The microbiota is the name given to all microorganisms that live in the digestive tract. The interactions between the host (the animal) and the flora it hosts (in its digestive tract) have been known for a long time. They are very numerous and can be beneficial or on the contrary cause health problems. What do we know today about the microbiota in fish? What is the significance of this for breeders and nutritionists?
What is a microbiota?
The microbiota refers to all the microorganisms living sedentarily or transiently in an animal's digestive tract.
The attention paid to the microbiota by nutritionists has increased considerably in recent years. The existence of links between the 'digestive flora' and health has been known for many years. The introduction of pathogenic bacteria into the digestive tract is responsible for some of the most important diseases in aquaculture (such as Yersinia ruckeri, the enteropathogen responsible for yerciniosis, or Ewardsiella ictaluri). Conversely, the ingestion of certain live bacteria can improve the digestive comfort of farmed animals. This is the case with probiotics, whose beneficial effects on fish and shrimp have been widely demonstrated.
The fish microbiota is mainly made up of bacteria. Its composition is dynamic. It changes according to parameters such as diet, water quality and temperature (in number and type). The use of antibiotics will also have a major impact on this population.
Microorganisms in the digestive tract are extremely numerous (around 106 to 109 Colony Forming Units per g of intestinal content). The concentration of microbes differs from one segment of the digestive tract to another. Generally speaking, in monogastric animals, the concentration of microorganisms increases from the stomach (which is inhospitable, particularly because of its low pH) towards the anus.
Microorganisms are often differentiated according to whether they are resident (characterised by their ability to colonise the surface of segments of the digestive tract) or transient (microorganisms that develop in the lumen of the digestive tract). A classification is also used based on the pathogenicity of the microorganisms to their host (the fish). These are referred to as commensal (neutral), symbiotic (beneficial to the host) or pathogenic (deleterious) microorganisms. The digestive comfort or discomfort of animals is largely the result of the dynamic balance between these different populations.
Role of the microbiota
We still know very little about the fish microbiota. However, data is beginning to accumulate and we now know some of its functions. Its role in animal nutrition is currently being investigated. Nutritional contributions include the production of vitamin B12, the production of exogenous enzymes and the supply of fermentation products such as volatile fatty acids. The microbiota also plays a major role in maintaining the health of the host. Knowledge of the various functions of the microbiota in fish is still in its infancy and still requires a great deal of research, but the possibilities are enormous.
It is still too early to define what constitutes a good or bad microbiota for a given species. Nevertheless, the data collected to date has shown the influence that factors such as raw materials, veterinary treatments, the living environment and certain feed additives can have on the microbiota and performance of farmed fish. Characterising the fine composition of the microbiota that enables the best performance could be a new tool available to nutritionists in the years to come.
Among the additives that influence the composition of the microbiota of farmed fish are probiotics and prebiotics. Probiotics are live bacteria whose ingestion improves the animal's performance. The theory is that a probiotic is administered live so that it transiently colonises the animal's digestive tract. Prebiotics, on the other hand, are sugars that cannot be digested by the host and which are metabolised efficiently by certain microorganisms in the digestive tract, thereby favouring their development to the detriment of other organisms.
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