Lactic acid bacteria are Gram-positive bacteria ferment carbohydrates, such as sugars, into lactic acid. Due to their numerous beneficial effects on fish and shrimp, aquaculturists employ them extensively.
Studies reveal that lactic acid bacteria can promote growth performance in fish and shrimp, regulate the ecological balance of gastrointestinal microflora, improve gastrointestinal function, inhibit the growth of pathogenic bacteria in the intestine, and ultimately enhance immunity and increase shrimp farming success rates.
Lactic acid bacteria can produce various enzymes, such as amylase, lipase, and protease. They break down large, difficult-to-absorb molecules into smaller, more easily digestible components for shrimp. Additionally, they synthesize essential nutrients for shrimp growth, such as niacin, pantothenic acid, folic acid, vitamin B1, vitamin B6, and vitamin B12, thereby promoting growth.
By colonizing the intestinal wall of shrimp, lactic acid bacteria reduce the pH of the intestine, stimulate intestinal motility, and accelerate the expulsion of pathogenic bacteria. They also exert inhibitory effects on certain pathogenic bacteria.
Lactic acid bacteria can strengthen shrimp immunity through various pathways, mainly by interacting with aquatic animals' intestinal mucosal immune systems. They activate T2 cells and enhance SIgA antibody secretion in cellular immunity while stimulating macrophages, NK cells, and B lymphocytes and increasing the production of leukocyte interleukins in humoral immunity. Moreover, they enhance the activity of immune enzymes, such as catalase, lysozyme, and superoxide dismutase, in the animal's body or body fluids.
Lactic acid bacteria exhibit a strong adsorption effect on aflatoxin B1. By binding aflatoxin B1 to the bacterial cell walls components, such as polysaccharides and peptidoglycan lipids, a detoxifying aflatoxin B1-lactic acid bacteria complex is formed and expelled from the body. The adsorption capacity is proportional to the number of bacterial cells, independent of temperature, time, and bacterial viability.
They transform harmful organic matter, such as leftover feed, feces, and dead algae, into phosphates, nitrates, and carbon dioxide via oxidation, nitrogen fixation, ammonification, nitrification, and denitrification. This process purifies water quality, increases dissolved oxygen levels, maintains water cleanliness, and promotes the healthy growth of aquatic animals.
As heterotrophic anaerobic microorganisms, lactic acid bacteria thrive in low-oxygen or anoxic conditions. They cannot synthesize many organic compounds and require an external supply of nutrients and growth factors for optimal growth.
Lactic acid bacteria grow best in slightly acidic environments, with an optimal pH of 5.5 to 6.5. Extreme pH levels are unfavorable for their growth. As nutritionally fastidious strains, they have specific nutrient requirements, relying on monosaccharides and disaccharides as carbon sources, along with some amino acids and vitamins.
Organic matter in aquaculture water, such as leftover feed, feces, and dead algae, primarily consists of large-molecular-weight proteins, cellulose, and starch, mostly unsuitable for lactic acid bacteria to utilize. Therefore, when using lactic acid bacteria to treat water, it is necessary to add an appropriate carbon source (such as brown sugar or molasses) to increase the water's carbon-to-nitrogen ratio and promote bacterial growth.
Generally, it is advised to apply lactic acid bacteria as early as possible after daybreak. This maximizes the utilization of their metabolic products by waterborne algae and bacteria, enabling the rapid establishment of a healthy aquaculture water ecosystem.
