Health intestinal infections
What makes harmless bacteria evil
Status: 22.02.2024 | Reading time: 4 minutes
An EHEC researcher shows E. coli cultures. Some are harmless, some are very dangerous
Quelle: picture alliance/dpa/Guido Kirchner
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Everyone knows E. coli, friendly common unicellular organisms that, among other things, live in large numbers in the human intestine. But sometimes they turn into threatening germs. Researchers from Australia can now explain what turns harmless bacteria into such malicious ones.
In May 2011 the time had come: Germany no longer bought cucumbers. A mysterious and life-threatening infection is spreading, especially in the north of the country. People are suffering from life-threatening diarrhea and dozens are being hospitalized. The health authorities suspect imported vegetables from Spain.
In the end, 53 people are dead and it is clear that it was not imported cucumbers but domestic sprouts that were behind the outbreak. In addition, every German now knows this abbreviation: EHEC, the name of this pathogen. When written out it is: Enterohemorrhagic E. coli.
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E. Coli bacteria are actually friendly, common unicellular organisms that live in large numbers in the human intestine. Normally you have nothing to fear from them. “Enterohemorrhagic,” however, is a threatening word. It means that the germs cause the intestines to bleed. They produce certain toxins called “Shigatoxins,” which are very similar to the toxins secreted by cholera bacteria. The problem is that even if the bacteria are killed with an antibiotic, these substances continue to work for a while.
There is another group of potentially deadly E coli strains, they are called “extraintestinal pathogenic” E. coli, or ExPEC for short. Their name means: They can leave the intestines and then make their host very sick. ExPEC infiltrate internal organs and also the nervous system. Some of them also produce Shiga toxins, but not all. They are still dangerous.
Researchers led by molecular biologist Mark Schembri from the Institute for Molecular Bioscience at the University of Queensland can now explain why: These bacteria then move around because they have developed a specific genetic defect. They report this in the journal “Nature Communications”.
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E. coli are not only friendly, but also sociable; they live in colonies with other members of their species, the “biofilm”. It consists not only of the bacteria, but also of a mass with which the single-celled organisms surround themselves and through which they can exchange substances. The mass consists, among other things, of fibers and cellulose, the same molecule that makes up the wood of plants. The fibers hold the bacteria in the matrix.
“Good” bacteria become “bad” when they can no longer produce cellulose, explains Schembri in a university press release. Apparently they can then more easily break away from the community and go on a journey within the body. “The mutation gives the bacteria the opportunity to infect other organs such as the liver, spleen and brain.”
Inflammation makes the intestines leaky
There are also elements in the bacteria’s shell that can trigger the host’s immune system and are probably easier to access without the fiber matrix. In the study, once the bacteria mutated, there was increased inflammation in the intestinal tract.
And even if the immune system actually tries to ward off germs with such reactions, in this case it doesn’t help the host, it helps the bacteria, as Schembri observed. Inflamed wall cells can no longer keep the intestine as tightly demarcated as healthy ones. “The result was a breakdown of the intestinal barrier, allowing the bacteria to infiltrate the adjacent tissue,” explains the researcher.
He did not examine the phenomenon in humans, but rather in newborn rats. Just like human babies, they can die from ExPEC because they invade their nervous system and colonize the meninges. Bacterial meningitis in young children is feared because it can quickly lead to death. The hope is that the bacteria can be stopped in the future because we now understand what makes them so mobile.
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For Schembri, there was another reason to take a closer look: E. coli, both the good and the bad, play a central role in the spread of bacterial antibiotic resistance among all sorts of other germs. Some of these species live with you in the biofilm. “In 2019 alone, almost five million deaths worldwide were associated with bacterial antibiotic resistance,” says the infection researcher.
Mutated E. coli caused more than 800,000 deaths. Now the big question is how well or poorly the spread of resistance genes works when the bacteria change their fibrous matrix. This knowledge could also become a starting point for drugs that block the spread of resistance.