Biomanagement Systems Pty. Ltd., 315 Main Road, Wellington Point. Queensland 4160 Australia, and Department of Chemical Engineering, The University of Queensland. Qld. 4072 Australia.
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 microbial species composition in hatchery tanks or large aquaculture ponds can be changed by adding selected bacterial species to displace deleterious normal bacteria. Virulence of luminous Vibrio species can be controlled in this manner. Abundance of luminous Vibrio strains decreased in ponds and tanks where specially selected, probiotic strains of Bacillus species were added. A farm on Negros, in the Philippines, which had been devastated by luminous Vibrio disease while using heavy doses of antibiotics in feed, achieved survival of 80-100% of shrimp in all ponds treated with probiotics.
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.
I feel these problems have been exacerbated because the interactions of microbes, animals and their environment at intensive production scales have been considered primarily from a clinical pathology perspective. That is, the disease was treated rather than the underlying cause. When pathogenic bacteria or viruses are detected, farmers apply antimicrobial compounds to the feed and water. Many farmers also use antibiotics as prophylactics in large quantities, even when pathogens are not evident. This has lead to an increase in vibrios, and presumably other bacteria, having multiple antibiotic resistance and to an increase in more virulent pathogens. Many of the pathogens appear to have mutated to more virulent forms than were present a decade ago, and thus even when the shrimps are not stressed by poor water quality they succumb to attack. Thus I feel that the incidence of disease has been exacerbated by the actions of the shrimp farmers.
Microbial Interactions in Aquaculture
The solution lies in the field of microbial ecology, not in the field of pharmacology, i.e. in developing new antibiotics or vaccines . Shrimp farmers have to learn to live with a complex community of microbes and manage them. The use of beneficial bacteria (probiotics) to displace pathogenic bacteria by competitive processes is a better remedy than administering antibiotics. And it works!
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.
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” . In this discussion, the authors considered only human and land farm animals. In extending their definition to aquaculture, I suggest that it also applies to the addition of live, naturally-occurring bacteria to tanks and ponds in which the animals live, because these bacteria modify the bacterial composition of the water and sediment. The health of animals is thus improved by the removal, or decrease in population density, of pathogens and by improving water quality through the more rapid degradation of waste organic matter.
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 onset of shrimp disease due to exposure to high numbers of Vibrio, especially when pathogenicity has increased by overuse of antimicrobial compounds indicates that a defense is needed.
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. The farmer can manipulate the species composition by seeding large numbers of desirable strains of bacteria or algae; in other words, by giving chance a helping hand.
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 environment. 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.
Probiotics such as the Gram positive Bacillus offer an alternative to antibiotic therapy for sustainable aquaculture. Bacillus species are commonly found in marine sediments and therefore are naturally ingested by animals such as shrimps that feed in or on the sediment. An advantage of using Bacillus species is that they are unlikely to use genes for antibiotic resistance or virulence from the vibrios or related Gram negative bacteria. There are barriers at the transcriptional and translational levels to the expression of genes from plasmid, phage and chromosomal DNA of E. coli in B. subtilis .
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 this 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.
Author: David J. W. Moriarty