Understanding Your Pet's Gut Microbiome
The gut microbiome — the vast community of bacteria, fungi, viruses, and other microorganisms living in the gastrointestinal tract — has emerged as one of the most significant topics in veterinary medicine over the past decade. In dogs and cats, as in humans, the microbiome plays a central role in digestion, immune system regulation, nutrient absorption, and even neurological function via the gut-brain axis. When this microbial community is healthy and balanced, it supports overall wellbeing. When it is disrupted, the consequences can extend far beyond the gut itself.
What Does a Healthy Microbiome Do?
A balanced gut microbiome in dogs and cats performs a remarkable range of functions. Beneficial bacteria break down dietary fibre into short-chain fatty acids, which nourish the cells lining the intestinal wall and help maintain the gut barrier. A healthy gut barrier prevents pathogenic bacteria and their toxins from leaking into the bloodstream — a state known as intestinal permeability, or colloquially as "leaky gut."
The microbiome also plays a central role in immune education, training the immune system to distinguish between harmless and harmful agents. Approximately 70 per cent of the body's immune cells are located in or near the gut. Disruptions to the microbiome in early life have been linked to increased risk of allergic and autoimmune conditions. Additionally, gut bacteria produce certain B vitamins and vitamin K, and influence how effectively the body absorbs nutrients from food.
Prebiotics vs Probiotics: An Important Distinction
These two terms are frequently confused, but they refer to quite different things:
- Probiotics are live microorganisms — usually bacteria — that, when administered in adequate amounts, confer a health benefit on the host. In simple terms, they add beneficial microbes to the gut. Common probiotic bacteria used in pet supplements include Enterococcus faecium, Lactobacillus acidophilus, Bifidobacterium species, and Bacillus coagulans.
- Prebiotics are dietary compounds — typically specific types of fibre — that are not digested by the host but instead selectively feed and stimulate the growth of beneficial bacteria already present in the gut. Common prebiotics include fructooligosaccharides (FOS), inulin, and certain forms of pectin and psyllium husk.
A useful analogy: if the gut microbiome is a garden, probiotics are the seeds (adding new plants) whilst prebiotics are the fertiliser (helping existing plants to flourish). Both can be valuable, but they work differently and are not interchangeable. Some products are described as synbiotics, combining both prebiotics and probiotics in a single formulation.
Dysbiosis: When the Microbiome Goes Wrong
Dysbiosis refers to an imbalance in the gut microbial community — typically a reduction in diversity and beneficial bacteria alongside an overgrowth of potentially harmful organisms. In dogs and cats, dysbiosis is associated with a wide range of conditions, including acute and chronic diarrhoea, inflammatory bowel disease (IBD), food-responsive enteropathy, obesity, liver disease, and even skin conditions.
Common causes of dysbiosis include:
- Antibiotic treatment — even short courses can cause significant and prolonged disruption
- Dietary changes, particularly sudden switches between foods
- Gastrointestinal infections, including bacterial, viral, and parasitic causes
- Chronic stress, which can alter gut motility and microbial populations
- Underlying inflammatory conditions affecting the gastrointestinal tract
Diagnosis of dysbiosis can be supported by faecal microbiome testing, available through specialist veterinary laboratories. However, interpretation requires expertise, and a positive result for dysbiosis is not itself a diagnosis of any specific underlying cause — it signals an imbalance that must be understood in clinical context.
Antibiotics and Microbiome Disruption
Antibiotics are among the most powerful disruptors of the gut microbiome. Whilst they are often essential and life-saving, broad-spectrum antibiotics in particular do not discriminate between pathogenic and beneficial bacteria. Studies in dogs have shown that a single course of commonly used antibiotics such as metronidazole or amoxicillin-clavulanate can substantially alter the gut microbiome, with effects lasting weeks to months. In some cases, certain microbial populations may not fully recover.
This does not mean antibiotics should be avoided when clinically indicated — they must absolutely be used when needed. However, it does support the approach of using the most targeted antibiotic possible, for the shortest effective duration, and only when genuinely necessary. It also supports the use of probiotic supplementation during and after antibiotic courses to help support microbial recovery, though the evidence base for specific products and protocols is still developing.
Metronidazole in particular, despite being historically used very commonly for gastrointestinal signs in dogs and cats, has increasingly been found to cause significant dysbiosis and is now recommended less freely as a first-line treatment for uncomplicated diarrhoea in the veterinary field.
Probiotic Products for Pets
Several veterinary-specific probiotic products are available and have some evidence behind them. FortiFlora (manufactured by Purina) is one of the most widely studied, containing Enterococcus faecium SF68, a strain with demonstrated ability to survive transit through the gastrointestinal tract and support stool quality in dogs and cats with diarrhoea. It is also highly palatable, which helps with administration.
Protexin and similar multi-strain products offer a broader range of bacterial species in each dose, which may provide a more diverse seeding effect, though comparative clinical data between products is limited. When choosing a probiotic for your pet, look for products specifically formulated and tested for use in animals, as human probiotic products contain strains that may not colonise the dog or cat gut effectively.
Faecal Microbiome Transplant (FMT)
Faecal microbiome transplant — the transfer of faecal material from a healthy donor into the gut of a recipient — is an emerging treatment in veterinary medicine, particularly for cases of refractory (treatment-resistant) chronic diarrhoea, recurrent Clostridioides difficile-like infections, and inflammatory bowel conditions that have not responded to conventional therapy.
FMT works by introducing a diverse, healthy microbial community directly into the disrupted gut environment, potentially restoring balance more comprehensively than probiotic supplementation alone. In human medicine, FMT has shown remarkable efficacy for recurrent Clostridioides difficile infection, and veterinary research is beginning to build a similar evidence base for companion animals.
In the UK and Ireland, FMT in animals is typically performed by veterinary internal medicine specialists. Donor screening is essential — donors must be thoroughly tested to ensure they do not carry pathogens that could be transmitted to the recipient. FMT can be administered via enema, colonoscopy, or capsule form. It is not a first-line treatment and is generally reserved for cases where other interventions have failed, but it represents an exciting development in microbiome-focused veterinary medicine.
High-Fibre Diets and Microbiome Health
Dietary fibre is the primary fuel for beneficial gut bacteria. A diet rich in appropriate fibre sources supports microbial diversity and the production of short-chain fatty acids that nourish the gut lining. For dogs with chronic gastrointestinal issues, adding soluble fibre such as psyllium husk, inulin, or canned pumpkin (plain, unseasoned) can support microbiome health and improve stool consistency.
Many commercial pet foods include prebiotic fibres as standard. However, extremely high-fibre diets are not appropriate for all animals — cats in particular are obligate carnivores and should receive fibre only in amounts appropriate to their natural dietary profile. Always discuss significant dietary changes with your vet before implementing them, particularly in animals with diagnosed gastrointestinal conditions.
- Feed a consistent, high-quality diet and avoid frequent sudden changes
- Use antibiotics only when clinically necessary and consider probiotic support during and after courses
- Discuss veterinary-grade probiotic options with your vet for acute or chronic digestive issues
- Ask about FMT as an option if your pet has chronic refractory diarrhoea unresponsive to standard treatment
- Include appropriate fibre in your pet's diet to support beneficial bacterial populations