# CRISPR-Cas Systems: Prospects for Use in Medicine
## 4 CRISPR-Cas Implementation to Control Human Infectious Diseases  
The mortality rate from infectious diseases in the world is constantly increasing. Therapy for infectious diseases includes both treatment and prevention. Some of them (measles, polio, tetanus, etc.) can be prevented through vaccination, while the main approach in infectious diseases' treatment involves application of drugs specific to the pathogen [85; 86]. Besides drugs, targeting the pathogen directly, bacteriophages (phage therapy), blood preparations (hemotherapy), gammaglobulins, or immunoglobulins (serotherapy) are also used [87; 88; 89]. In addition, timely diagnosis is of great importance.  
Problems associated with drug therapy can be associated with both the patient (side effects and contraindications) and pathogen (drug-resistance) [90]. Another obstacle of drug use is the possible lack of specificity of new drugs to pathogens. Human symbiotic microflora is also often damaged by antibiotic therapy, while part of the pathogen population usually survives due to drug-resistance or persistence. One of the approaches to combat drug resistance and a nonspecific effect of drugs on symbiotic microflora is to search for new potential targets for the development of antibacterial agents [91].  
Attempts to establish the relationship between the phenomenon of drug resistance and CRISPR-Cas have been made for a long time. At first glance, it may seem that there is no direct connection between the systems of adaptive immunity and drug resistance in microorganisms. However, CRISPR-Cas is involved in the development of antibiotic resistance. For the first time, an inverse relationship between drug resistance and the CRISPR-Cas system was established in enterococci with two systems: CRISPR1-Cas, CRISPR3-Cas, and additionally a single CRISPR2 cassette lacking _cas_ genes. Drug resistance genes in enterococci are often located on transposons, which are actively transmitted in the bacterial population by conjugation and on plasmids, some fragments of which may be included as CRISPR spacers in the cassette. Cleavage of plasmid DNA upon entry into a cell thus prevents the pathogen from acquiring resistance [92]. A similar situation is typical for _Klebsiella pneumoniae_[93]. In addition, in a number of microorganisms, strains lacking CRISPR-Cas systems more actively form biofilms, which provide additional protection against antibiotic penetration. In _Enterococcus faecalis_,which is the cause of infection in dental canals, a gene, encoding a surface aggregation factor cytolysin (a virulence factor, which interacts with host immune system), is often located in prophages and pathogenicity islands (PAI), and is able to be transmitted in the population similarly to the drug resistance determinants. The loss of the CRISPR cassette results in an increased chance of acquiring mobile genetic elements carrying those genes [94].  
In 2017, mutations in the genes of the adaptation module (_cas1_ and _cas2_) were shown to be associated with drug-resistance emergence in the intestinal pathogen _Shigella_. These data are experimental, and there is currently no explanation of this phenomenon [95]. CRISPR-Cas is involved in the formation of the cell wall in _Francisella novicida_ through BLP regulation. This leads to the development of antibiotic tolerance [16].  
However, despite the fact that the CRISPR-Cas system could be involved in the development of drug resistance, they do not represent interest as drug targets, because, despite their essential role in maintaining bacterial immunity, they are not essential for cell survival; moreover, CRISPR-Cas can have structural variants within the same species and differ in different strains.  
On the other hand, the use of CRISPR-Cas systems seems promising as a tool for combating drug resistance, since the field of their application is very wide.