Gut: The Inside Story of Our Body's Most Underrated Organ

by Giulia Enders

if the floor is just covered in dirty footprints, water and a drop of cleaning fluid are all you need. That combination is already enough to reduce the bacteria population of your floor by 90 percent, and it leaves the normal, healthy population of the floor a chance to recolonize the territory. What remains of any harmful elements is so little as to be negligible.

One example of bacteria dilution in the home is washing fruit and vegetables. Washing dilutes most soil-dwelling bacteria to such a low concentration that they become harmless to humans. Koreans add a little vinegar to the water to make it slightly acidic and just that bit more uncomfortable for any bacteria. Airing a room is also a dilution technique.

Kitchen sponges should only be used for cleaning the worst of the dirt off. Plates, cutlery, and so on should then be rinsed briefly under running water. The same is true for dish towels or drying-up cloths if they never get a chance to dry out. They are more useful for spreading a nice even layer of bacteria on your utensils than for drying them. Sponges and cloths should be thoroughly wrung out and allowed to dry—otherwise they become the perfect place for moisture-loving microbes.

BACTERIA CANNOT BREED on dry surfaces. Some cannot survive there at all. A freshly mopped floor is at its cleanest after it has dried.

The optimum temperature for your fridge is something below 41 degrees Fahrenheit

damp kitchen cloths, a load of underpants, or sick people’s laundry should be washed at 140 degrees Fahrenheit (60 degrees Celsius) or more. Most E. coli bacteria are killed by temperatures above 104 degrees Fahrenheit (40 degrees Celsius), and 158 degrees Fahrenheit (70 degrees Celsius) is enough to kill off tougher Salmonella bacteria.

CLEANING MEANS REMOVING a film of fats and proteins from surfaces. Any bacteria living in it or under it will be removed along with the film.

we use the same system when we use hand cream. We trap microbes in a film of fat and hold them captive there. When we wash the film off, we rinse the bacteria away with it. Since our skin produces a natural coating of fat, soap and water are often enough to achieve this effect. Some of the fat layer remains, aiding its replenishment after washing. Too frequent hand washing makes no sense—and the same is true of too frequent showering. If the protective fat layer is rinsed away too often, our unprotected skin is exposed to the environment.

That gives odor-producing bacteria a better foothold, making us smell more pungently than before, creating a vici ous circle.

When researchers were planning a study of the effect of antibiotics on the flora of the gut, they were able to find only two people in the entire San Francisco Bay Area who had not taken antibiotics in the previous two years. In Germany, one person in every four takes antibiotics once a year on average. The main reason for taking them is colds. This is like a knife in the heart of any microbiologist. Colds are often not even caused by bacteria, but by viruses!

doctors can perform a procalcitonin test, which indicates whether cold-like symptoms are caused by a bacterial or viral infection.

Antibiotics can alter our gut flora significantly. Our microbe collection becomes much less diverse and the abilities of the bacteria in it can change. Changes include the amount of cholesterol they can absorb, their ability to produce vitamins (like skin-friendly vitamin H), and the type of foodstuffs they can help us digest.

Antibiotics can be problematic for children and old people. Their gut flora is already less stable and less able to recover after treatment with antibiotics. Research in Sweden showed that the gut bacteria of children were still significantly altered two months after taking antibiotics. Their guts contained more potentially harmful bacteria and fewer beneficial types like Bifidobacteria and Lactobacilli.

Bacteria develop resistance strategies. Some install tiny pumps in their cell walls to pump the antibiotic out like emergency workers pumping water out of a flooded cellar. Others prefer to disguise themselves so the antibiotics cannot recognize their surface and so cannot pepper them with holes. Yet others use their ability to split things—they build tools to cut the antibiotics to pieces.

Every year, many thousands of people die because they are infected with bacteria that have developed resistances that no drug can counter.

In countries like India or Spain, for example, there is almost no regulation of the amount of antibiotics given to animals. This turns the animals’ guts into giant breeding zoos for resistant bacteria.

It was not until 2006 that the European Union banned the use of antibiotics in animal feed as performance enhancers. The kind of performance being enhanced here is the ability of an animal to not die of infections in a crowded and dirty pen. Antibiotics are a great way to “enhance” this “performance.”

Wash fruit and vegetables thoroughly. Animal feces are a popular fertilizer, and liquid manure is used in vegetable fields.

Take care abroad. One traveler in four returns home carrying highly resistant bacteria.

Southern Europe also has its problems. “Cook it, peel it, or leave it” is not only a good rule for avoiding diarrhea, it also protects against unwanted resistant souvenirs for you and your family.

When parts of plants snap off or become perforated, they need to produce antimicrobial substances at the location of the damage. If they did not do this, they would immediately become a feast for any bacteria in the vicinity.

concentrated plant antibiotics to treat developing cold symptoms, urinary infections, and inflammations in the mouth and throat. Some products contain mustard seed or radish seed oil, for example, or chamomile and sage extract.

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.

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.

Some things are fermented using bacteria but then heated to kill off the microbes—shop-bought sauerkraut, for instance.

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.

These bacteria break down the sugar in milk (lactose) and produce lactic acid (lactate), so they are classified as lactic-acid bacteria.

Studies have shown that old people and high-performance athletes, in particular, are less prone to catching colds if they take probiotics regularly.

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.

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.

To gain the benefits they offer, just one condition must be met: good bacteria must already be present in the gut. These can then be encouraged by eating prebiotic food, which gives the good bacteria more power over any bad bacteria that may also be present.

Some doctors who recommend their patients eat more dietary fiber do not even really know the reason, which is that they are prescribing a hearty meal for your bacteria that will benefit you, too. Finally your gut microbes get enough to eat, so they can produce vitamins and healthy fatty acids or put the immune system through a good training session.

This is exactly how prebiotics can help: they are roughage that can only be eaten by nice bacteria.

(here’s a tip: rinse endive leaves briefly under warm water—this leaves them crispy but removes some of the bitterness),

Resistant starches form, for example, when potatoes or rice are boiled and then left to cool. This allows the starches to crystalize, making them more resistant to digestion. This means that more of your potato salad or cold sushi rice reaches your microbes untouched.

Eating these dishes regularly has an interesting side effect—it causes regular cravings for just such foods.

Bifidobacteria and Lactobacilli, for instance, do not produce any unpleasant odors. People who never need to break wind are starving their gut bacteria and are not good hosts for their microbe guests.

Toxins can develop, for example, if Joe eats too little dietary fiber and the fiber is all used up at the beginning of his large intestine. The bacteria at the end of his gut will then pounce on any undigested proteins. Bacteria and meat can be a bad combination—we know it’s never a good idea to eat rotten meat. Too many meat toxins can damage the large intestine and, in a worst-case scenario, can even cause cancer. The end of the gut is more prone to cancers on average than the rest of the organ.

Ninety percent of the roughage in human breast milk is GOS, and the remaining 10 percent is made up of other indigestible fiber. In cow’s milk, GOS accounts for only 10 percent of the fiber content. So it appears there is something about GOS that is important for human babies.

there are more than a hundred different kinds of sugar. If, historically, we had developed a sugar industry based on endive sugar, our sweets would not cause tooth decay.

other substances used in typical diet products are also used in factory farming to fatten pigs.

“psychobiotics” was coined. It describes microbes that have psychological effects—and which may even be useful in treating conditions like depression.

Lactobacillus casei Shirota (a bacterium you may know from the yogurt drinks available at most supermarkets) improved the disposition of the most ill-tempered third of their test subjects after they had taken it for only three weeks, from “depressive” toward “cheerful.”

Above all, studies have repeatedly shown that the least favorable reaction is fretful brooding—about who might be to blame for our woes, for example.

those who took the real bacteria improved their scores—in particular, in two areas, anger and brooding—by about 10 percent at least (which means roughly half the questions in those areas were answered one degree more positively).

WHILE MOOD CAN be seen as originating in various parts of our nervous system, stress is better described as the state the nervous system is in. A stressed nervous system is like a taut bowstring—in a constant state of alertness and sensitive to any external stimulus.

microbes could help reduce the physical effects of stress, such as stress hormone levels, nervous stomach aches, nausea, diarrhea, and susceptibility to colds.

Those who slept less in the run-up to an exam were always more stressed. This effect was less pronounced among subjects who swallowed a daily dose of Bifidobacterium bifidum. They were still stressed—but a little less so than their peers who slept equally badly but did not get the bacterium.

brain structure that stores our memories and links them to each other (the hippocampus) is very densely populated with sensors that detect stress hormones. If the hippocampus registers large amounts of such hormones, the brain cuts back the level of activity there. After all, if you are running away from a wild animal, you don’t need to waste energy remembering which plants you pass by. During stressful times in our lives, we develop a kind of tunnel vision—to enable us to direct our attention to the problem at hand.

If we engage in it enough, brooding over problems that cannot be changed can result in feelings of stress. The resultant stress hormones serve to increase that tunnel vision. And it becomes increasingly difficult for us to see anything beyond our own problems. And this, in turn, further increases our level of stress. Voilà: we have a vicious circle.