In Regulation (EU) No 37/2010, fish are described as “minor species”, for which only a limited number of veterinary preparations are officially approved. This shortage of therapies makes it very difficult for aquaculture operations to effectively treat these animals in case of illness in fish stock. Veterinarians may repurpose standard commercial antibiotics for use in fish, but this is only permissible if a risk to the health of people and animals can be ruled out and a waiting period of at least 28 days until the fish are marketed is complied with. In addition to impacts on growth and possible fish losses due to disease, the affected companies consequently also suffer financial losses due to delayed harvesting, which can significantly disrupt the rhythm of breeding cycles. For these reasons, more and more companies are turning to preventive measures that strengthen the well-being and condition of the fish, thus preventing diseases.
Substances that influence the so-called microbiome are the subject of particular interest. This term is derived from the Greek words “mikrós” for small and “bios” for life, and it means the totality of all of the microorganisms that naturally live on and in fish and other multicellular organisms. Most microorganisms live in the digestive tract as intestinal flora. We now know, however, that many bacteria also live on the surface of the skin (skin flora) and mucous membranes of the body. Some microbiome organisms are only commensals. This means that they live alongside their host fish without discernibly harming or particularly benefiting them. However, a much larger number of microorganisms have developed a symbiotic relationship with their host during the process of evolution, which is manifested in complex, multilayered relationships at the level of metabolic processes. For example, the intestinal microbiome contributes to increasing the value of nutrients consumed. It synthesizes vitamins that are important for life, produces short-chain fatty acids that positively influence the intestinal environment, supports the breaking down of food components that are difficult to digest and fights inflammation. Also, most of the immune cells of the body are found in the intestines, and their function is regulated by the useful symbiotic bacteria in the intestines.
The intestinal microbiome influences fish health
The extent to which microbiota influence the metabolism, the effectiveness of the immune system and the physiological performance of fish has long been underestimated. However, the results of recent research have significantly advanced our understanding of the links between changes in the intestinal microbiome and chronic inflammatory illnesses. Any measures that prevent negative shifts in the species composition or number of bacteria could therefore be worthwhile therapeutic approaches for preventing or alleviating dysbiosis. Dysbiosis, also known as dysbacteriosis, is a pathological disruption of the balance of microorganisms in the intestine.
In the past, the options for interventions at the microorganism level were primarily antibiotics. These active agents negatively affect the metabolism of bacteria, usually very effectively and in a highly specific way, so that they ultimately die. However, antibiotics do not differentiate between useful microbiota and pathogenic bacteria, which is why they are helpful on the one hand, but on the other hand also cause great damage to the microbiome and can trigger dysbiosis. This knowledge and the continuing criticism of the use of antibiotics in aquaculture have fuelled the global search for alternatives for the preventive health protection of fish. And with significant success, despite the large increase in aquaculture production, antibiotics consumption is declining according to the FAO (Food and Agriculture Organization) and WHO (World Health Organisation). Improved husbandry conditions, the establishment of hygiene plans, the development of specific vaccines and modern treatment strategies with pre- and probiotics, synbiotics and postbiotics have been the main contributors to this. The development of these therapeutic approaches to treating or preventing dysbiosis of fish is at quite an early stage, but appears very promising. The demand for corresponding products and research results regarding options for use is constantly growing.
Targeted support for bacteria species with a positive effect
What are prebiotics? Prebiotics are substances that are consumed with food and not digested, but that nevertheless have positive effects on the fish, because they stimulate the activity of bacteria naturally occurring in the intestines. They modify the conditions in the gastrointestinal tract, helping specific species of bacteria to grow more efficiently, strengthening the immune system and reducing the susceptibility of fish to pathogens. To put it simply, prebiotics are a kind of delicacy for specific intestinal bacteria such as lactobacilli and bifidobacteria, which are selectively ingested and contribute a health advantage. These substances are usually indigestible carbohydrates (oligosaccharides) and fibres containing dextrin, inulin, lignin, waxes or beta-glucan. Although there have only been a few studies into the use of prebiotics in fish and crustacean cultivation, it has already been shown that they have very positive effects on growth, feed utilisation and the intestinal microbiome. They improve resistance to pathogenic bacteria, reinforce the natural immune system and also appear to prevent cell damage. These effects make prebiotics promising candidates for feed additives in aquaculture that promote fish health while also being environmentally friendly.
In one study, for example, the effects of the commercial prebiotic Levabon aquagrow E, made from autolysed yeast cells (Saccaraomyces cerevisiae), which contains potentially immunomodulating substances such as chitin, chitinase, nucleotides, mannan sucrose and mannoproteins, on the growth of young European seabass (Dicentrarchus labrax) were tested under stress conditions. There was a tendency for fish fed with the prebiotic to exhibit better growth and greater vitality.
Prebiotics are naturally present in many plant-based aquaculture feeds that have been processed very little or not at all. They can be manufactured industrially from a variety of input materials, including lactose (milk sugar). The known prebiotics in animal feed include di-, oligo- and polysaccharides, inulin, lactulose, lactitol, raffinose, stachyose as well as fructans and oligofructose. It is usually characteristic of these substances that they are resistant to digestive enzymes and stomach acids and therefore reach the intestines without being digested. There they promote a healthy balance in the intestinal microbiome as well as optimising the species composition of the intestinal flora.
In contrast to prebiotics, which work by the targeted “feeding” of bacteria that are naturally present in the intestines, probiotics are living microorganisms that are mixed into feed and ingested orally by fish. If the species of probiotic bacteria selected and the quantities administered are right, these substances are also claimed to have a health-promoting influence on the host organism. The extent of these effects is, however, frequently disputed or at least not as pronounced as is sometimes claimed. However, the probiotic effects of lactobacilli as well as some yeasts and other species are relatively certain and recognised. Both pre- and probiotics are components of different raw materials that are part of healthy and complete nutrition for fish and crustaceans. If the intestinal flora are weakened due to disease or unhealthy food, harmful microorganisms can proliferate and displace the beneficial intestinal bacteria. If the intestinal environment is damaged, the immune barrier becomes porous and pathogens can easily penetrate into the body. With probiotics, an attempt can be made to populate the intestines with beneficial bacteria in order to correct the imbalance present and to regenerate the microbiome. This works relatively well for some gastrointestinal illnesses.
The advantages of probiotics in improving general health have long been underestimated. However, biotechnology companies are now working globally and intensively on new therapeutic approaches with probiotics to prevent disease and to support the healing process. In some areas of commercial aquaculture, probiotics are already being used routinely. Normally they are mixed into fish feed. Through the combination of protective vaccinations and regularly adding probiotics to feed, modern aquaculture can almost completely do without drugs, for example. The fish grow much better and are more tolerant to captivity-related stress. Lactobacilli create lactic acid from carbohydrates as part of their metabolism, and they play a very important role in this context. They are particularly useful in the cultivation of young fish, with skin and intestinal microbiomes that are still insufficiently developed, which can be targeted with beneficial bacterial cultures for protection from pathogens. The lactobacilli compete with potential pathogenic bacteria for nutrients and oxygen and prevent them from growing, thus preventing infections. Some lactobacilli have beta-glucan in their cell walls, which stimulates the immune system. Lactic acid production also lowers the pH of the intestines, making it difficult for harmful bacteria to survive. The “good” bacteria multiply, while the “bad” decline in number.
Probiotics require regular follow-up doses
Probiotics can of course also have positive effects on adult fish, which have already developed a bacterial community in their intestines. Some probiotic bacteria can therefore only hold their ground for a short time in the existing intestinal flora and are soon displaced. In such cases, probiotics can only exert their protective effect if they are regularly administered in food, or “topped up”. However, current studies show that this can be worth the effort. Probiotics increase the efficiency of feed utilisation and the animals excrete less, which means less pollution of water and the environment. For reproducing fish, fertility is significantly increased and more embryos survive.
Some critics, however, doubt the health-promoting properties of some probiotic strains, particularly when they are touted and marketed as “miracle cures” for infections. The promised therapeutic effects have not been conclusively demonstrated in every case, and there is still a great need for further research. Regarding the use of probiotics for human food, the EU therefore put an end to unproven health-related claims with the Health Claims Regulation at the beginning of 2007. Since then, the European EFSA authority has also scientifically tested a range of probiotics for which health effects were claimed. Positive assessments were published for most probiotic strains, but there were also cases where health claims could not be credibly proven. It is also often unknown exactly how these substances work. As far as we know, probiotics stimulate or modulate the mucous membrane of the intestines. Some may also regulate the production of immunoglobulins, which strengthens immune defences. It is probable that some probiotic bacteria can also produce bacteriocins themselves that inhibit the growth of other competing bacteria.
In aquaculture, the range of probiotics used is already much wider than in land-based agriculture, and their positive effects can no longer seriously be doubted. In shrimp farms in particular, they are frequently the method of choice for prevention of infections in animal stocks. In multiple studies of aquatic animals, clear indications have been found that bacterial strains with probiotic effects can potentially suppress diseases. One of these studies investigated the influence of probiotics on sick rainbow trout. Over the course of the treatment, the probiotic bacteria suppressed the pathogens and the fish became healthy. The feed manufacturer BioMar, with its Larviva products, has developed and brought to market a product range that is supplemented with the probiotic bacteria species Pediococcus acidilactici (Bactocell) These feeds are approved in the EU for all aquaculture species. According to BioMar, they significantly reduce the frequency of malformations in fish larvae and spawn.
Syn- and postbiotics have comparable effects
In accordance with the “better safe than sorry” principle, synbiotics, which combine prebiotics with probiotics, thus reinforcing their positive effects, are also frequently used in practice. While prebiotics have a positive growth-promoting effect on microorganisms already found in the intestines, probiotics are used to supplement the intestinal flora with additional helpful bacteria species.
Postbiotics are the remaining instrument in the toolbox of modern therapeutic options. This term covers beneficial molecules and substances created by microorganisms. These include, for example, mannan oligosaccharides (MOS), natural sugar complexes (carbohydrates) that are isolated from the cell walls of brewer’s yeast, among other sources. They improve intestinal health and immunity by the targeted stimulation of growth and activity of bacteria species in the intestines, in particular lactobacilli. MOS also reduce the growth of pathogens by preventing them from establishing themselves in the intestinal mucous membrane. Studies have shown that MOS additives in feed can significantly reduce the number of Vibrio bacteria, which cause great harm to many fish species. MOS also have a positive effect on the intestinal structures of the fish. A comparison between intestinal cross-sections of trout that had and had not been fed with MOS supplements clearly shows that the number and size of the intestinal microvilli is significantly increased by MOS. This increases the surface area of the intestines and optimises digestive performance, which is especially beneficial for many fish species in low water temperatures. MOS additives in feed also improve the immune system of fish, which is why more and more companies are also using postbiotics as a preventive measure for keeping their animal stock healthy.
The conclusion to be drawn as to the use of beneficial microorganisms in aquaculture is therefore quite clear. While the use of antibiotics is constantly declining, interest in the use of dietetic pre-, pro-, syn- and postbiotics is growing. Using them is a sensible strategy to promote the health and wellbeing of many fish and crustacean species and they represent environmentally friendly alternatives to antibiotics. They improve intestinal flora and optimise food utilisation. They inhibit or eliminate pathogenic bacteria and strengthen the immune system, which reduces the number of drug treatments required to combat bacterial infections. They also reduce the stress levels of fish and ensure improved water quality. All of this makes probiotics and bacterially effective substances valuable tools in the health concepts of many aquaculture operations. With increasing global demand for sustainable fish and seafood products that are cultivated in an environmentally friendly way, the use of microorganisms with a probiotic effect is likely to increase.