What's Actually in Your Probiotic? A Practitioner's Guide

Walk into any health shop and you’ll find dozens of probiotics, all claiming to be the best. Billions of CFU. Multiple strains. Gut health in a capsule. But if you turn those bottles around and actually read the labels, most of them are remarkably vague about what’s inside and why.

That matters, because probiotic strains aren’t interchangeable. Lactobacillus rhamnosus does different things from Lactobacillus acidophilus. Bifidobacterium longum serves a different purpose from Bifidobacterium breve. A probiotic with the wrong strains, or the right strains at inadequate doses, won’t give you the results you’re looking for.

So let me walk you through what’s actually in a good probiotic and what each component does.

The two main families

Most probiotic supplements contain strains from two bacterial families: Lactobacillus and Bifidobacterium. These aren’t competitors they work in different parts of your gut and serve different functions.

Lactobacillus species primarily colonise the small intestine. They support early-stage digestion, produce lactic acid (which creates an environment hostile to harmful bacteria), and play important roles in immune function and nutrient absorption.

Bifidobacterium species primarily colonise the large intestine (colon). They help break down dietary fibre, produce short-chain fatty acids that nourish your gut lining, and support long-term microbiome stability and immune regulation.

A good probiotic contains strains from both families, because your gut isn’t just one environment it’s a series of distinct ecosystems that each need support.

Key strains and what they do

Here are the strains I encounter most often and the research behind each:

Lactobacillus plantarum

One of the most well-researched probiotic strains available. It’s particularly effective for reducing bloating and abdominal discomfort. Research shows it supports gut barrier integrity meaning it helps maintain the tight junctions between gut cells that prevent unwanted substances from crossing into your bloodstream. It’s also one of the most resilient strains, with a strong ability to survive stomach acid.

Lactobacillus rhamnosus

Probably the most studied probiotic strain in the world. It has strong evidence for supporting immune function, reducing the duration of diarrhoea (particularly antibiotic-associated), and supporting digestive comfort. It adheres well to the gut lining, meaning it’s more likely to colonise rather than just pass through.

Lactobacillus acidophilus

The household name of probiotics and for good reason. It’s one of the most researched strains for supporting lactose digestion, maintaining vaginal health (it produces hydrogen peroxide, which inhibits harmful bacteria), and supporting general digestive comfort.

Lactobacillus paracasei

Supports immune modulation helping your immune system respond appropriately rather than overreacting. Research has also linked it to improved skin health, likely through its effects on reducing systemic inflammation originating in the gut.

Lactobacillus gasseri

An interesting strain with emerging research on metabolic health. Studies have shown associations between L. gasseri supplementation and reduced abdominal fat, improved insulin sensitivity, and anti-inflammatory effects. It’s naturally found in human breast milk, suggesting an evolutionary role in early gut colonisation.

Bifidobacterium longum

One of the first species to colonise the infant gut and one that persists throughout life in a healthy microbiome. It helps break down carbohydrates, produces B vitamins, and has research supporting its role in reducing anxiety and stress through the gut-brain axis.

Bifidobacterium lactis

Well-researched for immune function and digestive comfort. Studies show it can reduce the frequency and severity of respiratory infections and improve regularity of bowel movements. It’s also one of the more robust Bifidobacterium strains, with better survival through stomach acid than some of its relatives.

Bifidobacterium bifidum

Specialises in supporting the mucosal layer of the gut, the protective barrier that lines your intestines. When this layer is compromised, you’re more susceptible to inflammation, food sensitivities, and immune dysfunction. B. bifidum helps maintain and restore this critical defence.

Bifidobacterium breve

Particularly effective at breaking down plant fibres that other species can’t handle. This makes it valuable for improving the diversity of short-chain fatty acid production in the colon, which in turn nourishes the gut lining and supports immune function.

Beyond the bacteria: what else matters

Prebiotics

As I’ve explained in our article on synbiotics vs probiotics, bacteria without food don’t colonise. A good probiotic should include prebiotic fibre typically inulin or FOS to feed the bacteria once they arrive. Without this, you’re sending organisms into an environment where they can’t sustain themselves.

CFU count

CFU (colony forming units) tells you how many live bacteria are in each dose. More isn’t always better, but there are minimums below which you’re unlikely to see meaningful effects. For a multi-strain probiotic, I’d generally look for a total count of at least 10–20 billion CFU per serving, with meaningful amounts per strain not 19 billion of one strain and trace amounts of the others.

Capsule technology

I’ve covered this in detail elsewhere, but it bears repeating: if your probiotic uses standard gelatin capsules, a significant percentage of the bacteria will be killed by stomach acid before they reach the intestine. HPMC (vegetable cellulose) capsules or enteric coating provide acid resistance, ensuring the bacteria arrive alive.

What’s NOT in it

A clean probiotic should have a short “other ingredients” list. No artificial colours. No artificial sweeteners. No unnecessary fillers. The active ingredients should be the star, not the supporting cast.

What we put in the fromel synbiotic (and why)

I chose nine strains across both Lactobacillus and Bifidobacterium families:

• L. plantarum (5 billion CFU) — the highest dose, because it’s the most effective strain for the bloating and gut barrier issues most of my clients experienced
• L. rhamnosus (3.5 billion CFU) — immune support and strong colonisation ability
• L. paracasei (2.5 billion CFU) — immune modulation and skin health support
• B. longum, B. lactis, B. bifidum, B. breve (1.5 billion CFU each) — comprehensive colon support covering fibre breakdown, mucosal health, immune function, and gut-brain signalling
• L. acidophilus (1.5 billion CFU) — lactose digestion and vaginal health
• L. gasseri (1.5 billion CFU) — metabolic health and anti-inflammatory support

Paired with 400mg insulin as prebiotics, 5mg zinc for gut barrier support, and delivered in acid-resistant HPMC capsules.

Every strain is there for a reason backed by published research. Nothing is there for label decoration.

That’s what should be in your probiotic. If your current one doesn’t measure up, now you know what to look for.

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Note: This article is for educational purposes only. Always consult a healthcare professional before starting any new supplement, particularly if you’re pregnant, breastfeeding, or taking medication.