There are a large number of bacteria living in the human intestinal tract, and the total number can reach more than 10 times the total number of human cells. Most of them are normal members of the gut environment, the "good bacteria". However, the intestinal tract is sometimes infected by pathogenic bacteria, causing various symptoms such as abdominal pain, diarrhea, and blood in the stool. These pathogenic bacteria are "bad bacteria". So what is the relationship between "good bacteria" and "bad bacteria"?
The intestinal tract is also an "ecosystem"
In fact, the relationship between "good bacteria" and "bad bacteria" is not static, and in some circumstances, normal members of the intestinal tract can also cause disease. In other words, "good bacteria" may also become "bad bacteria" under certain conditions. In short, the relationship between gut flora and the human body is very complex. An "ecosystem" is formed between the bacteria living in the gut and the body's immune cells. Therefore, for the gut, a healthy ecosystem is the most important. If the ecological system is out of balance, whether it is because of flora imbalance or immune deficiency, it is possible to turn some normal "good bacteria" into "bad bacteria", thereby causing harm to the human body.
We can use the macro-ecosystem as an analogy. In a normal forest ecosystem, plants, herbivores, and carnivores should be in a state of dynamic balance. If humans wipe out all carnivores, herbivores, such as voles, rabbits, and deer, may multiply in large numbers, resulting in excessive gnawing of plants, destruction of forests, and ecological degradation. The intestinal tract is also like a forest ecosystem. When the ecological balance is disrupted, there will be problems in the intestinal tract. If the intestinal tract maintains a relatively normal ecological environment, then the normal flora in the intestinal tract will not only pose no harm to the human body, but also resist the invasion of foreign pathogenic bacteria. Under normal circumstances, each ecological niche in the gut will be occupied by normal flora, and foreign invaders must compete with a large number of normal flora if they want to successfully colonize. If the ecological environment of the intestinal tract changes and the normal flora is destroyed, the invading pathogenic bacteria will not have to face such competition, and it will be much easier to colonize the intestinal tract. Pathogens entering a dysbiotic gut environment are like foreign organisms invading a fragile ecosystem. For example, invasive alien species (e.g. cats, rabbits) may overpopulate and cause ecosystem damage due to the absence of indigenous species in Australia. Intestinal flora imbalance can lead to unchecked and rapid growth of pathogenic bacteria invading the intestinal tract, resulting in disease.
In addition to directly competing for resources, the metabolic activities of normal flora will also synthesize products that are not conducive to the growth and reproduction of pathogenic bacteria, thereby inhibiting their growth. A recent scientific study found that when the intestinal tract of mice was infected with the pathogenic bacterium Klebsiella pneumoniae, the mice had a certain resistance to the subsequent infection of the bacterium. Interestingly, this resistance did not come from the mice's immune system, but from the normal flora in their guts. The study found that Klebsiella pneumoniae infection would change the intestinal flora of mice, so that the proportion of "good bacteria" that can utilize taurine in the intestinal flora increased significantly. The metabolites of taurine contain hydrogen sulfide, which can inhibit the aerobic respiration of bacteria. Since Klebsiella pneumoniae requires aerobic respiration, hydrogen sulfide inhibits the growth of this "bad bacteria" and thus prevents reinfection of the gut.
"Bad bacteria" that sneak in
The development of genomics research has allowed scientists to analyze a large number of gene sequences to study gut bacteria with different pathogenic potential at the genetic level. For example, by comparing the genetic differences between the generally non-pathogenic Bacteroides polymorpha and the pathogenic vancomycin-resistant Enterococcus faecalis, scientists have discovered the source of the "behavioral differences" between the two bacteria in the gut. The study found that the genome of Bacteroides polymorpha contains a large number of genes encoding hydrolase. These hydrolases degrade carbohydrates that cannot be digested by the human digestive system. The degraded products can not only be used by the human body as nutrients, but also can be transformed into substances that regulate the human immune system, such as short-chain fatty acids (SCFA). Therefore, as a normal intestinal flora, Bacteroides polymorpha is beneficial to human intestinal health.
In contrast, the genome of pathogenic vancomycin-resistant E. faecalis contains two gene sequences directly related to its pathogenicity. One of the genetic sequences, known as the "pathogenicity island," includes genes encoding adhesion molecules and hydrolases that metabolize bile acid, and the other contains the vancomycin resistance gene. Vancomycin is an important antibiotic and, in many cases, an effective weapon against multidrug-resistant infections. Therefore, the ability to resist vancomycin is obviously conducive to the growth and reproduction of the pathogenic Enterococcus faecalis in a specific intestinal environment.
So what makes this particular gut environment so special? Under normal circumstances, the ability to resist vancomycin or any other antibiotic (resistance) does not confer any survival advantage to pathogenic bacteria. However, after antibiotics are used on the human body, the normal intestinal flora without drug resistance will be eliminated. In the case of intestinal flora imbalance, "bad bacteria" with drug resistance genes will have certain advantages. Therefore, this mechanism can explain the phenomenon of enteritis caused by the use of antibiotics. When antibiotics kill the normal flora that is sensitive to antibiotics, the ecological niche is vacated for drug-resistant pathogenic bacteria, and "bad bacteria" can take advantage of it. So while antibiotics can treat bacterial infections, they also open the door to drug-resistant pathogenic bacteria.
The dangers of overuse of antibiotics
Unfortunately, at present, human beings seem to be inseparable from antibiotics, and there is also a phenomenon of antibiotic abuse. For example, antibiotics are used to treat diseases that should not use antibiotics (such as influenza). Altogether, the widespread use of antibiotics has greatly increased the likelihood of difficult-to-treat drug-resistant enteric bacterial infections.
In clinical research, the more typical example is Clostridium difficile, which can cause serious problems if antibiotics are used to treat the disease for a long time. The bacteria can cause severe intestinal inflammation and is difficult to treat with antibiotics because it is often resistant. C. difficile infection is often fatal.
However, there is now a new approach that can be used to treat Clostridium difficile infection. This method is "fecal microbiota transplantation", which is to transplant the intestinal flora of healthy people into the intestinal tract of patients to rebuild a balanced intestinal flora ecosystem, so as to control Clostridium difficile infection. Studies have shown that the effective rate of fecal bacteria transplantation in the treatment of Clostridium difficile infection is close to 90%, which undoubtedly provides a new way to treat this fatal disease.
Therefore, we should not only distinguish between "good bacteria" and "bad bacteria", but also regard the intestinal tract as a forest in the human body, just like treating the virgin forest in nature, to create a sustainable intestinal ecological environment . A correct understanding of the ecological balance of the intestinal tract will have a profound impact on human physical and mental health.
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