Probiotics and gut flora: strains, CFU and how to choose

Few topics in nutrition and food supplements are discussed as intensively as gut flora. Bacterial cultures, strain names with cryptic codes, billions-figures on the packaging: anyone looking for a suitable product quickly loses track in the jungle of numbers and technical terms. This guide sorts out the key terms, explains CFU, bacterial strains and gastric resistance, and shows you what really matters when choosing a product. And because we stay honest, we also give you a clear picture of the legal situation around the word "probiotic".

What are probiotics?

Probiotics is the term used for preparations or foods that contain live microorganisms, primarily certain bacteria, and sometimes yeasts too. The basic idea: these cultures are the same species, or closely related ones, to those naturally found in the human gut. You already encounter such cultures in everyday life, for example in yoghurt, kefir, sauerkraut or kimchi. As a food supplement, they come in concentrated form as capsules or powder, often with a far higher number of cultures and a defined, consistent composition than a food can offer. This is exactly where the practical difference lies: while the culture content in fermented foods fluctuates considerably, in a reputable preparation it is stated and defined for the whole shelf life.

The gut is home to an enormous number of microorganisms, known as the gut flora or, in more modern terms, the gut microbiome. Behind this lies a complex ecosystem of hundreds of bacterial species, which differs from person to person and changes with diet, lifestyle, age and many other factors. Microbiome research is one of the most exciting, and at the same time newest, fields of nutritional science, and much of it is only just beginning to be understood. If you'd like to dive deeper, our guide Gut & microbiome: probiotics, fibre & diet offers a broader overview.

Important bacterial strains

Not all bacteria are the same, and this is exactly where it gets interesting. On product labels, you'll mostly come across two major genera:

• Lactobacillus (today partly reclassified, for example as Lacticaseibacillus or Limosilactobacillus): lactic acid bacteria traditionally associated with fermented foods. Well-known representatives include Lactobacillus acidophilus, L. rhamnosus and L. plantarum.
• Bifidobacterium: a genus that plays a major role especially in the large intestine and already makes up a significant share in infants. Common strains include Bifidobacterium bifidum, B. lactis and B. longum.
• Other cultures: depending on the product, genera such as Streptococcus (for example S. thermophilus), Lactococcus or the yeast Saccharomyces boulardii may also be included.

What matters is this: a bacterium is precisely described on three levels – genus, species and strain – for example Lactobacillus rhamnosus GG. The code at the end designates the specific strain. Two products with the same species can therefore contain different strains. For you, this mainly means: transparent, complete disclosure of the strains contained is a mark of quality.

What does CFU mean?

CFU stands for colony forming units. The figure indicates how many viable – that is, live – microorganisms are contained in a defined amount of the product. You'll often see figures such as "10 billion CFU per capsule" or, for powders, "100 billion CFU per gram".

It's important to understand that a higher CFU figure doesn't automatically mean "better". It makes more sense to look at the overall picture of strain selection, dosage and processing. Nevertheless, the CFU figure is central, because without it a product is hard to assess at all. A reputable preparation states the CFU figure clearly, ideally in relation to the best-before date rather than just the manufacturing date, because live cultures lose activity over time. For comparison: some fermented foods contain anywhere from a few million to a few billion cultures per serving, while concentrated preparations can reach markedly higher, and above all clearly declared, figures.

Prebiotics vs. probiotics

These two terms are often confused, but they mean different things:

• Probiotics are the live cultures themselves.
• Prebiotics are specific, usually fibre-rich components of food that can be metabolised by microorganisms in the gut, such as inulin or oligofructose. Put simply: prebiotics are the "food", probiotics are the cultures.

Combinations of both are sometimes referred to as synbiotics. We go into more depth on how the two differ and complement each other in our article Probiotics or prebiotics: the difference.

What to look for when choosing

When comparing products, four sober criteria will help you more than any marketing claim:

• Strain diversity and transparency: are the strains contained stated fully and traceably? A broad, clearly declared selection is a good sign. You'll find more detail on this in Recognising probiotic quality: CFU, strains & gastric resistance.
• CFU figure: clearly stated and ideally related to the best-before date.
• Gastric resistance: live cultures must survive passage through the stomach in order to arrive alive in the gut at all. Some products rely on gastro-resistant capsules or robust, acid-tolerant strains for this.
• Storage and processing: some cultures are shelf-stable, others should be kept refrigerated. Manufacturer instructions on storage are a sign of care. We explain what a testing label such as "lab-tested" actually means in Lab-tested: what does that mean?.

Whether a capsule or powder suits you better is, in the end, also a matter of preference and practicality; we've compared the two in Probiotics: capsule or powder?.

Intake

There's no universal rule for intake; it makes sense to look at the manufacturer's recommendation for the specific product. Taking it with plenty of water is common, sometimes before or with a meal. Cultures don't take effect overnight: many people take them over a longer, continuous period. Whether a time-limited course or ongoing use makes more sense depends on your goal; you'll find guidance on this in Course or ongoing intake?. If you're combining cultures with other food supplements, it's worth looking at Combining supplements correctly. And because the question of absorption often comes up with nutrients: the basics are covered in Bioavailability explained simply.

Honestly assessed

Now for the part many suppliers would rather leave out: in the European Union, the term "probiotic" or "probiotikum" is not authorised as a health claim on foods and food supplements. Authorities regard the word as a claim that suggests a health effect, and such claims require official authorisation, which has not been granted to date. That's why, on compliant products, you'll find the cultures described factually – for instance as a "culture complex" or by naming the bacterial strains contained – rather than as a promise of efficacy.

That's not a weakness, simply the applicable legal position, and we stick to it. We deliberately make no health claims here and don't assert that a preparation "strengthens the gut" or "the immune system". What we do instead: make the composition transparent, explain the technical terms, and make choosing easier for you. It's best to decide whether and how a product makes sense for you personally while well informed, and when in doubt, in conversation with a doctor. Anyone who regularly takes medication, is pregnant, or has pre-existing health conditions is generally well advised to briefly consult a doctor before starting any new food supplement.

Matching products from Scheunengut

If you're looking for a broadly diverse, transparently declared culture, we offer the Flora Intenso culture complex with 23 bacterial strains and 100 billion CFU/g. This preparation combines a high strain diversity with a clearly stated CFU figure, and all strains contained are fully declared. Exactly the criteria we've highlighted for you above: traceable composition, clear figures, no promises hidden between the lines.

Frequently asked questions (FAQ)

What's the difference between gut flora and microbiome?

At their core, both terms mean the same thing: the entirety of the microorganisms in the gut. "Gut flora" is the older, more figurative term, while "microbiome" is the more modern, scientifically precise term, which also includes the genetic material of these microorganisms. In everyday use, they're often used interchangeably.

What does CFU mean on the packaging?

CFU stands for colony forming units and indicates how many viable microorganisms are contained in a given amount. Check whether the figure relates to the best-before date, since live cultures lose activity over time. A higher number alone, however, doesn't yet say anything about quality.

Why don't products say "probiotic"?

Because the term "probiotic" is regarded in the EU as a health claim, and no authorisation exists for it. Compliant products therefore describe the cultures they contain factually, for example by naming the bacterial strains or as a "culture complex". This is a legal requirement, and a sign that a supplier operates properly.

What's the difference between prebiotics and probiotics?

Probiotics are the live bacterial cultures themselves. Prebiotics are specific, usually fibre-rich food components such as inulin, which are metabolised by microorganisms in the gut. Put simply: prebiotics are the food, probiotics are the cultures. Combinations of the two are called synbiotics.

How long should you take cultures for?

This depends on the product and your personal goal; there's no universal rule. Many people take cultures over a longer, continuous period rather than just a few days. We explain whether a limited course or ongoing use suits you better in the guide "Course or ongoing intake?". When in doubt, a doctor's advice can help.