We swallow many billions of living bacteria every day. They are on raw food, a few survive on cooked food, we nibble on our fingers without even thinking about it, we swallow our own mouth bacteria or kiss our way through the bacterial landscapes of others. A small proportion even survive the acid bath of the stomach and the aggressive digestive process to reach our large intestine alive.

No one knows the majority of these bacteria. They presumably do us no harm, or perhaps even benefit us in some way we have not yet discovered. A few are pathogens, but they usually cannot harm us because their numbers are just too small. Only a fraction of these bacteria have been thoroughly checked out by scientists and given the official seal of approval. These bacteria can proudly call themselves probiotics.

We often read the word “probiotic” on the yogurt we find on our supermarket shelves without having any real idea what it means or how they work. Most will recall television commercials telling us they strengthen our immune system, or showing us a constipated aunt relieved of her woes and recommending this brand to everyone she knows. That all sounds good. You don’t mind spending a little extra for a healthy product like that. And before you know it, those probiotics are in your shopping basket, then in your fridge, and eventually in your mouth.

People have been eating probiotic bacteria since time immemorial. Without them, we would not exist. A group of South Americans had to learn that through bitter experience. They had the clever idea of taking pregnant women to the South Pole to have their babies. The plan was that the babies born there could stake a claim to any oil future reserves as natives of the region. The babies did not survive. They died soon after birth or on the way back to South America. The South Pole is so cold and germ-free that the infants simply did not get the bacteria they needed to survive. The normal temperatures and bacteria the babies encountered after leaving the Antarctic were enough to kill them.

Helpful bacteria are an important part of our life, and we are constantly surrounded and covered by them. Our ancestors had no idea of their existence, but they intuitively did the right thing: protecting their food from the bacteria that make it rot by handing it over to the care of good bacteria. In other words, they used bacteria to preserve their food. Every culture in the world includes traditional dishes that rely on the help of microbes for their preparation. Germany has its sauerkraut, pickled gherkins, and sourdough pretzels; the French love their crème fraîche; the Swiss have their hole-riddled cheese; salami and preserved olives come from Italy; Turks swear by a salty yogurt drink called ayran. None of these delicacies would exist if it weren’t for microbes.

There are many, many examples from Asian cuisine: soy sauce, kombucha drinks, miso soup, Korean kimchi. Indians have lassi and Africans have fufu… the list is endless. All these foods rely on bacteria for a process we call fermentation. The process often results in the production of acid, which makes the yogurt or vegetables taste sour. This acid, along with the many good bacteria, protects the food from dangerous microbes. Fermentation is the oldest and healthiest way of preserving food.

The bacteria used in this technique were as varied around the world as the foods they helped to produce. The soured milk drunk in rural Germany was made using different bacteria from those used to make the ayran enjoyed in Anatolia. In the warmer countries of the south, bacteria were used that prefer to work under higher temperature conditions; in the chilly north, bacteria were chosen that like to do their job at room temperature.

Yogurt, soured milk, and other fermented products came about by accident. Someone left the milk outside, bacteria found their way into the churn (either directly from the cow or from the air during milking), the milk thickened, and a new kind of food had been invented. If a particularly delicious yogurt bacteria jumped into the mix, people added a spoonful of that yogurt to the next batch to make more of the same. Unlike today’s yogurt products, however, traditional types were the work of whole teams of different bacteria—not just selected individual species.

The diversity of bacteria in fermented foods has fallen sharply. Industrialization has resulted in standardized production processes using single bacteria species isolated in laboratories. Today, milk is briefly heated soon after it leaves the udder to kill off any potential pathogens, but this also kills any potential yogurt bacteria. That’s why you can’t just leave modern shop-bought milk to go sour in the hope that it will eventually turn into yogurt.

Many foods that used to be full of bacteria are now preserved using vinegar, as is the case with most pickled gherkins today. Some things are fermented using bacteria but then heated to kill off the microbes—shop-bought sauerkraut, for instance. Fresh sauerkraut is usually only sold in specialist health-food stores these days.

Scientists in the early twentieth century suspected that good bacteria were of great benefit to us. That is when Ilya Metchnikoff appeared on the yogurt scene. The Nobel Prize winner spent his time observing Bulgarian mountain peasants. He realized they often lived to be a hundred years old or more, and they were unusually contented. Metchnikoff suspected the key to their longevity lay in the leather bags they used to transport the milk from their cows. The peasants had to walk long distances, and their milk often turned sour or transformed into yogurt in the bags before they reached home. Metchnikoff became convinced that the secret to their long lives was their regular consumption of this bacterial product. In his book The Prolongation of Life, he asserted the claim that good bacteria can help us live longer, better lives. From then on, bacteria were no longer just anonymous components of yogurt, but important promoters of health. However, Metchnikoff’s timing could hardly have been worse. Shortly before, it had been discovered that bacteria cause disease. Although the microbiologist Stamen Grigorov identified the bacterium described by Metchnikoff as Lactobacillus bulgaricus in 1905, he soon turned his efforts to the fight against tuberculosis. The successful use of antibiotics in fighting disease from around 1940 meant that the idea was fixed in most people’s minds: the fewer bacteria, the better.

We have babies to thank for the fact that Ilya Metchnikoff’s idea and Grigorov’s bacillus eventually found their way onto our supermarket shelves. Mothers who were unable to breast-feed their babies found they often had a problem with their bottle-fed offspring. The babies were getting diarrhea much more often than they should. This took the formula-milk industry by surprise, because they had taken care to make sure their product contained the same substances as real breast milk. What could be missing? The answer was, of course, bacteria. And, specifically, the bacteria that live on milky nipples and which are particularly common in the guts of breast-fed babies: Bifidobacteria and Lactobacilli. These bacteria break down the sugar in milk (lactose) and produce lactic acid (lactate), so they are classified as lactic-acid bacteria. A Japanese scientist used Lactobacillus casei Shirota to create a special yogurt, which mothers could initially buy only in pharmacies. When they fed it to their babies every day, the infants contracted diarrhea much less frequently. Industrial food research returned to Metchnikoff’s idea—with baby bacteria and more modest aims.

Most normal yogurt contains Lactobacillus bulgaricus, although it is not necessarily precisely the same sort as that in the yogurt of the Bulgarian peasants. Today, the species discovered by Stamen Grigorov is more correctly known as Lactobacillus helveticus spp. bulgaricus. These bacteria are not particularly good at resisting digestion and only a small number of them reach the large intestine alive. That is not so important for some effects on the immune system—often just the sight of a few empty bits of bacteria wall is enough to prompt our immune cells into action.

Probiotic yogurt contains bacteria that researchers were inspired to use by the case of the bottle-fed babies with diarrhea. They are meant to reach the large intestine alive. Examples of bacteria that can resist being digested are Lactobacillus rhamnosus, Lactobacillus acidophilus, and the abovementioned Lactobacillus casei Shirota. The theory is that a living bacterium will have a greater effect on the gut. There are studies that show their effects, but they are not sufficient to satisfy the European Food Safety Authority. It has banned companies like Yakult and Danone from claiming their products promote health in their advertising.

These doubts are compounded by the fact that it is not always possible to know whether enough probiotic bacteria are reaching the large intestine alive. A break in the cold chain, or a person with a particularly acidic stomach or slow digestion, might kill off these microbes before they reach their intended destination. That is not harmful, of course, but it means consuming a probiotic yogurt may have no effect that a normal yogurt doesn’t also have. To make any difference to the huge ecosystem in our gut, about a billion (109) bacteria need to make it through the system and arrive there intact.

To summarize: any yogurt can be good for you, although not everyone can tolerate milk protein or too much animal fat. The good news is that there is a world of probiotics beyond yogurt. Researchers are busy in their laboratories examining selected bacteria. They dribble bacteria directly onto gut cells in petri dishes, feed mice with microbial cocktails, or get volunteers to swallow capsules full of living microorganisms. Probiotic research has roughly defined three areas in which our good bacteria can display fascinating abilities.

  1. Massaging and Pampering
    Many probiotic bacteria take good care of our gut. They possess genes that enable them to produce small fatty acids like butyrate. This soothes and pampers the villi in the gut. Pampered villi are much more stable and likely to grow bigger than unpampered ones. The bigger the villi grow, the better they are at absorbing nutrients, minerals, and vitamins. The more stable they are, the less rubbish they let through. The result is that our body receives more nutrients and fewer damaging substances.
  2. Security Service
    Good bacteria defend our gut—it is, after all, their home and they do not willingly surrender their territory to bad bacteria. Sometimes they defend the gut by occupying the very places pathogens like to infect us most. When a bad bacterium turns up, it finds them sitting in its favorite spot with satisfied grins on their faces and their handbags on the seats next to them, leaving no room for anyone else to take up residence. Should that signal not be explicit enough—no problem! Security service bacteria have more tricks up their sleeves. For example, they can produce small amounts of antibiotics or other defensive substances that drive unfamiliar bacteria out of their immediate vicinity. Or they use various acids, which not only protect yogurt and sauerkraut from rotting bacteria, but also make our gut a less inviting environment for bad bacteria. Another trick is to snatch the bad bacteria’s food away (anyone with siblings may be familiar with this strategy). Some probiotic bacteria seem to have the ability to steal bad bacteria’s food from right under their noses. Eventually, the bad guys have had enough and give up.
  1. Good Advice and Training
    And, last but not least, the best experts in all things bacterial are bacteria themselves. When they work together with our gut and its immune cells, they provide us with insider information and useful advice. What do the different bacteria’s outer walls look like? How much protective mucus is needed? What quantity of bacterial defense substances (defensins) should the gut cells produce? Does the immune system need to be more active in its reaction to foreign substances or should it sit back and accept newcomers?

A healthy gut contains many probiotic bacteria. We benefit every day and every second from their abilities. But sometimes our bacterial community faces attack. That can be from antibiotics, a bad diet, illness, stress, and many, many other causes. Our guts are then less well cared for, less protective, and less good at giving advice. When that is the case, we can be thankful that some of the results of laboratory research have made it onto pharmacy shelves. Living bacteria are available that can be used like temporary workers brought in to help during times of heavy workloads.

Good for treating diarrhea. This is the number-one use for probiotics. Gastroenteritis (stomach flu) and diarrhea caused by taking antibiotics can be helped using various pharmacy-bought bacteria. They can reduce the length of such a bout of diarrhea by about a day. At the same time, unlike most diarrhea medications, they are almost free of side effects. That means they are particularly suitable for small children and old people. In conditions like ulcerative colitis and irritable bowel syndrome, probiotics can increase the intervals between diarrhea attacks or inflammatory flare-ups.

Good for the immune system. For people who tend to get sick often, it can be a good idea to try different probiotics, especially when colds are rife. If that is too expensive, eating a pot of yogurt a day may be enough, since bacteria don’t necessary have to be alive to trigger some mild effects. Studies have shown that old people and high-performance athletes, in particular, are less prone to catching colds if they take probiotics regularly.

Possible protection against allergies. This is not as well documented as the effect of probiotics on diarrhea or a compromised immune system. Still, probiotics are a good option for parents of children with an increased risk of developing allergies or neurodermatitis. A large number of studies show they can offer significant protection. In some studies, the effect could not be proven, but that may be because each study used a different kind of bacterium. Personally, I think the “better safe than sorry” approach is appropriate here. Probiotics certainly do no harm to children with a high risk of developing allergies. Some studies showed an improvement in the symptoms of those already suffering from allergies or neurodermatitis.

As well as the well-researched areas like diarrhea, gastrointestinal disease, and the immune system, there are other areas that are only now undergoing scientific scrutiny. Digestive complaints, traveler’s diarrhea, lactose intolerance, obesity, inflammatory joint disease, and diabetes are all promising areas of research.

If you ask your pharmacist to recommend a probiotic product to help with one of these problems (for example, constipation or flatulence), she will not be able to give you one that has been scientifically proven to work. The pharmaceutical industry and academic research are equally behind in this area. What remains for you is to try out different products for yourself until you hit upon a bacterium that seems to help. The packaging should always include the name of the bacteria the product contains. Try it for about four weeks and if you see no improvement, give a different bacterium a go. Some gastro-enterologists will advise you about the kind of bacteria that are more likely to be the ones you are looking for.

The rules are the same for all probiotics. You should try them for about four weeks and make sure they are still within the best-before date (otherwise sufficient bacteria may not have survived to have any effect on the huge ecosystem of the gut). Before buying probiotic products, you should find out whether they are intended by the manufacturer for the complaint you are hoping to treat. Different bacteria have different genes—some are better at advising the immune system, others are more belligerent about ridding the gut of diarrhea-causing bugs, and so on.

The best-researched probiotics to date are lactic-acid bacteria (Lactobacilli and Bifidobacteria) and Saccharomyces boulardii, which is a yeast. This yeast has not received the attention it deserves. It is not a bacterium, so it is not one of my favorites either, but it does have one huge advantage: as a yeast, it has absolutely nothing to fear from antibiotics.

So, while we are massacring our entire bacteria population by taking antibiotics, Saccharomyces can move in and set up house without a worry in the world. It can then protect the gut from harmful opportunists. It also has the ability to bind toxins. However, it does cause more side effects than bacterial probiotics. For example, some people have an intolerance to yeasts, which can cause them to break out in a rash.

The fact that almost all the probiotics we know—give or take a yeast or two—are lactic-acid bacteria shows how little we have yet discovered in this area. Lactobacilli are normally less common residents in the guts of adults, and Bifidobacteria are unlikely to be the only health-promoting microbes present in the large intestine. At the time of writing, there is only one other probiotic bacteria species that is as well researched as the two mentioned above: E. coli Nissle 1917.

This strain of E. coli was first isolated from the feces of a soldier returning from the Balkan War. All the soldier’s comrades had suffered severe diarrhea in the Balkans, but he had not. Since then, many studies have been carried out to show that this bacterium can help with diarrhea, gastrointestinal disease, and a compromised immune system. Although the soldier died many years ago, scientists continue to breed his talented E. coli in medical laboratories and package it up for sale in pharmacies so it can work its wonders in other people’s guts.

There is one limitation to the efficacy of all current probiotics we take: they are isolated species of bacteria bred in the lab. As soon as we stop taking them, they mostly disappear from our gut. Every gut is different and contains regular teams that help each other or wage war on each other. When somebody new turns up, they are at the back of the line when it comes to allocating places. So, currently, probiotics work like hair conditioner for the gut. When you stop taking them, the regular flora folk have to continue their work. To achieve longer-lasting results, researchers are now looking at the possibilities of a mixed-team strategy: taking several bacteria together, so that they can help each other to gain a foothold in unknown territory. They clear away each other’s waste and produce food for their colleagues.

Some products you can buy at the pharmacy, drugstore, or supermarket already use this strategy, with a mix of our trusty old lactic-acid friends. And it seems they really do work better as a team. The idea that we might be able to encourage these bacteria to settle permanently in our gut is a nice one, but it has not yet been seen to work well… to put it mildly.

But if you persist doggedly with the teamwork strategy, it can have impressive results, for example, in treating Clostridium difficile infections. Clostridium difficile is a bacterium that can survive treatment with antibiotics and then colonize the entire area left free by the bacteria killed by treatment. Infected people often have bloody, slimy diarrhea for many years, which does not respond to further treatment with antibiotics or probiotics. This can put great strain not only on the body, but also on the mind.

In difficult situations like this, doctors really have to use all their creativity. A few brave medics have now begun transplanting experienced teams of bacteria, including all possible good and true gut bacteria, from the guts of healthy donors to those with Clostridium difficile infections. Luckily, such a transplant is relatively easy to carry out (it has been used for decades by veterinary practitioners to cure many diseases). All you need are some healthy feces, complete with bacteria, and that’s it. The treatment is known as fecal bacteriotherapy, or more directly, a fecal transplant. The feces used in medical fecal transplants are not pure, but they are purified. Then, it does not really matter whether they enter through the front or back door, so to speak.

Almost all studies show a success rate of around 90 percent in treating previously incurable diarrhea caused by Clostridium difficile. Few medical drugs have such a high success rate. Despite these positive results, the treatment can currently be used only on patients with truly hopeless cases of incurable diarrhea. The danger is that other diseases or potentially harmful bacteria might be transplanted along with the donor stool. Some companies are already working to develop artificial stool that they can guarantee is free of any harmful elements. If they succeed, the therapy is likely to become much more widespread.

Probiotics’ greatest potential probably lies in transplanting beneficial bacteria to the gut, where they would settle permanently and grow. Such transplants have already helped patients with severe diabetes. Scientists are currently investigating whether they can also be used to prevent patients from ever developing full-blown type 1 diabetes.

The connection between stool and diabetes may not be immediately obvious to everyone. But, in fact, it is not as absurd as it seems. It is not just defensive bacteria that are transplanted with the stool, but an entire microbial organ that plays an important part in regulating the body’s metabolism and immune system. We are still completely ignorant of more than 60 percent of these gut bacteria. Looking for species that may have probiotic effects is time-consuming and difficult, just as the search for medicinal plants was in the past. Only this time, the search is going on inside us. Every day we live and every meal we eat influence the great microbial organ inside us—for better or for worse.

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