Bacillus-based probiotics shaping a resilient intestinal environment

The use of probiotics aligns with the growing demand for antibiotic-free poultry products, making it a sustainable and consumer-friendly option in modern poultry farming.

Bacillus-based products are popular in poultry production because of several key advantages. They form resilient spores, which can survive harsh environmental conditions, including feed processing and the acidic environment of the gastrointestinal tract. Some strains, such as Bacillus subtilis DSM 29784 (Bs29784, Alterion NE, Adisseo) can germinate rapidly, giving rise to a substantial number of viable vegetative cells in the intestines. These cells will then secrete a diverse array of metabolites specific to the bacterial strain.

Several of these metabolites, such as surfactins, fengycins, subtilins and bacillaene, exhibit antibacterial and antifungal properties. They enable Bacillus to compete with other microorganisms in its natural environment. It is therefore unsurprising that commercial probiotic strains are often selected based on in vitro assays that assess their antimicrobial properties against enteric pathogens in poultry, such as toxic Clostridium perfringens strains, or zoonotic species such as certain Salmonella serovars, which can cause foodborne gastrointestinal disease in humans.

While such assays are useful to select promising bacterial strains from a microbial library, research into host-microbiota interactions suggest that the beneficial role of probiotic additives in vivo is not, or not solely, due to a direct and selective effect against a few pathogenic bacteria in the intestine.

Indeed, to understand the potential effect of probiotics on enteric pathogens, it is important to consider how these pathogens have evolved to take advantage of any disruptions in intestinal homeostasis.

Salmonella taking advantage of inflammation

Salmonella, for instance, is a member of the Enterobacteriaceae, whose relative abundance is typically low in the intestine of birds with a healthy developed microbiota. In the hindgut of chickens, they will be outnumbered by anaerobic bacteria that break down non-digestible fibres and produce anti-inflammatory short-chain fatty acids (SCFA).

However, intestinal inflammation will change enteric cell biological pathways in a way that Salmonella can exploit. For example, oxygen delivered by blood vessels in the lamina propria is normally consumed by epithelial cell to oxidise SCFA. Inflammation, however, can lead to vasodilation and an impaired epithelial barrier, allowing oxygen to diffuse into the lumen. There it will have a toxic effect on the anaerobic SCFA-producers, reducing competition for Salmonella.

In addition, inflammation triggers the production of reactive oxygen species (ROS), which will enter the intestinal lumen where it can react with endogenous sulphur-containing compounds to generate tetrathionate. This molecule can be used by Salmonella as an electron acceptor, giving Salmonella a metabolic resource that it can use to proliferate and further outcompete the remaining beneficial microbes.

Clostridium perfringens: breaking cells to liberate nutrients

Pathogens taking advantage of a disrupted intestinal homeostasis is a common feature of several gastrointestinal diseases. Certain Clostridium perfringens strains, for example, secrete toxins and enzymes that degrade the protective mucus and epithelial lining of the gut. This leads to inflammation and cell death and nutrient leakage into the lumen, fuelling proliferation of Clostridium perfringens and triggering a vicious cycle of further inflammation, tissue damage and pathogen growth.

These dynamics highlight that, for a probiotic to be able to minimise the impact of intestinal pathogens, it is crucial it can make the natural intestinal ecosystem as robust as possible, keeping it unfavourable for opportunistic pathogens. In practice, this means controlling inflammation and luminal redox status and maintaining epithelial health.

Bs29784 eliminates opportunities for opportunistic pathogens

To assess the potential of the probiotic strain Bs29784 in maintaining gut health under enteric challenge, several experiments where conducted. In one trial, Wang and co-workers (2021) used a subclinical necrotic enteritis model in broilers. In the jejunum, this Clostridium perfringens-induced challenge increased apoptotic markers and inflammatory cytokines, but Bs29784 supplementation markedly reduced these effects.

The positive impact of strengthening these natural pathogen-restricting mechanisms was made clear in a trial where Bs29784 and/or an antimicrobial growth promotor (BMD) was fed to broilers exposed to a higher bacterial environmental load via dirty litter. After 42 days, caecal content showed that Bs29784 supplementation reduced E. coli, Salmonella, and C. perfringens levels as effectively or better than BMD (Figure 2). Notably, unlike BMD, Bs19784 increased the amount of Lactobacillus.

In summary, Bs29784 can help to control enteric pathogens by improving overall gut health, thereby contributing to healthier flocks and more efficient production systems.