Formation of gut microbiota in aquatic animals
It is generally believed that newly hatched aquatic seedlings do not have relevant bacterial strains colonizing their intestines. When they come into contact with water, microorganisms in the water environment will preferentially adsorb into the intestines, gradually forming a gut microbiota. Sugita et al. first reported in 1985 that the gut microbiota of juvenile tilapia (Oreochromis mosambicus) was established within 20-60 days after hatching, and inferred that these microorganisms may come from water bodies, bottom sediments, or feces of other fish. Gatesoope et al. have also reported similar findings, indicating that bacteria can enter the gut through the oral cavity of aquatic animals, and even some strains can become dominant bacteria in the gut at the first feeding start of aquatic animals. Due to the fact that most aquatic animals breathe through their gills and consume food through their mouths, the food they feed and the bacteria they come into contact with in the water environment enter their bodies and settle in their intestines after adapting for a period of time, thereby affecting the formation of gut microbiota in aquatic animals.
Composition of gut microbiota in aquatic animals
The gut microbiota of aquatic animals is divided into two categories: normal microbiota and transient microbiota. The normal microbiota is composed of inherent bacteria that regularly colonize the intestinal tract of fish. Under normal circumstances, it does not cause disease to fish and is beneficial and necessary for aquatic animals. Probiotics are one of the normal microbiota. The gut microbiota of different marine and freshwater aquatic animals is composed of different types of bacteria, and the number of bacteria contained is not fixed. The types and quantities of bacteria may vary with breeding time, food composition, main diet, hunger and satiety status, and living environment. Numerous studies have shown that the gut bacterial species of marine aquatic animals are mainly Bacillus, Vibrio, and Escherichia, with slightly higher gut bacterial counts ranging from 106 to 108 CFU/g. In contrast, the gut bacterial species of freshwater aquatic animals are mainly Lactobacillus, Bifidobacterium, and Aeromonas, with gut bacterial counts ranging from 105 to 107 CFU/g.
