The topic of gut health and gut stability has become increasingly important in poultry farming over the last few decades, certainly since the ban on the use of antibiotic growth promoters in 2006 (1). A healthy gut plays a crucial role in an animal’s well-being and has a positive effect on its overall health. Furthermore, the topic of gut health also plays an important role in terms of flock productivity and food safety (2). Anyone who looks into the topic of gut health in more detail will soon come across the terms gut microbiome and dysbiosis. But what exactly is the gut microbiome? What is its function? And when do we speak of dysbiosis?

The community in the gut – the gut microbiome

In all our farm animals, just as in humans, the gut is colonised by a multitude of different microorganisms. These include bacteria, as well as viruses, fungi and protozoa (3). The totality of all these organisms in the gut is referred to as the gut microbiome or gut flora. Compared to other livestock species such as cattle and pigs, the gastrointestinal tract in poultry is relatively short in relation to body length. The time it takes for the feed mash to pass through the gut is therefore also significantly shorter. As the residence time of the feed in the gut has a major impact on the composition of the microbiome, it is now known that the composition of the poultry gut microbiome differs significantly from that of other farm animals (4).

Colonisation of the gut by microorganisms begins immediately after hatching. A wide variety of microorganisms are taken up from the environment, during the handling of the chicks in the hatchery and also when the chicks first consume feed and water, before subsequently multiplying in the gut (3,5,6).

The diversity of microorganisms in the gut increases steadily during the first few weeks. Only after a certain period does a balance between the different microorganisms become established. In both broiler chickens and turkeys, this process takes three to four weeks (5, 8).

In chicks whose gut colonisation was prevented as part of scientific studies, the intestinal wall was thinner and the entire intestinal tract lighter compared to animals that came into contact with microorganisms from the environment. The gut microbiome therefore appears to promote the development of the gut in chicks (4).

How can the development of the gastrointestinal tract and the formation of a stable gut microbiome be supported?

It is important that chicks have access to high-quality feed and water as early as possible (3, 7). Until a stable balance has been established in the gut, the animals are more susceptible to pathogens. It is therefore particularly important during this early stage of life to ensure good feed and drinking water hygiene.

The composition of the gut microbiome can be influenced by the administration of prebiotics and probiotics. Prebiotics are feed additives that are indigestible to the animal but can stimulate the growth of beneficial microorganisms in the gut (9). Probiotics, on the other hand, are cultures of live microorganisms intended to contribute to the stability of the gut flora or to promote the proliferation of beneficial gut organisms (9).

Studies in broiler chickens have shown that the administration of lactic acid bacteria, Bacillus species or yeast cells, as well as hard-to-digest carbohydrates, has a positive effect on the development of microvilli in the intestinal mucosa (4). Microvilli are projections of the intestinal mucosa. They increase the surface area of the intestine and help to ensure that more nutrients can be absorbed.   

The gut microbiome – little helpers in digestion

The feed contains various forms of carbohydrates. Easily digestible carbohydrates are broken down in the small intestine of poultry with the help of enzymes and absorbed through the intestinal wall. However, not all carbohydrates can be broken down by this process. Hard-to-digest carbohydrates, such as crude fibre, are converted in the caecum into short-chain organic fatty acids such as butyrate. Butyrate is an important source of energy for intestinal cells and stimulates their growth. However, birds rely on the help of microorganisms in the gut for this conversion process (4).

Initial studies also suggest that the composition of the gut microbiome in poultry improves the activity of digestive enzymes (4). Furthermore, a high level of species diversity in the gut also appears to have a positive effect on feed conversion (5).

The gut microbiome thus influences the digestibility of various feed ingredients, meaning that the microorganisms contribute to better energy utilisation of the feed. Overall, a stable gut microbiome therefore has a positive effect on the productivity of a flock (5, 6).

The gut microbiome – competition for many infectious agents

Maintaining gut health is also a function of the gut microbiome. The ‘good’ gut bacteria compete with classic gut pathogens such as Clostridia, which can lead to very severe inflammation of the gut wall. To this end, gut bacteria have developed various mechanisms that help prevent gut infection by pathogens:

• In healthy animals, the intestinal mucosa is covered with a film of bacteria. These occupy binding sites on the intestinal mucosa which the pathogens, in turn, require to penetrate the intestinal mucosa. Pathogens are, so to speak, displaced by the bacteria of the microbiome. This is referred to as ‘competitive exclusion’ (4). This mechanism of action is also utilised in the use of probiotics.
• Bacteria in the microbiome promote the formation of a mucus layer on the intestinal mucosa. This mucus layer makes it more difficult for pathogens to penetrate the intestinal wall (4).
• Some gut bacteria produce substances that inhibit the growth of other bacteria, including pathogens (4).

The immune system also benefits from a stable microbiome

The bacteria of the microbiome not only compete with pathogens, they also support the immune system. For instance, chicks whose intestines were free of microorganisms showed lower antibody concentrations in the gut during examinations. Furthermore, the Peyer’s patches in these animals were less well developed (4). Peyer’s patches are part of the intestinal lymphatic tissue. They form part of the adaptive immune system, which only develops after the animals hatch upon their first contact with pathogens. The proper development of Peyer’s patches is therefore important for successfully combating intestinal infections. The results suggest that the microbiome also has stimulating effects on the development of the animals’ immune system.

Furthermore, the microbiome promotes the formation of so-called defensins. These antimicrobial substances are produced by the intestinal cells and serve as a further defence mechanism against pathogens (5).

Dysbiosis – when the balance is disrupted

An imbalance in the gut is referred to as dysbiosis. Dysbiosis is therefore not a specific disease caused by a pathogen (6). Nevertheless, infections with intestinal pathogens are also associated with the development of dysbiosis.  

There are many factors that influence the balance in the gut and can lead to the development of dysbiosis (6, 8):

• Poor development of the gut
• Change in diet
• Vaccinations
• Poor quality of feed / raw materials
• Mycotoxins
• Lack of biosecurity
• Problems with house climate (ventilation, temperature)
• Hatching conditions
• Infections
• Poor drinking water quality

Dysbiosis is observed in poultry primarily in connection with stress (e.g. due to poor house climate), infections with pathogens such as coccidia, or as a reaction to a change in feed (6,8).

They lead to changes in the permeability of the intestinal wall (1). More water enters the intestine and the consistency of the faeces becomes more liquid. This is accompanied, amongst other things, by an increase in litter moisture (6), which promotes secondary conditions such as footpad inflammation.

In addition, the increased permeability of the intestinal wall allows substances from the interior of the intestine to migrate into the intestinal wall, where they can trigger inflammation (1).

Dysbiosis increases the risk of (6):

• reduced nutrient absorption
• poorer feed conversion
• lower live weights

How can gut stability be maintained?

The foundation for good gut stability in animals is therefore laid during the first few weeks of life. Poultry farmers should ensure that the animals consume good-quality feed and water as soon as possible after being placed in the house. Only in this way can colonisation and, consequently, the development of the gut proceed. Stress factors, such as vaccinations or changes in feed, cannot be avoided. However, care should be taken to ensure that vaccination and feed changes are not carried out at the same time, in order to keep the stress on the animals as low as possible.

Studies have shown that prebiotics and probiotics can have positive effects on the composition of the gut microbiome. There is now a wide range of products on the market that can be administered via both feed and water. These products can be used to preventatively promote the development of a stable gut microbiome (6).

In cases of dysbiosis, for example, probiotics and the administration of organic acids can help to restore a suitable environment in the gut and establish a balance between the microorganisms (6).

It is important to note that many factors contribute to good gut stability. Therefore, when dealing with gut health issues, one should not only consider infectious diseases but also take a look at the current management of the herd.