Here we clarify the terminology used regarding probiotics, prebiotics and synbiotics, and provide an overview of the main categories of commercially available products along with their relative strengths and drawbacks.
- A probiotic is “a live organism which, when given in sufficient quantities, confers a benefit to the host” (FAO/WHO, 2001). Probiotics are also called direct-fed microbials, or DFMs.
- A prebiotic is a non-digestible additive often consisting of natural dietary fibers such as fructooligsaccharides (FOS) that stimulate the growth and activity of beneficial bacteria in the colon, thus improving host health (Gibson and Roberfroid, 1995).
- A synbiotic is a combination of probiotic and prebiotic products (Patterson and Burkholder, 2003).
Categories of commercial probiotics
Currently, there is a range of probiotic products available globally. These products fall into several categories; single strain, multi-strain, multi-strain / multi-species and synbiotic products. There are also a few multi-genus products available, but due to the complexity of cultivation and stabilization, these products are rare. Table 1 provides an overview of the major probiotic bacteria species used. The modes of action of the various probiotic species differ between performance enhancement and gut health improvements, including competitive exclusion of pathogens.
Table 1: Main species of probiotic bacteria
|Genus||Major species used|
|Bacillus||B. subtilis, B. licheniformis|
|Lactobacillus||L. acidophilus, L. bulgaricus, L. reuteri, L.salivarus, L. sobrius|
|Bifidobacterium||B. animalis, B. bifidum|
Two major categories of probiotics
Most commercial probiotic products fall into two major categories. First, the sporulated Bacillus spp., both single and multi-species, which remain in the lumen or outer mucus layer (dark green in Figure 1). Second, the lactic acid producing bacteria which, depending on their origins and mucus attaching capabilities, are either able to colonize the firmly attached mucus (light green in Figure 1) and the underlying epithelial wall itself like most Bifidobacterium animalis strains, or they are transient organisms like most Lactobacillus acidophilus strains, which are not colonizing said areas, similar to Bacillus spp..
Figure 1. Mucus layers of the gastrointestinal tract
Adapted from Atuma et al., 2001.
Understanding Bacillus spp.
Starting with the small intestine, many of the Bacillus species remain in the lumen or the upper layer of the loosely adhering mucus layer of the gut, and excrete proteases or non-starch polysaccharide enzymes, thereby increasing nutrient digestibility. This reduces the availability of easily fermentable nutrients to pathogenic bacteria, especially in the hindgut. Bacillus species also have the ability to secrete some bacteriocins (metabolites that selectively inhibit bacterial growth), which has a positive effect in modulating pathogenic bacteria like Clostridium perfringens coming from the feed.
As described above, Bacillus spp. are not colonizing bacteria but transient microorganisms. This means they are unable to attach to the epithelial layer of the gastrointestinal tract, thereby inherently limiting any direct immune modulation in the bird. In addition, many Bacilli are isolated from soil, not poultry, limiting the host immune interaction further. However, the increase in protein digestibility combined with the bacteriocin production reduces the risk of gut inflammation from C. perfringens, thereby improving the bird’s health and performance, and reducing the need for therapeutic antibiotic treatments in an indirect fashion.
Although Bacillus spores provide the natural protection of a probiotic, the spores are not metabolically active. Consequently, they need to be activated throughout the digestive process in the chicken, which is done with temperature over time and through the presence of free amino acids in the small intestine. There are ongoing debates as to the time it takes for spores to germinate in order for their protective and digestive capabilities to take effect in a bird’s gastrointestinal tract.
Scientific studies indicate that spore germination is triggered by moisture (here saliva), temperature over time and free amino acids, as well as a sensitivity to acid of the viable Bacillus. This indicates that most activity can be expected past the gizzard, when the pH of the digesta is close to neutral, starting in the duodenum.
Lactic acid producing bacteria
The lactic acid producing bacteria (LAB), for example Lactobacillus spp., Pediococcus spp. and Enterococcus spp., can be derived from various sources, which may not be particularly poultry. This may have an impact on the ability of the individual bacteria species to adhere to the firmly attached mucus layer (light green in Figure 1) and intestinal cell lining, and therefore reducing the ability of the bacteria to competitively exclude pathogenic bacteria from attachment sites.
This attachment is important for the early development of the immune system as it is a time when the system is being imprinted in regard to its function. With 70% of the chicken’s immune system in the intestine, rapid development of this system is important to the future gut health of the bird.
As the name indicates, lactic acid producing bacteria have the ability to produce organic acids, primarily acetic and lactic acid, but in a few cases butyric acid as well. They are generally produced along with a large variety of bacteriocins. Acetic acid is also used by some of the other commensal bacteria in the cecum as a food source, for example it is utilized to produce butyric acids as a metabolite. This may help explain why there is an increase in natural butyrate production with probiotics, even when no butyric acid producing bacteria are included in the probiotic mix itself.
In addition, many of the pathogenic bacteria, such as avian pathogenic E. coli or Salmonella spp., are pH sensitive, so even localized production of organic acids in the firmly attached mucus layer, along with the bacteriocin secretion of probiotic bacteria will have a modulating effect of pathogens in the gastrointestinal tract. Colonization by a beneficial Bifidobacterium strain can improve a bird’s immune development, limiting the need for immune response, thereby reducing the nutrient loss for this process.
Two key factors for prebiotics
The success of a prebiotic relies on two factors:
- Being indigestible to pathogenic bacteria but able to stimulate the growth of beneficial bacteria, such as Bifidobacterium and Lactobacillus
- Being able to restrict the growth and colonization of pathogenic bacteria
The process of restricting pathogens while favoring beneficial bacteria is known as competitive exclusion, often abbreviated to CE. Competitive exclusion can only be achieved with live organisms through activity in the digestive tract.
Contention over true synbiotics
Some companies combine probiotic bacteria with prebiotic mixtures in order to produce a synbiotic product. The prebiotics advance the rapid establishment of the probiotic strains through the provision of an additional nutrient source. However, with low inclusion levels and localization in close proximity to the probiotic, the prebiotic effects will be more limited to the probiotic bacteria rather than the general microbiota, which can also utilize them. There is ongoing discussion that immune-modulating prebiotics, e.g. those used in combination with probiotic bacteriocin producing strains, where the prebiotic (e.g. immune modulatory yeast cell walls) is not in direct support of the probiotic, is, strictly speaking, not a true synbiotic.