Postbiotics: what inactivated bacteria are and what the research shows

It is Tuesday morning. A man around fifty leaves a box of probiotics on the counter. He asks me whether it will do the job for his father, who has just finished a course of antibiotics after heavy chemotherapy. Instead of handing him the receipt, I surprised him with a question rarely heard in a pharmacy: 'Are you sure that right now you want to give him live bacteria of all things?' During chemotherapy the immune system is strongly suppressed. In that state even the beneficial live microorganisms can surprise the body and carry a small but entirely real risk of infection. It is exactly in such delicate moments that a word we see on packaging more and more often steps on stage: postbiotic.

In the lines ahead you will learn what the word means according to official science, why inactivated bacteria could have any effect at all, what the data show so far and how to read the inscriptions on the packaging without paying for marketing. I will be honest: I will look at when a postbiotic makes sense and when it does not.

To keep them apart, let us separate them by the official definitions of the International Scientific Association for Probiotics and Prebiotics (ISAPP), the organisation that sets the standards in this science: the probiotic is the live bacterium itself. The prebiotic is its favourite food (special fibres such as inulin), which it feeds on in order to grow. And the postbiotic is the newest finding of science: a preparation of inanimate bacteria or their components. The three terms may sound alike, but they describe three entirely different stages of caring for our gut.

To simplify completely: if the probiotic is the worker on shift and the prebiotic is his lunch, then the postbiotic is the finished work and the tools this worker has left behind. He is no longer active, but everything useful from him remains available to our body. For the live options and why the strain decides everything there, see the dedicated article.

In 2021 an ISAPP expert panel fixed the exact frame: a postbiotic is a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host. Inactivation happens through heat, irradiation or mechanical disruption of the cell. Two details of the definition protect your money. First, pure metabolites are not postbiotics; they are written under their real chemical names, and the butyric acid your gut produces from fibre remains butyric acid. Second, a pasteurised starter culture does not become a postbiotic automatically: to earn the name, the manufacturer must have clinical studies in black and white for exactly this product.

In practice this means that the word 'postbiotic' on a random ferment is simply an inscription on a label, not a scientifically documented and studied product. So always look for the name of the specific strain on the packaging of a given product and the evidence behind it.

The immune cells in the gut do not check whether the microbes are alive; they recognise specific molecules on their surface, such as elements of the cell wall, peptidoglycans and surface proteins. This mechanism works on the key-and-lock principle: the receptors of the immune system activate when they match the microbial molecular shape, regardless of whether the bacterium itself is active. For that reason heat treatment does not always neutralise the effect of a given strain and in some cases even stabilises its properties.

A classic example is the Amuc_1100 protein from the envelope of Akkermansia muciniphila. It stays active after pasteurisation and keeps interacting with the TLR2 receptor, which takes part in maintaining the gut barrier. The difference compared with the live probiotic lies in the very mechanism of action: live bacteria settle temporarily in the gut and produce metabolites directly on site. The inanimate postbiotic, on the other hand, simply passes the molecular signal to the immune system without colonising the gastrointestinal tract.

The most categorical evidence in support of inanimate bacteria comes from a large clinical trial in Germany. It involved 443 patients with irritable bowel syndrome (IBS), distributed across 20 medical centres. For eight weeks they took the heat-inactivated strain Bifidobacterium bifidum MIMBb75. The trial's primary goal was at least a 30% reduction in abdominal pain plus adequate relief of overall symptoms for at least half of the time. The results show that this goal was reached by 34% of those taking the inanimate strain versus only 19% in the placebo group, with no serious side effects reported. For full objectivity, however, it should be noted that the trial was funded by the strain's manufacturer, a fact that calls for a more critical reading of the data.

A second interesting signal comes from a pilot study with 32 volunteers with overweight and insulin resistance. In them, three months of taking pasteurised Akkermansia muciniphila improved insulin sensitivity by about 29% against placebo and lowered total cholesterol by close to 9%. Although the sample is small and the period short, the detail that caught the attention of the scientific community is that the pasteurised (inanimate) strain showed better results than the live one.

Despite the promising data, the scientific frame calls for a realistic approach. Studies in this sphere are still few and at an early stage. As of now the European Union has authorised no official health claim for either probiotics or postbiotics. It must be stressed that no such product treats irritable bowel syndrome, obesity or diabetes. These conditions require mandatory medical follow-up by a physician, and food supplements can serve only as accompanying support at their discretion.

Compatibility with antibiotics: antibiotics destroy the live bacteria they come into contact with, but do not affect the inanimate cellular structures. For this reason this type of preparation can be taken throughout the whole antibiotic course, without observing hourly pauses between the intake of the two agents. More on when antibiotic use is justified and when it is not is in our article on colds and flu.

Convenience when travelling: these products do not need refrigeration. The amount of useful components in them stays stable over time and their shelf life is not affected by the higher temperatures in your luggage or bag.

Safety with a compromised immune system: in patients on chemotherapy, after transplantation, with a central venous catheter fitted or in premature babies the immune defence is severely weakened. The medical literature describes rare but real cases in which a live probiotic (most often the yeast Saccharomyces boulardii) enters the bloodstream of severely ill patients and causes an infection. In such conditions the therapeutic decision is always taken by the treating physician, and the inanimate version is a logical and safer alternative to consult them about.

Three things should be stated clearly: the strain with its full code, the method of inactivation and the amount in cells or cell equivalents. The CFU marking does not apply here, because it counts live units and this product contains none.

Red flags: words like 'fermentate' or 'lysate' with no strain name, a promise to treat any disease and a price justified only by advertising. Since the European Union has authorised no health claims for this group, every medical promise on the packaging is a sign of unseriousness.

A healthy person with a varied diet that includes fermented foods rarely needs a bacterial supplement, live or inanimate. A postbiotic does not replace nutrition and is not a compulsory element of everyday life.

Persistent abdominal complaints, blood in the stool, involuntary weight loss or night pain call for an examination, not a supplement. In pregnancy, childhood and chronic diseases every supplement is agreed with the physician.

The science of inanimate bacteria is young but serious. Postbiotics do not displace probiotics; they extend the toolkit where the live form is inconvenient or risky. When you see the word on the packaging of a product, ask about the strain and the study behind it. And if you wonder whether a specific product passes these criteria, write to me in the chat. I answer personally and free of charge, for informational purposes, without diagnosis and without changing therapy.

This article was written by Donka Arabadzhova, Master of Pharmacy, and confirmed by the following sources: ISAPP consensus 2021: definition and scope of postbiotics, Nature Reviews Gastroenterology & Hepatology; ISAPP consensus 2014: definition and appropriate use of the term probiotic; ISAPP consensus 2017: definition and scope of prebiotics; Andresen et al., Lancet Gastroenterology & Hepatology 2020: heat-inactivated Bifidobacterium bifidum MIMBb75 in irritable bowel syndrome, 443 patients; Depommier et al., Nature Medicine 2019: pasteurised Akkermansia muciniphila in volunteers with overweight; Review of reported infection cases from live probiotics in at-risk patient groups; European Commission: register of authorised health claims for foods.