GM bacteriophages against antibiotic resistance
Genetically engineered antibiotics
Alexandra Bruter, <url>Since Alexander Fleming discovered penicillin, a new era has begun in medicine.
Diseases and wounds, which often turned out to be fatal before, no longer look so threatening. The risk of developing dangerous infections after surgical interventions has been reduced to a minimum. But antibiotics have two problems: non-specificity and antibiotic resistance. Both of these problems will be solved in one fell swoop by the approach proposed by the American-French group of scientists in the journal Nature Biotechnology (Bikard et al., Exploiting CRISPR-Cas nucleases to produce sequence-specific antimicrobials).
Antibiotics have a certain mechanism of action. Some pierce the bacterial cell wall, others inhibit the work of a certain bacterial enzyme, and others prevent the bacteria from synthesizing protein. Antibiotics are relatively harmless to humans (compared, for example, with drugs for cancer chemotherapy), because the molecular mechanisms of bacteria and humans are very different. A substance that inhibits the work of the bacterial ribosome will not inhibit the human ribosome.
But there are many non-harmful or even beneficial bacteria living in the human body. Antibiotics act on them in the same way as on pathogenic bacteria. Their death is undesirable for humans. The death of symbiotic bacteria in the gastrointestinal tract can lead to problems with the digestion of food. In addition, in the human body, on the skin and mucous membranes, populations of different types of bacteria and fungi are in a delicate balance. Representatives of different species restrain each other, the immune system monitors them all, and no one interferes with each other. A reduction in the number of bacteria of one antibiotic-sensitive species often entails the expansion of bacteria of another species or fungi that did not bother in peacetime, and the body has to fight another infection. Of course, all this is not so scary as to refuse to take antibiotics when there are indications for it, but it is quite unpleasant.
The problem associated with antibiotic resistance is much more serious. The DNA of bacteria, like the DNA of any other organism, mutates. Under stressful conditions, even faster than usual. For example, if the antibiotic used should inhibit a bacterial enzyme, then a mutation may occur in the gene encoding this enzyme in such a way that the resulting new enzyme will also cope with its function, and will no longer be inhibited by the antibiotic. Having received this mutation, the bacterium will successfully multiply in the presence of an antibiotic, and the patient will not recover.
In the most advanced cases, the bacterium becomes resistant to several antibiotics at once. In such cases, they talk about polyresistance. The polyresistance of the tuberculosis bacterium has already become a very noticeable problem. Cases have become more frequent when, with all the successes of modern medicine, the patient cannot be helped. The problem of antibiotic resistance is reinforced by the ability of bacteria to exchange genes among themselves. The gene that gives bacteria resistance may be encoded not in the bacterial genome, but in a plasmid, a small DNA molecule that exists separately from the main genome. Plasmids can get from one bacterium to another, or from a bacterium to the environment (for example, after its death), and from there to a new bacterium. Many bacteria, feeling that their affairs are bad, can even increase their susceptibility to plasmids from the outside world.
The method developed by the authors of the article allows killing only pathogenic bacteria or destroying plasmids inside bacteria that give resistance to antibiotics.
Scientists have adopted the CRISPR/CAS9 system, borrowed from bacteria. In bacteria, CRISPR/CAS9 plays the role of the immune system. Due to the presence of sequences in the genome that match the DNA of the enemies of bacteria – bacteriophages, bacteria identify bacteriophages at an early stage of infection, and the Cas9 protein cuts the DNA of phages. The mechanism of this is similar to the mechanism of RNA interference. After previous infections by a similar bacteriophage, fragments of its DNA remain in the genome. RNA is synthesized from them. According to the principle of complementarity, this RNA interacts with the genome of a new phage, the Cas9 protein recognizes this structure with the help of other proteins and cuts it. If you artificially synthesize the necessary RNA, complementary to any pre-selected sequence, and introduce it into cells together with a construct encoding the Cas9 protein, then any DNA in the cell will be cut at the selected location.
The authors synthesized genetic constructs encoding the Cas9 protein and sequences complementary to the genes responsible for pathogenicity and antibiotic resistance in Staphylococcus aureus. They packed these constructs into bacteriophages and infected harmless strains of S. aureus, pathogenic strains and antibiotic-resistant strains with these bacteriophages. It turned out that the bacteria of the non-pathogenic strain did not suffer, the pathogenic ones died, and the resistant ones lost their resistance. Non-pathogenic and unstable strains after such treatment could no longer become pathogenic or resistant, capturing the plasmid from the environment, because the structure remained in the cells forever.
Of course, this method is much more expensive than chemically synthesized antibiotics, but some infections no longer leave a choice.
Portal "Eternal youth" http://vechnayamolodost.ru06.10.2014