What are Probiotics?

Dr Kate Stephens PhD Food and Microbial Sciences; Gut Microbiology (University of Reading), BSc Medical Microbiology

Probiotics are live bacteria and yeasts that are ‘friendly’ and can offer health benefits. They crowd out harmful bacteria to keep the gut healthy. With around 70% of your immune system based in your gut, digestive health is a key factor to our overall wellbeing.

Read on to find out more about what are probiotics and how they can help you:

The digestive tract is home to trillions of microorganisms, collectively known as the microbiome. Some of these are good for us, some are harmful and some are neutral. Disruptions to the good bacteria in the gut can compromise our health. The resulting imbalance of good and bad bacteria is known as ‘dysbiosis’ and can result in poor digestion, lowered natural immunity and a large variety of conditions, read more about this here: Gut Health - All You Need To Know

probiotic strains under the microscope
Probiotic strains under the microscope - from left to right: B. lactis BB-12, L. acidophilus LA-05 and B. infantis Rosell-33

What do probiotics do?

Many people are confused about what are probiotic supplements, and what probiotics do. There are many questions about the role of probiotics in health, which we answer in this article.

Well, research confidently supports the fact that probiotics do support digestive health by producing specific enzymes1 needed in the digestion of food and aiding the breakdown of foods substances.
Probiotics are also known to improve the absorption of vitamins & minerals into the bloodstream, and even produce B complex vitamins & vitamin K2. B vitamins are vital for maintaining energy levels and brain function3, while vitamin K plays a key role in bone health4. In addition, ‘bad’ strains of bacteria, known as pathogens, can be crowded out by probiotics, the ‘good’ strains of bacteria, so they help to complement and support the health of the gut microbiome.

Probiotics are also thought to support immunity by stimulating the body’s natural defences, and by lining the intestines with a protective layer of friendly bacteria that bars pathogenic substances in the gut from harming the body. As two-thirds of the body's immune system is managed in the gut, it may be important to keep one's probiotic levels high.

There is mounting research to support the role of probiotics in a host of other health conditions and parts of the body - read on to find out more about how probiotics support the skin, vagina, mood, and heart health

Read more about what are probiotics and how they work in our articles How do probiotics work? and Benefits of probiotics

What is a probiotic strain?

It is important to point out that there are many different types of probiotics, called strains, and each one has its own special function. Research shows that different probiotic strains have different benefits and can help support different areas of health. Read more about this here: Are all probiotics the same?

But what is a probiotic strain? A strain denotes a type of bacteria - and informs us on a very specific, in-depth level (more specific, than a species, for example). Take a look at the image below to understand the context of genus, species and strain.

Diagram explaining genus, species and strain. Probiotics Learning Lab

GENUS: A genus is a biological classification of living organisms. The term comes from the Latin genus meaning group. A genus contains one or more species. Examples of common probiotic genera include Bifidobacterium and Lactobacillus.

SPECIES: ‘Species’ refers to a type of microorganism existing within a genus or family. For example, acidophilus is the name of a species within the Lactobacillus genus.

STRAIN: A probiotic ‘strain’ is a genetic variant or subtype of a species. Different probiotic supplements contain different strains, which may be classified under the same species and genus. However, one acidophilus is not equal to another acidophilus. It’s the strain level that matters when you choose a probiotic in order to help with a specific health condition. If you're looking to take a probiotic, make sure to look out for robust, well researched strains when choosing the right for you.

Diagram to explain strain break down

After all, when choosing a dog there is a difference between embarking on life with a Labrador or a Dachshund.

Is there a difference between live cultures and probiotics?

Whilst live cultures and probiotics are often used interchangeably, strictly speaking, they are not exactly the same thing. We love live cultures and fermented foods and drinks such as yoghurt, kimchi, kombucha and kefir. However, if we take the probiotics definition as microorganisms that are proven to have health benefits on the human body, then it's worth pointing out that live cultures found in foods and drinks are not usually clinically trialled, and therefore cannot be linked to a specific health benefit. As mentioned earlier, there is also rarely a guarantee of the types or numbers of live cultures found in fermented foods. It may therefore be more accurate to refer to most fermented foods as containing 'live cultures’ and reserve the term 'probiotics' for foods and supplements that have been clinically studied in humans and proven to have health benefits. For more information on this topic take a look at the Food Myth.

If you are thinking about taking a probiotic or prebiotic and are unsure where to start or what to take, ask for help from a specialist. Choosing the right one should depend on the specific health concerns you have, as different types of probiotics have been shown to help support different areas of health.

What’s the difference between Probiotics, Prebiotics, and Postbiotics?

Probiotics definition is the beneficial microbes which live in our gut, whereas prebiotics are a source of food for probiotics. Prebiotics help our good gut bacteria to grow, multiply and thrive. For more information, take a look at our article What are Prebiotics?  This name for a combination of probiotics and prebiotics in a supplement is 'synbiotic' .

Postbiotics are still a developing area of the category - in simple terms, they are the beneficial end products of bacteria.

Diagram of gut and bacteria. Probiotics Learning Lab.

Key takeaways:

  • The gut microbiome is the collection of microscopic bacteria and yeasts that naturally live in our gut
  • Probiotics are the bacteria that can be beneficial for our health. They are made up of a genus, species and a strain, and are all very different.
  • Probiotics can be taken in supplement form and they can also be found in fermented foods and drinks.
  • The best probiotic supplement for you depends on the condition you are looking to support - immune health, digestive health, mental health, skin health and more.

We hope we have answered all of your questions about what are probiotics. 

Here are some Frequently Asked Questions about probiotics:

How do I determine which specific probiotic strain is best for my health needs?

Consider the research behind each strain and their specific benefits. Choose a strain that aligns with your health needs.

Are there any potential side effects associated with taking probiotics?

While probiotics offer many potential benefits with minimal side effects, it's important to be aware of mild symptoms like bloating or gas, especially when starting a new supplement. Probiotics are not recommended for use in a few serious medical conditions including those who are severely immunosuppressed.

Can probiotics be harmful if taken in excessive amounts?

It's important to follow recommended dosage guidelines and focus on consuming probiotic-rich foods in moderation to maintain a healthy balance. Excessive consumption of probiotics isn’t usually a problem, but may disrupt the natural balance of gut bacteria.

The Probiotics Database provides more detailed information on some of the most researched strains in the world, and explores the clinical research behind different strains, for different circumstances. Health professionals can read about probiotic strains on the Probiotics Database 

Interested in finding out more? Read these articles:

Which Probiotics are the Best for Women?

The Gut-Brain Axis and Probiotics

Gut Health – All You Need to Know

What are Human Strains


  1. Markowiak P and Śliżewska K (2017) Effects of Probiotics, Prebiotics, and Synbiotics on Human Health. Nutrients, 9(9): 1021
  2. Magnúsdóttir S, Ravcheev D, de Crécy-Lagard V, Thiele I (2015) Systematic genome assessment of B-vitamin biosynthesis suggests co-operation among gut microbes. Frontiers in Genetics, 6, 148
  3. O’ Kennedy D (2016) B Vitamins and the Brain: Mechanisms, Dose and Efficacy—A Review. Nutrients, 8(2): 68
  4. Rodríguez-Olleros C, Curiel MD (2019) Vitamin K and Bone Health: A Review on the Effects of Vitamin K Deficiency and Supplementation and the Effect of Non-Vitamin K Antagonist Oral Anticoagulants on Different Bone Parameters. Journal of Osteoporosis, Article ID 2069176