Animal Nutrition

Probiotics, prebiotics, and phytogenic substances for optimizing gut health in poultry. Part 1

To read more content about AviNews March 2024

Guillermo Tellez-Isaias

Hafez M Hafez

Juan D. Latorre


The permeability of the intestinal tract controls the uptake of nutrients and the transport of unwanted extracellular substances such as bacteria and xenobiotics, in addition to the nondigested substances. Therefore, gut health plays an essential role in the pathogenesis of various intestinal disorders.

The permeability of the intestine is controlled by gut microbiota, digestive secretions, physical barriers (mucin, intestinal epithelial cells lining and tight junctions), and chemicals such as cytokines.


Under normal conditions, the symbiotic relationship between the gut microbiota and the host crucially determines intestinal health. However, a disturbance in the gut microbiota can lead to an imbalanced host–microbe relationship, which is called “dysbiosis”.

  • Several factors, such as antinutritional factors in feed, heavy metals, toxic substances, bacterial toxins, herbicides, and antibiotics, can disrupt the gut microbiota.
  • These impacts can lead to localized inflammation, extensive infection, or even intoxication.
  • Additionally, the intestinal epithelium forms tight connections, acting as a biological barrier that controls the paracellular transit of different materials across the intestinal epithelium, including ions, solutes, and water.
  • It also functions as a barrier of extracellular bacteria, antigens, and xenobiotics.


The impaired intestinal barrier function, commonly known as “leaky gut”, is a condition in which the small intestine lining becomes damaged, leading to infiltration of luminal contents such as bacteria and their associated components including toxins to pass between epithelial cells.

  • These conditions subsequently lead to cell damage and/or inflammation of the intestine, characterized by increased levels of bacteria-derived endotoxins in blood.
  • This inflammatory process consumes significant amounts of nutrients, and, subsequently, has negative effects on metabolic responses, in particular on immunometabolic and endocrine responses.
  • As a result, animal performances are severely reduced.

Additionally, field observations in Europe showed that the poultry industry faced several problems after the ban of antibiotic growth promoters (AGPs), including negative impacts on performance, animal welfare aspects, and general health issues.

In this review, we discuss the role of these alternatives in maintaining gut function through modulation of the gut microbiota and the related effects benefitting health and quality of poultry.

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Microorganisms that live in animals’ gastrointestinal tracts (GITs) are a prime example of beneficial bacteria. Indeed, the GIT is the home of a diverse and plentiful microbial community providing essential functions to their host animals.


Impact on development and function

Although the intestine is exposed to microflora components from birth or hatching, little is known about their impact on healthy development and function.

Most of these intestinal microflora’s species cannot be cultured when they are removed from their niches, as is the case with most complex ecosystems.

Colonization of avian guts could already start during embryogenesis and progresses to the formation of a complex and dynamic microbial society.

Comparing germ-free rodents that were raised without exposure to microorganisms to those that built up a microbiota since birth, or those that were colonized with microbiota components during or after postnatal development, a variety of host functions influenced by indigenous
microbial communities were identified.

The microbiota, for example:

GutFigure 1. Factors affecting the gut microbiota composition modified according to Carrasco et al. (figure was created with, accessed on 15 December 2021).

Besides, Proteins and protein breakdown products, sulfur-containing substances, and endogenous or foreign glycoproteins can all be metabolized by the microflora.

More than 90% of all gut microbiota species in humans and animals belong to the phyla Bacteroidetes, Firmicutes and Actinobacteria, others are Fusobacteria, Proteobacteria, Verrucomicrobia, and Cyanobacteria.

In chickens, the phyla Bacteroidetes and Firmicutes are the most predominant representatives in the gut. In human and several animals, the ratio between Firmicutes and Bacteroidetes is a health/metabolism-associated marker.


It is also assumed that the molecular principles that aid in the modification and maintenance of normal physiological functioning of the gut microbiota are mainly derived from food and its supplements, such as nutraceuticals.

Nutraceuticals can include everything:

These nutraceuticals also aid in the prevention of infectious diseases of the host.

Additionally, several multidrug resistance bacteria have emerged making this crisis global. Nutraceuticals will be required to reduce the use of antibiotics.

Lactic acid bacteria

Lactic acid bacteria have been used as feed supplements since pre-Christian times when humans ingested fermented milk.

Progress in bacteriology and the easier availability of germ-free animals helped to assess the impact of newly identified intestinal occupants on the host.

GutFigure 2. Microbiota in chickens, summarized from Shang et al. (figure was created with, accessed on 15 December 2021).


Enterocytes are the cornerstone of the intestinal mucosal monolayer that protects the host from the external environment. A scheme of the intestinal epithelial barrier and some interactions with intestinal microbiota is shown in Figure 3.

The formation and function of tight junctions are controlled by intracellular signal transduction pathways:

The disruption of tight junctions by bacterial factors can occur in the following steps:

Finally, these processes lead to an increase in intestinal permeability. Thus, leaky gut syndrome develops as a response to pathogens, feed deprivation, and stress.

Figure 3. Intestinal epithelial barrier and intestinal microbiota interaction.


The interactions between the epithelial barrier function, intestinal inflammation, and the microbial environment influence gut health. Therefore, the discovery of reliable, widespread biomarkers to measure intestinal inflammation and barrier function is an important ongoing area of research. A summary of some of the known biomarkers related to intestinal health is presented in Table 1.

Inflammation can also be associated with oxidative stress and changes in the expression of genes related to oxidative stress, indicating that oxidative stress may have a critical role in the physiological intestinal function.

One quantitative technique that is used to evaluate the integrity of tight junction proteins in epithelial cell monolayers is the measurement of transepithelial electrical resistance (TEER). Mitochondrial respiration is required to maintain TEER, implying that oxidation plays a critical role in Caco-2 cell tight junction stability.

According to Janssen-Duijghuijsen et al., reduced mitochondrial ATP production resulted in a decrease of intestinal permeability and an increase in occludin and claudin-1 gene expression, but a decrease in claudin-2 and claudin-7 gene expression.

Biomarkers of oxidative stress

Often, oxidative stress is quantified by examining metabolites formed during or after an oxidative process.

Table 1. Potential biomarkers to evaluate intestinal health. Adapted from Chen et al.and Baxter et al.

Biomarkers of intestinal health

Biomarkers for the evaluation of intestinal health can also be related to monitoring intestinal function.

Biomarkers related to the immune activity

In the case of biomarkers related to immune activity that can influence intestinal health.

Ultimately, both innate and adaptive immune responses are likely to provide viable biomarkers for assessing intestinal health.

Histomorphological analysis

The histomorphological analysis is another type of evaluation closely influenced by an adequate balance of the intestinal environment.

Biomarkers of intestinal permeability

Bacterial translocation and gene expression of TJ such as claudins, occludins, and zonula occludens (ZO-1) are intestinal permeability biomarkers used to evaluate gut health.

Bacterial translocation has been related to diseases such as chondronecrosis with osteomyelitis in broiler and broiler breeders, suggesting the migration of enteric pathogens to the thoracic vertebrae.

TJs such as occludin have shown to be downregulated in human patients with inflammatory bowel diseases (Crohn’s disease), and in chickens under nutritional gut health challenge condition models, therefore revealing the fundamental role of TJs such as occludin in maintaining intestinal barrier integrity.

A different set of biomarkers candidates include the fatty acid binding proteins (FABP), which are intracellular lipid chaperones in charge of orchestrating lipid metabolism and critical lipid-sensitive pathways in macrophages and adipocytes.

Another well-known biomarker that has been utilized in poultry to evaluate intestinal permeability is the measurement of fluorescein isothiocyanate dextran (FITC-d) in the serum.

Some non-invasive biomarkers that are currently being studied in fecal samples by different research groups are fibronectin, calprotectin, and lipocalin. These biomarker candidates have shown promising results in chickens; nevertheless, there have been also inconsistencies between studies.

Ultimately, the objective is to continue searching for intestinal health biomarkers that can be easily measured from samples that do not require an extensive preparation time or cost.




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