If I don't have me, I have bacteria

About 14^10 cells live in our body microorganisms, most of them bacteria. Where are they hiding, what is their benefit, when do they become dangerous and is yogurt with probiotic bacteria healthier?

Dror Bar-Nir

If you've ever wondered what our body is made of: the human body is made up of trillions of cells, more precisely the order of magnitude of 13^10 cells. But apart from these cells, about 14^10 more cells have joined our body since it emerged into the world. These cells, all of micro-organisms (mostly bacteria - and we will mainly refer to them here; a minority of them are fungi and protists) - exist in the body in a ratio of about 10 guest cells to each of our cells! (And this without mentioning the 50 "descendants of bacteria", on average, that reside in each of our body's cells - the mitochondria). Because of their size (the bacterial cell is very small compared to a eukaryotic cell), these guests make up only 2-1 percent of the body's mass.

Where are all these guests hiding? They are not hosted in each cell separately (they do not penetrate the cells), but inhabit several areas of our body. The main mass of our guests is concentrated in our digestive system - in the small and large intestines (about 14^10). Smaller masses inhabit the oral cavity and nose (10^10), the skin (12^10), and in women also the vagina (7^10).

How did they reach us? A fetus in the womb is completely sterile (lacks microorganisms). He "acquires" the first microorganisms from his mother already during the passage through the birth canal - then the bacteria begin to populate the skin and the mouth and nose. Later, the digestive system is populated by various bacteria that come with the food and drink. The bacteria change in their composition and multiply in their living places according to the environmental conditions that our bodies provide for them. These conditions include the food we eat and our hormonal status - therefore they differ between different people and between the same people at different times in their lives.

Bacteria do not inhabit our internal organs, and these remain sterile as long as we are healthy. Our blood is also generally sterile, but any injury, even a slight one (for example while brushing teeth), causes bacteria to enter it. These bacteria are detected and destroyed within a few hours by the leukocytes, the white blood cells.

A question of definition

How will we define these guests? Since they live with us and within us, it is clear that they are symbionts (from the word symbiosis - living together) and most of them are endosymbionts (in Greek, endo = parrot). There is an ongoing debate among researchers whether this is a commensal symbiosis (commensalism - in which one symbiont benefits while the other is not harmed; commensalism in medieval Latin means "together at the table"), or a mutual symbiosis (where both parties benefit from the partnership).

Bacteria free animals

To avoid infection in the birth canal, bacteria-free animals are removed from the womb under completely sterile conditions by caesarean section. They are then kept in a sealed chamber, where the air penetrates through filters that are impermeable to microorganisms of any kind, and the food and water are sterilized. It is possible to inject certain species (or a combination of species) of bacteria into these animals in a controlled manner, thus trying to know the specific contribution of these bacteria to the animals. Animals that know exactly which bacteria they have are called gnotobiotic animals.

To answer the question, a series of experiments was performed on animals free of bacteria (mostly mice and rats - see box). In the first experiments, these animals developed vitamin deficiency symptoms, especially vitamin K, and even died as a result. Later it turned out that some of the intestinal bacteria are the exclusive suppliers of this vitamin. It also turned out that they also produce and excrete vitamin B12. The lactic acid bacteria provide other vitamins from the B group. The artificial addition of vitamins to the food solved the problem, and the impression was created that the animals were growing and developing normally. But once the animals were removed from the sterile conditions and exposed to the world's air (and the microorganisms accompanying it), they did not survive beyond a few hours (and see one of the possible reasons below).

If so, it turns out that these bacteria are absolutely necessary for our existence, and their importance is expressed in several areas.

The natural flora and the digestive system

The intestinal bacteria contribute to us, besides vitamins, additional nutrients (certain fatty acids). They also help us digest certain foods. With their help, the absorption of iron ions, calcium and magnesium from the digestive system into the blood is also optimized.

Bacteria and their secretions have a very important effect on the development of the walls of the digestive system: they influence the development and differentiation of the epithelial cells lining the intestinal wall, which play an important role in the system's activity. In bacteria-free animals, the amount of epithelial cells is significantly reduced, and the function decreases accordingly.

A "disadvantage" of the bacteria of the natural flora is their energy consumption - they actually "waste" energy that could be used by the host. If you compare the food consumption of "normal" animals and animals free of bacteria, then the latter consume less food. Because of this, huge amounts of antibiotics are used in farms to save part of the food consumed by farm animals. (On the resistance problem caused as a result, see "Antibiotics", "Galileo" 7).

The natural flora and the immune system

The bacteria "train" our immune system and prepare it for dealing with the real world, full of microorganisms. The immune tissues of the digestive system (galt - gut associated lymphoid tissues) are highly developed and contain a large pool of cells. In bacteria-free animals, these tissues degenerate and do not function properly. Stimulation of these immune tissues by certain antigens, which in normal animals results in the creation of secreted antibodies (IgA), in bacteria-deficient animals results in the creation of allergenic antibodies (IgE), which cause the release of pharmacological substances from the mast cells. These substances cause symptoms known to us as allergies: cough, runny nose, rash, shortness of breath, and in extreme cases even death (which probably occurs in some of the bacteria-free animals upon their sudden exposure to the environment).

The bacteria that live on our skin

If we want to get to know the bacteria that inhabit our skin, we will use normal hand soap. We will take a growth plate with a rich substrate for growing bacteria. We will divide the plate into two areas; We will touch our fingertips to one area, and after thoroughly washing our hands with soap and water, we will touch the other area. After incubation at room temperature for two to three days, we can see that each area has different bacterial colonies: the area we touched before washing our hands grew colonies of environmental bacteria that we collected when we touched different objects. Bathing with soap and water removed from our skin the environmental bacteria we collected and the fatty layer secreted from our skin, revealing our symbiotic bacteria - and these grew colonies in the other area.

The skin bacteria and their secretions contribute to a significant part of the odors that characterize us, especially when we forget what a shower looks like, or to change socks.

Introducing bacteria from the natural flora to the bacteria-free animals before stimulation can restore the situation to normal, but this is only if the bacteria-free animals are very young. These and other findings led the researchers to hypothesize that the allergy symptoms known to us in about 10% of the population result from insufficient early childhood exposure to certain environmental bacteria (for example, Mycobacterium vaccae), and they even suggested vaccinating the entire population with these bacteria.

The very presence of the bacteria of the natural flora in all openings of our body makes it difficult for disease-causing bacteria to populate the system and cause us diseases. To infect and cause intestinal disease in "normal" mice, at least 106 Salmonella bacteria are needed. To cause the same disease in bacteria-free mice, 10 salmonella bacteria will be enough! Some of the bacteria of the natural flora secrete substances (called bacteriocins) that inhibit the growth of certain pathogenic bacteria.

The natural flora of the vagina

An important example of the protective effect of the bacteria of the natural flora is the vaginal bacteria. Until the age of childbearing, and even after, the bacterial population in the vagina is mixed and similar to other bacterial populations in the woman's body. But the activity of estrogens during the fertile age causes the secretion of the polysaccharide glycogen. The lactic acid bacteria in the vagina ferment the glycogen and secrete organic acids that lower the pH in the area. The low pH does not allow bacteria other than the lactic acid bacteria to exist in the vagina. In this way, infections of the vagina with bacteria that come from the outside (for example during sex) or from the digestive system through the nearby opening, the anus, are avoided.

The importance of the location of the natural flora

It is important to emphasize that the bacteria of the natural flora are indeed bacteria that benefit us (or are not harmful) as long as they are in their place and do not penetrate other parts of the body, and that the condition in the body is stable and is not changed by external factors. Injury to the walls of the digestive system by a foreign object that entered the system can cause bacteria to enter the blood system and the development of life-threatening sepsis and/or other diseases.

Even bacteria living on the skin, which penetrate during an injury or injection without proper disinfection, can cause a local infection or sepsis.

Extensive antibiotic treatment, during an illness caused by some bacteria, also harms some of the bacteria of the natural flora and therefore may harm the shelter as well. Clostridium difficille bacteria, for example, are part of the natural flora. When their competitors are harmed, they may thrive and increase their relative abundance, causing a serious intestinal disease known as pseudomembranous colitis, which can be fatal.

Escherichia coli, the famous bacterium of the digestive system (which is a marker of fecal infection), may pass from the digestive system to the urinary system and cause an unpleasant inflammation.

Even some of the streptococci, which are among our oral bacteria, can cause heavy damage when they enter the blood following invasive dental treatment (root canal treatment or complicated extraction). With the blood, the streptococci reach internal organs, and can cause serious diseases in the heart (especially in people whose heart valves are damaged) and brain. That is why some recommend receiving, before such treatments, preventive antibiotic treatment, which will prevent these bacteria from reaching internal organs.

The natural flora and the microbiome project

In May 2006, the project to determine the nucleotide sequence of the human genome was completed. A careful scan of the genome content revealed 223 foreign genes - all of bacterial origin! It seems that these genes originate from bacteria that were hosted in the bodies of our ancestors.

But is the project really completed? What about the genome of all our regular guests - it also has an important contribution to our biology. The late Nobel laureate microbiologist Joshua Lederberg, who passed away at the beginning of last February, called all of these guests the microbiome.

Indeed, in December 2007 the NIH announced the microbiome project, for the implementation of which 115 million dollars were allocated for the next five years. In the first step, the genome sequence of 600 known bacterial species from the natural flora will be determined. And this is just the beginning…


In the last decade we have witnessed an advertising attack, mainly in the dairy industry, encouraging us to consume products (mainly yogurts) containing probiotic bacteria. What is probiotics? Who are the probiotic bacteria, and are they really able to contribute to our health?

Probiotics (the meaning of the name in Greek - for the sake of life) is a term indicating a variety of bacteria that is usually added as a food additive to the various yogurts. These are mainly three species of the genus Lactobacillus: L. rhamnosus, L. acidophilus and L. casei. and in two species of the genus Bifidobacterium: B. bifidum and B. longum, which originate from the bacteria of the natural flora in the intestine. (One species, L. acidophilus, also inhabits the vagina). Some of them are found in yogurt naturally (they are the ones that ferment the milk sugar, lactose, into organic acids).

There is no debate that in various disease states - mainly in the digestive system, but also in the vagina - treatment with live, and sometimes even dead, probiotic bacteria given orally contributes significantly to recovery (see Marit Salvin's article in "Haaretz"). Even after antibiotic treatment, which damages the natural flora, it is possible to use them to restore the natural flora.

But the main argument raised against this advertising attack, which recently even reached the court, is the illusion that the advertisers are trying to sell to consumers, that daily consumption of dairy products enriched with bacteria may contribute to the health of a healthy person and prevent him from diseases.

The amount of probiotic bacteria given to patients with diseases of the digestive tract is tens of times greater than the amount of bacteria found in yogurt (either naturally or as an artificial supplement).

The amount of probiotic bacteria added to a cup of yogurt (about 10^10) is negligible compared to the amount of bacteria in the digestive system (14^10). When there is a medical need to treat probiotic bacteria, use the amount equivalent to the amount of bacteria added to 150-100 cups of yogurt.

We will conclude that probiotic bacteria are a good remedy for certain intestinal and other diseases, but when added to a cup of yogurt, they are not much different from the original yogurt bacteria.

And in conclusion

14^10 The bacteria that inhabit our body greatly influence its health and function. The human body is their home. Some consider the combination of the body's cells and the cells of the bacteria that reside in it to be one biological entity, functioning in harmony. Without them, we will not survive.

Dr. Dror Bar-Nir teaches microbiology and cell biology at the Open University.