furthermore, are cause for concern in promoting transfer of antibiotic resistance to human pathogens. Probiotic technology provides a solution to these problems.
The UN FAO estimates that half of the world’s seafood demand will be met by aquaculture in 2020, as wild capture fisheries are overexploited and are in decline. Shrimp (or prawn) culture is widespread throughout the tropical world. It is in an industry set for a period of strongly growing demand and is currently worth around US$10 billion. Penaeus monodon, the black tiger shrimp, is the most widely cultured species.
In much of the world, however, the shrimp aquaculture industry is beset by disease, mostly due to bacteria (especially the luminous Vibrio harveyi) and viruses. The high density of animals in hatchery tanks and ponds is conducive to the spread of pathogens, and the aquatic environment, with regular applications of protein-rich feed, is ideal for culturing bacteria.
The use of beneficial bacteria (probiotics) to displace pathogenic bacteria by competitive processes is a better remedy than administering antibiotics. The microbial species composition in aquaculture ponds can be changed by adding selected species to displace deleterious common bacteria. Success depends upon defining the ecological process or processes to be changed, the types of deleterious species that are dominant and the desirable alternative species or strains of bacteria that could be added.
Competitive exclusion is one of the ecological processes that allows manipulation of the bacterial species composition in the water, sediment and animal guts. Vibrio spp., especially the luminous V. harveyi, have been implicated as the main bacterial pathogens of shrimps. Antibiotics have been used in attempts to control these bacteria, but their efficacy is now, in general, very poor. In the Philippines, luminous Vibrio disease caused a major loss in shrimp production in 1996, and many farms have ceased to produce shrimps because survival was so poor. The Vibrio species were resistant to every antibiotic used, including chloramphenicol, furazolidone, oxytetracycline, and streptomycin, and were more virulent than in previous years.
In Thailand this year, a farmer who was using colloidal silver in all feeds experienced a large increase in mortality from vibriosis. This was managed for a while with large doses of norfloxacin in all feeds. However, when it was stopped all shrimps died within 2 days. Clearly, a highly virulent strain of luminous Vibrio had developed in response to the use of the silver and antibiotics.
Chlorine is widely used in hatcheries and ponds, but its use stimulates the development of multiple antibiotic resistance genes in bacteria. Some farmers in Thailand have reported that when chlorine is used in ponds to kill zooplankton before stocking shrimp, there is a rapid increase in Vibrio harveyi numbers after the chlorine is removed. This is to be expected as marine vibrios have very fast growth rates, and the chlorine treatment will lower the numbers of competitors for nutrients and kill algae, thus increasing food resources. It is likely, therefore, that the vibrios surviving after chlorine treatment are not only more resistant to antibiotics but are also pathogenic. Thus the problems have been exacerbated by the use of antimicrobial compounds.
If antibiotics or disinfectants are used to kill bacteria, some bacteria will survive, either strains of the pathogen or others, because they carry genes for resistance. These will then grow rapidly because their competitors are removed. Any virulent pathogens that re-enter the pond or hatchery tank, perhaps from within biofilms in water pipes or in the guts of animals, can then exchange genes with the resistant bacteria and survive further doses of antibiotic. Thus, antibiotic-resistant strains of pathogens evolve rapidly.
The transfer of resistance to human pathogens and gut bacteria is of major concern. Such transfers probably happen easily and often, as discussed by Salyers. Resistance plasmids encoding for many antibiotic resistance genes were transferred between pathogenic and non-pathogenic Gram negative bacteria in several environments including sea water. In the presence of tetracycline concentrations that were not high enough to kill the bacteria, the rate of gene transfer between Vibrio cholerae and Aeromonas salmonicida increased 100 times.
The use of beneficial bacteria (probiotics) to displace pathogens by competitive processes is being used in the animal industry as a better remedy than administering antibiotics and is now gaining acceptance for the control of pathogens in aquaculture. The term “probiotic” has been defined as: “a probiotic is a mono- or mixed culture of live microorganisms that, applied to animal or man, affect beneficially the host by improving the properties of the indigenous microflora”.
Unlike land animals, aquatic farmed animals are surrounded by a milieu that supports opportunistic pathogens independently of the host animal, and so the pathogens can reach high abundance around the animal. Vibrio grow attached to algae, and may reach high population densities after being ingested with the algae and then excreted with lysed algae in faecal pellets by zooplankton; they are gut bacteria in fish and shrimps as well as zooplankton. In aquaculture ponds, where animal and algal population densities are very high, Vibrio numbers are also high compared to the open sea.
The species composition of a microbial community, such as that in a pond, will be determined partly by stochastic phenomena, that is, chance, and partly by deterministic and predictable factors that allow one species to grow and divide more rapidly than others, and thus dominate numerically. Chance favours those organisms that happen to be in the right place at the right time to respond to a sudden increase in nutrients, e.g. from the lysis of algal cells or the decomposition of feed pellets that fall around them.
Competitive exclusion is one of the ecological processes that can be manipulated to modify the species composition of a soil or water body or other microbial environments. Small changes in factors that affect growth or mortality rates will lead to changes in species dominance. We are still a long way from knowing all the factors that control growth rates of particular species. The complete species composition in natural environments is largely unknown, but enough is known to argue that it is possible to change species composition by making use of competitive exclusion principles. Thus bacteria can compete by secreting antimicrobial compounds that do not necessarily kill all their competitors, but increase mortality rates just enough to tip the balance in resource utilization. For example, if a Bacillus strain were to produce an antibiotic that inhibited a Vibrio, then the Vibrio’s mortality rate would increase, shifting the dominance to the Bacillus, even if the antibiotic were not produced at high enough concentration to kill all or most Vibrio cells directly.
Microbial ecology and biotechnologies have advanced in the last decade, to the point that commercial products and technologies are available for treating large areas of water and land to enhance population densities of particular microbial species or biochemical activities. The practice of bioremediation (or bioaugmentation) is applied in many areas, but success varies greatly, depending on the nature of the products used and the technical information available to the end user. The bacteria that are added must be selected for specific functions that are amenable to bioremediation, and be added at a high enough population density, and under the right environmental conditions, to achieve the desired outcomes. Bioaugmentation and the use of probiotics are significant management tools for aquaculture, but their efficacy depends on understanding the nature of competition between particular species or strains of bacteria. They rely on the same concepts that are used successfully for soil bioremediation and probiotic usage in the animal industry.
Probiotic Applications in Aquaculture
Bacterial species composition in shrimp ponds, which are large water bodies up to a hectare or more in size, hatchery tanks and shrimp guts can easily be changed and thus result in an improvement in shrimp production. In particular, luminous Vibrio can be controlled in this manner. To my knowledge, there has not been any rigorous study made of Vibrio populations in shrimp on farms, in relation to antibiotic or probiotic usage. Thus the data referred to here are given as examples of what has been observed, but the conclusions need to be substantiated. An example is the Viveros farm in Negros, where losses from luminous Vibrio had been catastrophic, even though antibiotics were used in the feed and gave protection some of the time. Luminous Vibrio abundance in the pond waters of that region was often as high as 103 to 104 per ml within 2 – 3 weeks of filling and fertilizing ponds. However, when the probiotic bacteria were used, no disease was experienced and indeed survival was very high (80-100%), even in the presence of luminous Vibrio species.
In several ponds in the Philippines, luminous Vibrio numbers in the hepatopancreas of shrimps fell from around 1 x 104 per gut when antibiotics were used in the feed, to zero when probiotics were applied to the pond.
Luminous Vibrio was completely eliminated from the water column and from the sediment of ponds in Indonesia when probiotic strains selected for their direct inhibitory effect were used . In contrast, Vibrio numbers increased markedly from around 20 to over 200 CFU/ml in shrimp ponds where antibiotics were used in the feed. Survival, and thus production, was high in all ponds where probiotics were used.
These data show that the disease problems can be overcome by applying probiotic biotechnology, which is an application of microbial ecology. It makes use of the natural mechanisms by which bacteria compete against each other. In other words, shrimp farmers who learn to farm microorganisms will be far more likely to achieve successful harvests.
With the right combination of bacteria and aeration, water exchange can be minimized and water can be recycled between crops, thus lessening environmental impacts and the likelihood of introducing pathogens. The transfer of antibiotic resistance to human pathogenic bacteria, which is exacerbated by the abuse of antibiotics in the aquaculture industry, will decrease.
Source: David J. W. Moriarty
Biomanagement Systems Pty. Ltd., 315 Main Road, Wellington Point. Queensland 4160 Australia,
and Department of Chemical Engineering, The University of Queensland. Qld. 4072 Australia.