The discovery and mass use of antibiotics in medicine and agriculture in the second half of the XX century caused one of the main problems facing modern medicine – the emergence of resistant forms of pathogenic microorganisms. The therapeutic effectiveness and commercial success of the use of antibiotics have made our dependence on them absolute. We can no longer give up antibiotics, unless we want to return to the” stone age ” of high child mortality, hunger and reduced life expectancy. So can a natural antimicrobial system cope with this problem or can we expect complete dependence on antibiotics?
The set of resistance genes found in pathogenic bacteria for humans is only part of a huge repertoire of resistance genes, the main custodian of which is soil bacteria. In the scientific community, a set of such genes is called a resistor . The ability of pathogenic bacteria to exchange resistance genes with their soil “brothers” creates protection for them, which experts consider impenetrable.
In this light, the World Health Organization is rather pessimistic about the prospects for the existing arsenal of antibiotics. Why? The fact is that such recombination mechanisms under the continuing pressure of antibiotics give a selective advantage to microorganisms that simultaneously combine resistance to different classes of antimicrobial drugs. Doctors have already encountered such multi-resistant strains in their practice . And experts estimate as a real possibility of the appearance of the so — called “superbug” – a bacterium that will be resistant to all existing antibiotics. (Superbug exists in the same way as avian flu — that is, the fact of its existence is based on individual cases of the manifestation of a set of its signs. But the bacterium itself, as such, has not yet been isolated. Fortunately).
Relatively recently, another form of resistance, not related to gene transfer, was described — biofilms. It turned out that in addition to the well — studied planktonic form of the existence of microbial communities (when bacteria float freely in the environment), which allows them to quickly increase their numbers, there is another one-used by bacteria in unfavorable conditions. Some of the bacterial cells settle on the substrate and secrete biopolymers (proteins, carbohydrates, DNA) that form an intercellular matrix. The cells “embedded” in this matrix go into a state of metabolic rest, stop dividing and actively feed. A biofilm is formed — a community of bacterial cells interacting with each other. Who would have imagined that bacteria have complex mechanisms of chemical signaling, now called quorum sensing, and intercellular cooperation, allowing them to jointly resist the action of external factors. Even 20-30 years ago, it was believed that such levels of organization are peculiar only to multicellular living organisms.
The vast majority of antibiotics somehow affect the metabolism of the cell: it blocks the synthesis of protein, nucleic acids, components of the cytoplasmic membrane and cell wall. Therefore, a reversible decrease in the metabolic activity and reproduction rate of bacteria helps them avoid the toxic effects of antibiotics. In some cases, the effectiveness of antibiotics decreases many times during the transition of bacteria to the biofilm state.
The population of bacterial cells inhabiting the biofilm is heterogeneous. Among the bacteria, there are persistent cells that are maximally resistant to the action of an antimicrobial drug: even in the case of destruction of the biofilm and the death of most of the population, these cells quickly restore its number. Therefore, in some cases, in order to achieve the desired therapeutic effect, the concentration of the antibiotic would have to be increased to toxic for human cells, which means that it is impossible to continue using this antimicrobial drug in practice.
Bacteria can stay in the biofilm state for a long time, waiting out the effect of an unfavorable factor. It is generally believed that it is the ability of bacteria to form biofilms that underlies the phenomenon of chronic infections. Thus, there is a consensus among surgeons that the formation of a biofilm in a wound is guaranteed to complicate its healing. Moreover, it has already been established that biofilms are the root cause of a number of oncological and cardiovascular diseases, which were previously considered non-infectious diseases.
Scientists see a way out of this situation in the optimization of existing schemes for the use of antibiotics, the search for ways to block the transmission of resistance genes , the development of treatment methods that exclude the use of antibiotics, the creation of new materials for medical instruments that do not allow bacteria to gain a foothold.
Natural antimicrobial system – it is easier to borrow than to create
Many researchers consider combination therapy of bacterial infections to be one of the ways to solve the problem of resistance. At the heart of this campaign is the search for combinations of antibiotics that simultaneously affect the bacterial cell. The key criterion for the effectiveness of such combinations is synergy. The phenomenon of synergism consists in the fact that a combination of two substances has a property that its constituent components do not possess separately.
Meanwhile, the principle of comprehensive protection against bacteria is the basis for the functioning of all known immune systems. To resist foreign microflora, living organisms, starting with prokaryotes and ending with higher eukaryotes, use not single protective factors, but complexes of substances with antibacterial activity. This applies, in particular, to antimicrobial peptides-endogenous antibiotics that are ubiquitous in the animal world. In fact, each of these natural combinations can be considered as a prototype of a ready-made drug. In this sense, those living organisms that are in constant contact with human pathogens are of particular interest.
In 2015, in the journal PLoS One, our laboratory staff published an article describing the properties of the antimicrobial complex FLIP7, isolated from the larvae of the blue meat fly Calliphora vicina, infected with bacteria. We have studied the composition of the complex in detail, identified the substances included in it, established their structure and properties. FLIP7 includes four classes of antibiotics of a peptide nature: defensins, diptericins, cecropins and proline-rich peptides, each of which is represented by several isoforms. The spectrum of FLIP7 activity is very wide and includes such socially significant species as Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, etc.
In the course of research, we found one very interesting property of this complex. Enterobacteria, on which the antimicrobial activity of FLIP7 was studied, were unable to develop resistance to it for more than 20 passages (replanting), while the effective concentration of other antibiotics increased many times during the experiment. Note at the same time that not all bacteria are sensitive to FLIP7, but initially sensitive to it do not really develop resistance. Interestingly, this property is directly related to the multicomponency of FLIP7 — resistance to the antimicrobial components of the complex, taken separately, is formed quite quickly.
In another work, it was shown that FLIP7 does not lose its effectiveness on biofilm forms: the same concentrations of FLIP7 are sufficient for the destruction of biofilm as for the elimination of plankton culture. Moreover, when sub-inhibiting (not destroying it) concentrations of FLIP7 and an antibiotic are applied to the biofilm simultaneously, a pronounced synergistic effect is observed. The combination with certain antibiotics makes it possible to achieve complete destruction of the biofilm and elimination of persistent cells.
So, the natural antimicrobial system works, but for medicine, in turn, one of the urgent tasks is to find alternative methods of treating bacterial infections that are not based on “habitual” antibiotics.
Peptide antibiotics of natural origin have been in the focus of attention of scientists for more than 30 years and are considered as promising candidates for the role of antibiotic drugs (natural antimicrobial system), protected from the development of resistance. Researchers are also attracted by the high activity of antimicrobial peptides against biofilms. Meanwhile, no significant progress has been achieved in this area. The reason for the failure, apparently, is connected with a conservative approach to the development of antibiotic drugs, which is based on the effect of a single substance on a specific pathogen.