By Hui Gong*
After decades of impressive growth, global shrimp aquaculture production in 2012 reached 3.78 million tons. However, catastrophic disease outbreaks have hindered the growing pace of the shrimp industry from time to time and caused tremendous economic loss, in billions of USD$ collectively. The most recent disease problem was the outbreak of the Early Mortality Syndrome (EMS) or “Acute Hepatopancreatic Necrosis Syndrome” (AHPNS), which was first reported in 2009. EMS has affected the shrimp farming industry greatly in much of Asia and Mexico, mostly targeting young animals within one month after stocking in ponds. Estimated economic losses due to EMS are over USD$1 billion per year. Substantial research efforts have been invested in finding the causes and cures, and the causative pathogen agent of EMS (AHPNS) was identified as a specific strain of Vibrio parahaemolyticus, which now can be detected via PCR tests.
The mechanism of EMS pathogenicity is not fully understood yet. However, the mode of action is believed to be that the bacterium colonizes the shrimp gastrointestinal tract, producing a toxin that causes destruction and dysfunction of the hepatopancreas, which is the major digestive/absorptive organ in shrimp, consequently resulting in mass mortality. V. parahaemolyticus is a common inhabitant of coastal and estuarine environments all over the world, and closely related to various shrimp pathogenic luminous bacteria such as V. harveyi, V. campbelli and V. owensii. It has been proven to be very difficult to eradicate these bacterial pathogens, if not impossible. Maintaining a good balance of microbiota in the digestive system of shrimp and in the culture environment to prevent any imbalance favoring the propagation and domination of pathogenic bacteria is probably a more sensible approach. Application of probiotics could play an important role in this aspect.
What are probiotics?
The term “probiotic” comes from the Latin “for life”, and has had different popular meanings over the years. The term in modern aquaculture can often mean a bacterial supplement of a single or mixed culture of selected non-pathogenic strains. Parker described probiotics as organisms and substances that contribute to the intestinal microbial balance. Fuller provided a revised definition of probiotics as ‘a live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance.’ A newer version of the definition provided by the Food and Agricultural Organization and World Health Organization was ‘live microorganisms that when administered in adequate amounts confer a health benefit on the host.’ As for aquaculture, the bacteria present in the aquatic environment influence the composition of the gut biota as the host and microorganisms share the ecosystem. Probiotics may act as a microbial dietary adjuvant that beneficially affects the host’s physiology by modulating mucosal and systemic immunity, as well as improving the nutritional and microbial balance in the intestinal tract.
Application of probiotics in shrimp aquaculture
Application of probiotics in shrimp aquaculture is still in its very early stages and much more needs to be done as compared to uses in land animals and fish. Some probiotics- related research in shrimp and prawns is summarized in the Table below. Probiotics have been applied in shrimp during various developmental stages for different purposes. They have been used to condition the water in shrimp nursery tanks, to inhibit the growth of pathogenic agents and to maintain water quality in intensive aquaculture ponds. Studies have shown that administration of probiotics during early developmental stages can be more effective in preventing gastrointestinal associated disorders because the digestive tract and function are not fully developed and the immune system is still incomplete.
Applying probiotics in shrimp culture can be categorized in three major ways: through feed, through water, and through a combination of feed and water. The majority of probiotics used in previous studies in shrimp and prawns are Lactic acid bacteria (Gram-positive), Bacillus bacteria (Gram-positive), Vibrio alginolyticus (Gram-negative), Nitrobacter spp. (Gram-negative), and some yeasts, such as Saccharomyces cerevisiae, S. exiguous, Phaffia rhodozyma, etc. Functions of probiotics vary, to a large extent. Some probiotics stimulate the host’s immune response, produce vitamins, detoxify compounds in the diet and break down indigestible components, while others may inhibit the growth of pathogenic bacteria by producing wide-spectrum inhibitory compounds. Probiotics may also benefit shrimp by competing for available nutrient substrates, space and even Iron in the microbiota in gastrointestinal environment, and/or altering water quality in the external environment. Consequently, the beneficial effects of probiotics to shrimp have generally been suggested as enhancing feed conversion efficiency, improving growth rates, protecting against pathogens and improving water quality.
A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity of the gastrointestinal microflora that confers benefits upon host’s health conditions. In some cases, prebiotics may be beneficial for probiotic effects. Combined application of probiotics and prebiotics, is known as “synbiotics.” With prebiotic support, probiotics will thrive and survive well in hosts’ digestive systems. The synergism makes it possible for the application of synbiotics to achieve greater benefits than the application of probiotics alone. The application of synbiotics were found to provide beneficial effects such as improved disease resistance and growth, and enhanced survival post WSSV challenge in a couple of studies in Litopenaeus vannamei.
Quorum sensing (QS) is a process of bacterial cell-to-cell communication, which involves regulation of gene expression in response to fluctuations in cell-population density. Quorum sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of an autoinducer leads to an alteration in gene expression. QS cascade can occur both intra- and inter-species via specific autoinducers, which elicit specific responses. Although the nature of the chemical signals, the signal relay mechanisms, and the target genes controlled by bacterial quorum sensing systems differ in every case, the ability to communicate with one another allows bacteria to coordinate their gene expression, and therefore the behavior of the entire community. Virulent bacteria can launch an attack on the host with QS controlling the pathogenicity.
Gaining understanding of the mechanism how V. parahaemolyticus causes EMS in shrimp is crucial and developing specific chemical molecules that could either impede certain QS of this causative bacteria or promote some alternate QS might be helpful in coming up with new anti-infective strategies in shrimp aquaculture.
With the rapid advancement of biotechnologies in the recent years, there is no doubt that more in-depth knowledge of probiotics can be gained, and that proper application of probiotics/prebiotics could achieve greater positive impacts in shrimp aquaculture production.
Hui Gong, PhD, is an Associate Professor at the College of Natural and Applied Sciences at the University of Guam. Her expertise in shrimp aquaculture has built on 17 years of experience in applied research in both academic and industrial backgrounds.