What Are Probiotics? A Simple Guide to Friendly Bacteria

You’ve probably heard probiotics described as “good” or “friendly” bacteria. But what exactly are they, what do they have to do with your gut, and how do you know which probiotic to choose?

Our digestive system is home to a huge community of microorganisms, including bacteria. Together, these are known as the gut microbiome. While that might sound complicated, you can simply think of it as the community of microorganisms that naturally lives in your gut.

Probiotics are live microorganisms found in certain foods and supplements. Different probiotics contain different types and strains of bacteria, which is why choosing one can sometimes feel confusing.

Here, we explain what probiotics are, where you can find them and what to consider if you're thinking about adding a probiotic supplement to your routine.

What Are Probiotics?

Probiotics are live microorganisms, usually bacteria or yeasts, found in some fermented foods and dietary supplements.

Our bodies naturally contain trillions of microorganisms. A large number live in our digestive system, particularly the large intestine, where they form part of the gut microbiome.

Rather than thinking about bacteria as simply “good” or “bad”, it is more helpful to think of the gut as an ecosystem. It contains many different microorganisms living alongside one another, and the balance and variety of this community can be influenced by factors including diet, age, lifestyle and medication.

Probiotic foods and supplements are one way of adding particular live microorganisms to your diet.

What Is the Gut Microbiome?

The gut microbiome is the name given to the enormous community of microorganisms living in our digestive system.

Everyone's microbiome is different. It begins developing very early in life and continues to change throughout our lifetime.

What we eat is particularly important. A varied diet containing plenty of different plant foods provides the gut with a wide range of fibres and other nutrients.

This is one reason we're increasingly encouraged to think about gut health as more than simply taking a probiotic supplement. What we eat every day matters too.

What Do Probiotics Do?

This is where probiotics can become confusing because there isn't one single answer.

There are many different probiotic microorganisms, and they aren't all interchangeable. Researchers usually study specific strains for specific purposes, rather than treating every probiotic as though it does exactly the same thing.

That's important when you're shopping for a supplement. A product containing one probiotic strain isn't necessarily equivalent to another product simply because both are labelled “probiotic”.

You'll also see probiotic supplements marketed towards different ages, lifestyles and needs. The formulation, strains and amounts can vary considerably between them.

What Is a Probiotic Strain?

You'll often see long names printed on probiotic packaging, such as Lactobacillus or Bifidobacterium, sometimes followed by additional letters and numbers. These identify the microorganisms contained in the product.

An easy way to understand this is to think of probiotics as having a family, type and individual strain. Two bacteria may belong to the same broader family but still be different strains.

Why does this matter? Because research carried out on one particular strain can't automatically be applied to every other probiotic.

You don't need to become an expert in microbiology before buying a probiotic, but it can be useful to look for products that clearly identify the strains they contain rather than relying solely on a very large number on the front of the pack.

What Does CFU Mean?

Another term you may encounter is CFU, which stands for colony-forming units. This is a way of expressing the quantity of viable microorganisms in a probiotic product. You might therefore see supplements containing millions or billions of CFUs. A bigger number isn't automatically better.

The strains used, the formulation and how the product is intended to be taken are also important. Choosing a probiotic shouldn't simply become a competition to find the bottle with the largest number of billions printed on it.

What Foods Contain Probiotics?

Some fermented foods contain live microorganisms. Examples can include:

• live yoghurt
• kefir
• some fermented vegetables
• sauerkraut
• kimchi
• some kombucha

However, not every fermented food necessarily contains live cultures by the time you eat it. Processing methods such as heating can reduce or remove live microorganisms.

Fermented foods can still be part of a varied diet, but they shouldn't automatically be considered equivalent to a probiotic supplement containing specifically identified strains.

Probiotics vs Prebiotics: What's the Difference?

The names sound very similar, but probiotics and prebiotics aren't the same thing.

Probiotics are live microorganisms. Prebiotics are substances, often particular types of fibre, that are used by microorganisms in the gut.

Prebiotic fibres occur naturally in foods including onions, garlic, leeks, asparagus and a range of other plant foods.

You may also come across supplements combining probiotics and prebiotics in the same formula. For a more detailed explanation, read our Prebiotics vs Probiotics guide.

Do You Need a Probiotic Supplement?

Not everyone needs to take a probiotic supplement. A varied, balanced diet remains an important foundation for looking after your digestive health. That includes eating a good variety of fruit, vegetables, wholegrains, pulses, nuts and seeds.

Some people nevertheless choose to include a probiotic as part of their everyday routine.

If you do, it is worth choosing based on what the product contains and why you're taking it, rather than simply choosing the probiotic with the highest number of bacteria or the longest ingredient list.

How Do You Choose a Probiotic?

Walk into Evergreen's probiotic section and you'll quickly discover that there are lots of options. When comparing them, consider:

The strains - Look at which microorganisms the product actually contains. Different formulations use different strains.

The amount - Check the CFU count, but remember that more isn't necessarily better.

Who it's designed for - There are general probiotics as well as products formulated for particular age groups and needs.

The format - Probiotics are available as capsules, powders, liquids and other formats. Choosing something you'll find convenient to take can make it easier to use consistently.

Storage - Some probiotics need refrigeration while others are designed to remain stable at room temperature. Always follow the manufacturer's storage instructions.

When Should You Take Probiotics?

There isn't one timing rule that applies to every probiotic. Different products are formulated differently, so the best approach is to follow the directions provided by the manufacturer. Some recommend taking the product with food, while others may give different instructions.

Consistency is generally more useful than trying to find a universal “perfect” time of day.

Can You Take Probiotics Every Day?

Many probiotic supplements are designed for daily use, but this depends on the individual product. Always check the directions on the packaging and don't exceed the recommended amount.

If you have a health condition, take medication, are pregnant or breastfeeding, or are considering probiotics for a child, speak to an appropriate healthcare professional if you're unsure whether a particular product is suitable.

Are All Probiotics the Same?

No — and this is probably the most useful thing to remember when shopping for one.

“Probiotic” describes a very broad category. Products can differ in:

• bacterial species and strains
• number of strains
• CFU count
• formulation
• format
• storage requirements
• intended user

That doesn't necessarily make one universally “better” than another. It means it's worth looking beyond the word probiotic on the front of the pack.

Food First: Looking After Your Gut

Probiotic supplements are only one small part of the gut-health picture. Your everyday diet matters too.

Eating a varied diet containing plenty of different plant foods can provide fibre and other nutrients. Fruit, vegetables, wholegrains, beans, lentils, nuts and seeds can all contribute to that variety.

Fermented foods can also add another dimension to your diet if you enjoy them.

Rather than looking for one product to transform your gut health, we prefer to think about the bigger picture: a varied diet, healthy lifestyle and supplements where they're appropriate for you.

The Evergreen View

The world of probiotics can seem much more complicated than it needs to be.

The important thing to remember is that probiotics aren't all the same. Different products contain different microorganisms, in different quantities and formulations.

Start with the basics: look after your gut with a varied, balanced diet, and if you decide to add a probiotic supplement, choose one based on what it contains and what suits your needs rather than simply reaching for the biggest number on the pack.

You can explore our Probiotic Supplements collection or read our Best Probiotics: How to Choose the Right Probiotic for You guide if you'd like help comparing the different options available at Evergreen.

This article is for general information only and should not replace individual medical advice. Speak to your GP, pharmacist or another appropriate healthcare professional if you have ongoing digestive symptoms or are unsure whether a supplement is suitable for you.

Checked and updated 17th September 2026