The natural microbial community of fish is highly complex and has many effects on health, growth and immune response. However, it is also prone to bacterial infections which can be caused by various factors including environmental and behavioural influences.
Beneficial bacteria in Aqua Probiotics can directly adhere to the intestinal epithelial cells (enterocytes). They can then form a colony and competitively exclude harmful microbes and positively modulate the host’s immune responses.
1. Strengthen the Immune System
Probiotics are live microorganisms that can improve intestinal microbial balance and increase the host’s resistance against pathogens. They are important in aquaculture because they can replace antimicrobial drugs, which have many adverse effects. Furthermore, they are not effective against antibiotic-resistant bacteria.
Aside from their role in disease prevention, dietary probiotics can also strengthen the immune system in aquatic species. They can do this by increasing the production of antimicrobial peptides and regulating the activity of immunoregulatory cells in the gut. They can also enhance cellular immunity by reducing inflammatory responses and improving metabolic processes. In addition, they can increase lipid metabolism and reduce the accumulation of fat in the body by stimulating the expression of insulin-like growth factor-1 (IGF-1), which increases fat synthesis.
Several studies have shown that dietary probiotics can significantly enhance the reproduction of aquatic organisms. For example, in European seabass, a bacterial preparation containing Bacillus subtilis and Saccharomyces cerevisiae increased both the number of eggs per female and the total number of alevins. The preparation also decreased the concentration of the stress hormone cortisol and increased mRNA expression of antioxidative enzymes, which improved the survival of the embryos.
In another study, Abasali and Mohamad incorporated a commercial probiotic containing Lactobacillus acidophilus, L. casei, Enterococcus faecium, and Bifidobacterium thermophilum into the feed of four ornamental fish species—Poecilia reticulata, Poecilia sphenops, Xiphophorus helleri, and X. maculates—over a one-year experiment. They found that the presence of B. subtilis at a concentration of 106-108 cells g-1 of food significantly increased the number of eggs and total fecundity of the females in all four fish species. In addition, the amount of complex B vitamins synthesized by these bacteria remarkably increased fecundity and egg production.
2. Prevent Diseases
Many diseases are caused by pathogenic microorganisms that can have significant impact on aquaculture. In order to control the occurrence of such diseases, vaccines or immunostimulants are usually employed. However, owing to the increasing resistance of bacteria to antibiotics, there is a growing need to find alternatives for disease prevention. This is where aqua probiotics come into play.
Lactic acid bacteria and other species that produce beneficial metabolites have been considered potential candidates as aqua probiotics. One such bacterial strain is Enterococcus faecium, which produces bacteriocins, which are antimicrobial agents that can kill pathogenic microorganisms in the gut of aquatic animals. Enterococcus faecium is also known to colonize the intestinal epithelial cells, as well as to adhere to fish enterocytes. This means that the lactic acid bacteria can effectively prevent the entrance of other pathogens into the digestive tract by competitive exclusion
Studies have shown that dietary probiotic supplementation with Pediococcus acidilactici improves survival, body composition and feed conversion in Javanese carp (Cherax tenuimanus). Furthermore, Lactobacillus plantarum was found to be effective as an aqua probiotic in improving transport stress resistance and survival of first feeding turbot larvae. However, the selection of suitable bacteria is important to ensure high levels of viability and stability during production and storage, as well as to ensure that no antibiotic resistance or virulent plasmids are transmitted to the host gastrointestinal tract.
3. Prevent Infections
In addition to their role in stimulating the immune system, some aqua probiotics have direct anti-microbial properties and suppress pathogenic microorganisms. This mode of action can be through competitive exclusion (by occupying spaces or nutrients that could be used by the disease-causing bacteria), by producing inhibitory compounds, or by directly attacking the organisms through secreted toxins. Probiotics have been shown to be more effective than antibiotics in inhibiting the growth of aquaculture pathogens, with some studies showing protection against multiple different strains in just two weeks.
The use of probiotics in preventing diseases in aquatic species is an area of increasing interest, especially as an alternative to antibiotics. They can prevent infections in several ways, including modulating the hypothalamus-pituitary-interrenal axis and regulating the oxidative stress system. They can also prevent gill parasites by boosting mucosal immunity, although this effect may only last for a limited time after the treatment.
A probiotic bacterium, Pediococcus acidilactici, has been shown to reduce the number of dead and deformed alevins in a one-year experiment in Nile tilapia. This is thought to be linked to its ability to increase the number of mature flies, as well as by reducing the levels of cortisol in the blood, which can lead to stress and reduced immune response.
Probiotics can also increase the production of short chain fatty acids in fish, which improves gut digestion and immune function. In addition, a probiotic bacterium, Shewanella putrefaciens Pdp11, was found to improve condition and growth in Senegalese sole larviculture, possibly by regulating the microbiota and enhancing gut immunity.
4. Prevent Reproduction
Probiotics are beneficial microorganisms that can be ingested to increase the health of animals. They can stimulate growth, improve digestion and immune response, and prevent diseases. They can also help to improve water quality and pond management. Increasingly, the use of probiotics is being applied to aquaculture in order to elevate production.
Studies have shown that the bacteriocins produced by certain probiotic bacteria can inhibit the growth of pathogens and other unwanted organisms. This makes them a viable alternative to antibiotics for the control of diseases in aquatic species.
In addition, many of the same bacteriocins that inhibit the growth of pathogens also work to strengthen the immunity of the fish. This can be important for preventing disease in aquaculture species, particularly when there is overcrowding stress in the pond.
Some studies have even found that the administration of probiotics can prevent the occurrence of diseases in the pond that are associated with overcrowding stress, such as ulcerative skin disease. This may be because the bacteriocins from the probiotic bacteria can interfere with the inflammatory process that occurs in response to overcrowding stress. Other studies have found that the probiotic bacteria can prevent the overgrowth of cyanobacteria and other harmful algae. This can reduce the nutrient load of the water and help to improve water quality and habitat for fish.