Clostridium clostridium wolf
Clostridium difficile vs. C. scindens
Anna Gogleva, "Biomolecule"The human body is inhabited by trillions of microorganisms, the totality of which is called the microbiome.
The microbiome performs many important functions – from the synthesis of vitamins to the breakdown of complex components of food. "Healthy" microflora constantly competes for limited nutritional resources with pathogenic microorganisms, thereby suppressing their growth. However, due to taking antibiotics or other reasons, the normal composition of the microbiome may be disrupted, and then pathogens are able to multiply uncontrollably, causing diseases. One of these pathogens is the bacterium Clostridium difficile, the causative agent of pseudomembranous colitis. The fight against C. difficile is complicated by its resistance to most known antibiotics. But recently it has been shown that the growth of C. difficile can be successfully suppressed not with drugs, but with the help of a related species – C. scindens. This discovery will serve as the basis for the creation of "smart" drugs-probiotics: effective against C. difficile, but safe for beneficial microflora.
Antibiotics are effective in fighting many deadly diseases, but at the same time they cause significant damage to the human microbiome. After a course of antibiotics, a person is usually more susceptible to infection with various pathogens. Clostridium difficile is a gram–positive motile bacterium, the main causative agent of acute nosocomial intestinal infections. Over the past 15 years, the number of deaths caused by the reproduction of C. difficile has increased at least 10 times, especially among elderly and weakened people [1]. The fight against C. difficile is complicated by the fact that under unfavorable conditions this species forms spores that withstand the action of antibiotics. This feature allows bacteria to re-colonize the intestine a few weeks or even months after the end of treatment.
It has recently been shown that microbiome transplantation of healthy donors cures severe C. difficile infections [2]. However, it remained unknown which members of the microbiome restore the body's resistance to C. difficile infection and by what mechanisms. And finally an article was published that sheds light on a rather unusual mechanism of interspecific interactions in the microbiome [3].
About mice and peopleC.
difficile is not only a human problem, but also a mouse one. To begin with, the microbiomes of mice were poisoned with different antibiotics and looked at how susceptibility to C. difficile changes from this. In general, antibiotics do not so much reduce the total number of bacteria in the intestine, as they significantly disrupt the balance of forces – the ratio of different taxonomic groups. It turned out that susceptibility to C. difficile clearly correlates with a general decrease in microbial species diversity. It was possible to identify a handful of 11 conditional species (operational taxonomic units) associated with resistance to C. difficile infection (Fig. 1).
Correlation between the presence of bacterial taxa in the microbiome and resistance to C. difficile infection [3].
Many of them were also clostridium (Clostridium XIVa cluster). Scientists drew attention to one taxon, the presence of which correlated most strongly with resistance to C. difficile, even in animals with extremely low species diversity of the microbiome. The hero turned out to be Clostridium scindens.
But that's in mice. How are things with a person? To determine the species associated with resistance to C. difficile infection, the microbiomes of patients who underwent allogeneic hematopoietic stem cell transplantation were examined. Most of them underwent chemotherapy and/or radiation therapy at the same time as a course of antibiotics during transplantation. Weakened immunity and reduced microbiome species diversity make these patients an easy target for C. difficile. In humans, it was possible to find two main species with which C. difficile did not get along. The strongest inhibitor was C. scindens, as in mice. Like a victory?
To test the inhibitory effect, C. scindens, along with several other promising bacteria, were launched into the intestines of animals that had recently taken antibiotics (Fig. 2).
Correlation between the engraftment of candidate species in the microbiome and resistance to C. difficile [3].
It turned out that such microbial transplantation significantly facilitated the course of infection caused by C. difficile, and also had a positive effect on reducing mortality and increasing body weight compared to the control. The most noticeable resistance to C. difficile was provided, as expected, by C. scindens. The implanted bacteria engraftment in the microbiome was tracked by the presence of the corresponding 16S rRNA gene. Resistance to C. difficile grew in direct proportion to the abundance of C. scindens. That is, improving the survival rate of C. scindens can increase protection against C. difficile. It is noteworthy that such a neat replacement of a bad clostridium with a very similar, but good one does not violate the balance existing in the microbiome (neither qualitative nor quantitative). A kind of antidote therapy, only at the microbiome level. All this opens up sparkling horizons for the development of safe drugs against C. difficile. But how exactly does C. scindens oppose C. difficile?
Mechanism of inhibitory actionIt is known that some secondary bile acids can impair the growth of C. difficile in vitro [4].
A very large number of microbiome bacteria have the bsh gene encoding bile acid hydrolase. However, a rare bacterium (an awfully small proportion of microbiome organisms) has all the genes necessary to implement the full pathway of bile acid biosynthesis. A rare bacterium, of course, turned out to be C. scindens, which has the 7α-hydroxysteroid dehydrogenase gene critical for the synthesis of secondary bile acids (Fig. 3).
Correlation between resistance to C. difficile and the presence of gene families,
necessary for the synthesis of secondary bile acids [3].
It is on this unique biochemical feature that the protective mechanism of C. scindens against C. difficile is based. The introduction of C. scindens into the microbiome of animals susceptible to C. difficile restores the necessary level and ratio of secondary bile acids: deoxycholic (DCA) and lithocholic (LCA). Both of these acids inhibit the growth of C. difficile in proportion to their concentration. It is noteworthy that C. scindens increases the amount of secondary bile acids to a physiological level and prevents the growth of C. difficile even in animals that have undergone antibiotic therapy. This mechanism is conservative – it is implemented both in the mouse microbiome and in the human microbiome.
Therapeutic perspectivesC.
scindens suppresses the growth of C. difficile due to the synthesis of secondary bile acids (for the black deeds of intestinal saboteur microbes that reduce the pool of bile acids and stimulate the development of atherosclerosis, read the article "Do not trust advertising, or the potential relationship of L-carnitine metabolism and the development of atherosclerosis" – Ed.). However, the use of these acids as an independent medicine is unsafe, since some of them are associated with the development of duodenal cancer [5]. It is much more effective to use C. scindens itself and work, for example, to improve the survival rate of this species in the microbiome. Other bacteria can help here through additional mechanisms: competition for carbohydrates with C. difficile, activation of the host's immune defense or production of antimicrobial peptides. Such a rational approach will make it possible to use C. scindens as an effective weapon against C. difficile, which does not harm the rest of the microbiome.
Literature
- Lucado J., Gould C., Elixhauser A. (2012). Clostridium difficile infections (CDI) in hospital stays. Statistical Brief No. 124 (Healthcare cost and utilization project of the Agency for Research and Quality Control in Healthcare (AHRQ), USA);
- de Vrieze J. (2013). Medical research. The promise of poop. Science. 341, 954–957;
- Buffie C.G., Bucci V., Stein R.R., McKenney P.T., Ling L., Gobourne A. et al. (2015). Precision microbiome reconstitution restores bile acid mediated resistance to Clostridium difficile. Nature. 517, 205–208;
- Sorg J.A., Sonenshein A.L. (2008). Bile salts and glycine as cogerminants for Clostridium difficile spores. J. Bacteriol. 190, 2505–2512;
- Bernstein H., Bernstein C., Payne C.M., Dvorakova K., Garewal H. (2005). Bile acids as carcinogens in human gastrointestinal cancers. Mutat. Res. 589, 47–65.
Portal "Eternal youth" http://vechnayamolodost.ru17.02.2015