With the development of aquaculture large-scale, intensive and intensive techniques, the residual bait, excreta and other organic pollutants in the pond are also increasing. The decomposition of organic pollutants requires a large consumption of dissolved oxygen and a large amount of toxic and harmful substances. Such as ammonia nitrogen, nitrite nitrogen, sulfide and the like. With the increase of these toxic and harmful substances, it not only affects the growth and reproduction of aquatic animals, but also causes serious poisoning and death. The amount of pathogenic microorganisms in the water body is directly related to the concentration of ammonia nitrogen, nitrite nitrogen and sulfide in the water. For example, one of the pathogenic conditions of bacterial sepsis in freshwater fish is the deterioration of water body, and the ammonia nitrogen and nitrite nitrogen are obviously high. Therefore, how to effectively regulate the water quality of aquaculture water has become a key issue in the aquaculture industry. Since the 1980s, China has begun to use microecological preparations as water quality improvers in aquaculture. The main types of microecological preparations used are photosynthetic bacteria. Although photosynthetic bacteria have achieved good results in aquaculture production, they have shortcomings such as long production cycle, short shelf life, and unstable quality. Bacillus is widely used in aquatic animals because of its good stability, strong resistance, high temperature resistance, acid and alkali resistance, rich enzyme production, inhibition of pathogen reproduction, promotion of animal nutrition digestion and absorption, decomposition of organic pollutants, purification of water quality, etc. In culture, Bacillus subtilis and Bacillus licheniformis are the most widely used strains in aquaculture. These two species of Bacillus are listed as safe use strains by the Ministry of Agriculture.
1 Characteristics of the function of Bacillus
At present, aquatic microbial microorganisms mainly use Bacil-lus, Lactobacillus, Yeast, and Photosynthetic bacteria. Lactic acid bacteria, yeast and photosynthetic bacteria have poor resistance and short shelf life. In the process of feed processing, almost all of them are inactivated after high temperature and high pressure granulation, and the Bacillus products are stable in both granular materials and powder materials.
As a strain of aquatic microbial feed additive, Bacillus is used in production later than photosynthesis bacteria, nitrifying bacteria, lactic acid bacteria and other strains. With the application practice in aquaculture in recent years, Bacillus microbial feed additives have shown the following advantages:
(1) It has good stability, strong resistance, high temperature resistance, acid and alkali resistance, and resistance to extrusion. It can maintain high stability during industrial production and granulation and through the culture of animal acid stomach. Once formed, spores can withstand various adverse conditions, such as dry heat, damp heat, ultraviolet rays, radiation, strong acid, strong alkali, organic solvents, vacuum drying, oxidation, etc.;
(2) It can produce nutrients such as vitamins. Bacillus can produce B1, B2, B6 and other B vitamins, vitamin C and vitamin K2 during the growth and reproduction process, providing vitamin nutrition for aquatic animals. Daily consumption of Bacillus natto can produce a large amount of vitamin K2;
(3) in the form of dormant spores during storage, do not consume the nutrients of the feed, and ensure the quality of the feed;
(4) spores can quickly resurrect in the upper part of the intestine with the feed entering the intestine, and the resurrection rate is high, almost 100%;
(5) Revived spores can produce a variety of enzymes, which help to degrade some complex carbohydrates in plant feeds, thus maintaining the intestinal ecological balance of farmed animals;
(6) inhibiting the growth of pathogenic bacteria and promoting the growth of beneficial bacteria;
(7) It can be used as a water quality regulator to purify the aquaculture water environment.
2 Application of Bacillus in aquaculture
2.1 Reduce disease occurrence and promote animal growth
Disease problems are a major problem that plagues aquaculture. In the process of breeding, the occurrence of disease control can be considered from the following two aspects: on the one hand, it inhibits the increase of pathogenic bacteria and reduces the chance of disease occurrence; on the other hand, it improves the immunity of farmed animals and enhances the resistance to disease. Bacillus showed a certain effect in both aspects. Antagonism is a natural phenomenon that prevails between microorganisms. In the same niche, microorganisms inhibit the growth of other microorganisms through nutrient competition, spatial competition or secretion of antibiotics, bacteriocins, etc. When the beneficial bacteria predominate in the digestive tract to form a dense membrane flora, the pathogens adhering to the digestive tract can be inhibited. This helps to establish a normal beneficial microflora and to exclude or control potential pathogens. The probiotic effect of Bacillus is based on this mechanism. Sugita et al. isolated a strain of Bacillus with strong antibacterial activity from offshore fish. Further research found that it can produce a heat-labile iron carrier and compete with iron for pathogenic bacteria. Gatesoupe and other studies believe that Bacillus can inhibit the growth of pathogens by releasing antibiotics. Bacillus can also improve the cellular and humoral immunity of the body, such as increasing the activity of interferon and macrophages, and stimulating the production of non-specific immune factors.
A large number of studies have shown that Bacillus feeding bacteria have an effect of antagonizing intestinal pathogenic bacteria and improving disease prevention ability. Bacillus can produce volatile fatty acids such as acetic acid, propionic acid and butyric acid during its growth. These acids can reduce the intestinal pH of animals, effectively inhibit the growth of pathogenic bacteria, and consume anaerobic oxygen in the intestinal tract by biological oxygenation. The environment is conducive to the reproduction of the dominant bacteria of the intestines and maintain the normal ecological balance of the intestines. Rengpipat et al. fed the P. monodon with the Bacillus sp. S11 feed. After 100 days of feeding, the shrimp was placed in the water containing the shrimp pathogen Vibrio harveyi for 10 days. The results showed that the survival rate of the shrimp fed with Bacillus feed was 100%. The survival rate of the control group was only 26%. Pan Kangcheng et al reported that sprinkling Bacillus subtilis in water significantly increased the survival rate of Penaeus vannamei and reduced the mortality rate. The average survival rate of the experimental group was 31.05% higher than that of the control group.
2.2 Produce a variety of enzymes to improve the digestive enzyme activity of animals.
Bacillus produces a variety of enzymes with strong protease, amylase and lipase activities; it also degrades the enzymes of complex compounds in feeds, such as pectinase, glucanase, cellulase, hemicellulase , glucose isomerase, and the like. Many studies have shown that the addition of Bacillus in feed can significantly increase the activity of digestive enzymes in farmed animals. Liu Xiaogang et al added 0.2% Bacillus in the feed. The protease activities of the intestinal and hepatopancreas of the gibel carp were 466.2% and 85.4% higher than those of the control group, respectively. The addition of 0.1% Bacillus, intestinal and hepatopancreatic amylase The activity was 83.7% and 129.5% higher than the control group, respectively. The addition of Bacillus preparation to the feed showed that the protease, lipase and cellulase of the experimental group were significantly higher than that of the control group, and the survival rate and weight gain rate of the shrimp were better than the control group. Liu Bo et al. added Bacillus licheniformis to the diet to feed the gibel carp. The results showed that the addition of 2O0 mg/kg and 300 mg/kg of Bacillus licheniformis significantly increased chymotrypsin, amylase activity and intestinal cathepsin activity. Fish body weight gain rate and apparent digestibility of dry matter, crude protein and phosphorus, and reduced feed coefficient. It can be seen that the digestive enzyme activity can reflect the degree of digestion and utilization of feed nutrients by the body to a certain extent.
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