Bacillus species are a special case among probiotics, the live microorganisms given to influence the gut flora: they form spores, a dormant form that survives the heat of processing, storage and the trip through the stomach. In healthy dogs, studies find small changes in stool consistency and odour, and mixed results on digestibility. The European Food Safety Authority (EFSA) described the effect on faeces of one Bacillus strain authorised for dogs as small, and doubted how useful it was in practice for the owner. In dogs with diarrhoea or chronic enteropathy, it is not clear what the probiotic adds to a good diet.
What Bacillus are and how they differ from Lactobacillus and Enterococcus
Bacillus subtilis and Bacillus velezensis are two species in this group used in animal feed. When conditions are poor, the bacterium shuts itself inside a spore, which does not grow but survives. In a favourable environment, the spore can germinate and return to its active form.
That resistance is their practical advantage, and the dossier EFSA assessed in 2017 shows it: a B. subtilis strain added with the coating oil to an extruded dry food, or mixed into a pelleted feed at up to 74 °C, lost no viability after 25 months stored at 18 °C. The compression used to make solid treats did not affect it either.
Lactobacilli (now split across several genera, such as Lactiplantibacillus or Lacticaseibacillus) and enterococci do not form spores. They live as active cells, and whether they are still alive when the dog eats them depends on how the product is made and stored. Product quality is a problem across the board. A 2011 study tested 25 commercial probiotics for animals. Of the 15 that stated an amount of live bacteria, only 4 met or exceeded it, and the actual counts ranged from zero to 2 billion colony-forming units (CFU) per gram.
EFSA also assesses their safety differently, treating B. subtilis and B. velezensis as eligible for "qualified presumption of safety" (QPS). For these, it is enough to identify the strain properly and show that it produces no toxins and is not resistant to antibiotics that matter in human and veterinary medicine. Other strains need more data. In 2023, assessing a product for dogs with three lactobacilli and one Enterococcus faecium, EFSA could not confirm the identity of the strains and flagged a safety concern: the enterococcus carried an acquired antimicrobial resistance gene.
Stools, digestibility and diarrhoea
The strain EFSA assessed for dogs
In the EU, a probiotic in dog food is a zootechnical additive (a "gut flora stabiliser") that needs authorisation for that species. One well-documented example in dogs is strain DSM 15544, also known as C-3102. It was assessed as B. subtilis and authorised for dogs in 2017, and in 2021 EFSA reclassified it as B. velezensis. It is used at a minimum of 1 billion CFU per kg of complete feed.
EFSA reviewed four studies in dogs. In one, with 12 young beagles over 30 days, the faeces of the dogs given the strain had more dry matter (39.1% against 36.5%) and less ammonia, with no change in digestibility. The study's author considered the faeces of both groups to be optimal. In 14 puppies, the only difference in the faeces was more IgA, an antibody found on mucosal surfaces. In 16 adult beagles, faecal dry matter rose during the first two weeks and by week four was back level with the control group. EFSA concluded that there is a small but significant increase in faecal dry matter and an improvement in subjective faecal scores. It also said that the biological relevance of changes this size, and their practical use to the owner, are doubtful.
Stools and odour
In a 2020 study, half of the 16 dogs ate a dry food containing B. subtilis and B. licheniformis for 25 days. Their stools were better formed and less smelly, with fewer by-products of protein fermentation, and digestibility did not change.
Two trials on digestibility
In a 2025 crossover trial, 12 healthy beagles received a B. subtilis strain at 1 or 5 billion CFU a day, which tended to lower dry matter and energy digestibility slightly. Most faecal and blood values did not change, and the authors considered it safe. By contrast, in a 2021 trial with 10 beagles, Bacillus coagulans at 1 billion CFU a day raised the digestibility of energy and organic matter, with no change in stool quality. Each strain behaves differently, so a result with one does not tell you about another.
Dogs with acute diarrhoea
A 2025 randomised trial, run in five clinics around Barcelona by the company that makes the diet, compared two highly digestible dry foods in dogs with mild or moderate diarrhoea of less than a week. Of the 111 dogs that completed it, those on the diet with B. velezensis DSM 15544, fructo-oligosaccharides, plasma, yeast and sepiolite had normal stools after 3.6 days on average, against 5.9 days on the control diet. The authors acknowledge that, with five functional ingredients in the diet, nobody knows how much each one contributes.
A more cautious picture comes from a 2019 systematic review of 17 studies of probiotics for preventing or treating digestive problems in dogs, 12 of them on acute problems. The studies were small, with a moderate to high risk of bias. It concluded that in acute digestive problems the effect is very limited and may not be clinically relevant. In chronic disease, diet remains the main treatment and the probiotic does not appear to add a significant improvement.
Dogs with chronic enteropathy
A 2026 crossover trial fed 13 dogs with chronic enteropathy a hydrolysed protein diet with and without B. velezensis DSM 15544, over two 60-day periods. Their digestive signs improved on both versions. The effect of the Bacillus was left unclear, because the response to the diet was so strong that it may have masked any extra benefit.
Doses in the dog studies
A probiotic dose is given in CFU, the bacteria or spores able to grow. A reputable product states the species, the strain and the CFU.
| Study | Dogs | Microorganism and dose | Result |
|---|---|---|---|
| EFSA dossier, 2017 | 12 beagles, 14 puppies and 16 adult beagles, among others | B. subtilis (now B. velezensis) DSM 15544, 1 billion CFU/kg of feed; in the puppies, 300 to 700 million CFU per dog per day | Slightly drier faeces; relevance doubtful |
| Stools and odour, 2020 | 16 dogs | B. subtilis and B. licheniformis, 36.6 million CFU/kg of food of each, 25 days | Better formed and less smelly stools; digestibility unchanged |
| Digestibility, 2025 | 12 beagles | B. subtilis, 1 or 5 billion CFU/day, 4-week periods | Slightly lower digestibility; stools unchanged |
| Digestibility, 2021 | 10 beagles | B. coagulans, up to 1 billion CFU/day, 3-week periods | Slightly higher digestibility; stools unchanged |
| Acute diarrhoea, 2025 | 111 dogs | B. velezensis DSM 15544, at least 1 billion CFU/kg of food, together with four other ingredients | Normal stools sooner; five ingredients at once |
| Chronic enteropathy, 2026 | 13 dogs | B. velezensis DSM 15544, 1 billion CFU/kg of food, 60 days | No clear benefit over the diet alone |
Is it safe?
No adverse effects on general health turned up in the trials in healthy dogs. If the stools get worse soon after your dog starts a new product, stop it and ask the vet. See the vet rather than reaching for a probiotic if the diarrhoea has blood in it, comes with vomiting or lethargy, lasts more than a couple of days, or the dog is a puppy.
If your dog is on antibiotics, ask whether it makes sense to give the probiotic at the same time, and if it has weakened immunity (on chemotherapy or high-dose corticosteroids, for example), decide together with the vet. None of the studies cited here included immunosuppressed dogs; they were done in healthy dogs or dogs with digestive problems.
Sources
- Safety and efficacy of Calsporin® (Bacillus subtilis DSM 15544) as a feed additive for dogs. EFSA Journal, 2017.
- Safety and efficacy of a feed additive consisting of Bacillus velezensis DSM 15544 (Calsporin®) for piglets (suckling and weaned), pigs for fattening, sows in order to have benefit in piglets, ornamental fish, dogs and all avian species (Asahi Biocycle Co.). EFSA Journal, 2021.
- Safety and efficacy of a feed additive consisting of Lacticaseibacillus casei IDAC 210415-01, Limosilactobacillus fermentum IDAC 210415-02, Levilactobacillus brevis IDAC 051120-02 and Enterococcus faecium IDAC 181218-03 (K-9 Heritage Probiotic Blend®) for dogs (CanBiocin Inc.). EFSA Journal, 2023.
- Assessment of commercial probiotic bacterial contents and label accuracy. The Canadian Veterinary Journal, 2011.
- Bacillus subtilis and Bacillus licheniformis reduce faecal protein catabolites concentration and odour in dogs. BMC Veterinary Research, 2020.
- Effects of Bacillus subtilis ATCC PTA-122264 on apparent total tract macronutrient digestibility and fecal characteristics, metabolites, and microbiota of healthy adult dogs. Journal of Animal Science, 2025.
- Evaluation of graded levels of Bacillus coagulans GBI-30, 6086 on apparent nutrient digestibility, stool quality, and intestinal health indicators in healthy adult dogs. Journal of Animal Science, 2021.
- A new highly digestible prescription diet containing Bacillus velezensis DSM 15544, fructo-oligosaccharides, plasma immunoglobulin, yeast and sepiolite for the management of acute diarrhea in dogs: a randomized double-blinded, controlled trial. Frontiers in Veterinary Science, 2025.
- Clinical effect of probiotics in prevention or treatment of gastrointestinal disease in dogs: A systematic review. Journal of Veterinary Internal Medicine, 2019.
- Clinical and Faecal Outcomes in Adult Dogs with Chronic Enteropathy Fed an Hydrolysed Protein Diet with and Without Bacillus velezensis DSM 15544: A Randomised, Double-Blind, Cross-Over Trial. Animals, 2026.