It is not easy to kill bacteria, but there are viruses that specialize in this. Can they be used in the treatment or prevention of diseases caused by bacteria?
By: Dror Bar-Nir
Viruses attack bacteria
The bacteriophages - viruses that attack bacteria - cause an explosion (lysis) of the bacterial cells in which they multiply. The first evidence of their existence was published in 1896, when the English bacteriologist (researcher of bacteria) who worked in India, Ernest Hankin, claimed that a mysterious factor in the waters of the Ganges and Jumna rivers caused the cholera bacteria to disappear from the cultures he studied.
Henkin realized that this factor was not a bacteria, because he filtered the water and removed bacteria from it. Two years later, Russian bacteriologist Nikolai Gamaleya observed a similar phenomenon in Bacillus subtilis bacteria. Additional sightings of the strange phenomenon were reported from time to time, but there was no follow-up to them.
About two decades later, in 1915, the British researcher Frederick Twort reported on the isolation of microorganisms that could pass through filters that prevent the passage of bacteria (that is, smaller than 200 nanometers), with which he was able to create clear foci on a mat of bacteria in a plate.
Foci of bacteriophages
Bacteriophages, as well as other viruses, are too small to be seen by the human eye, even with the best light microscopes (apart from a few exceptions, such as the smallpox virus); To see them, electron microscopes must be used. But most virologists (researchers of viruses) do not use an electron microscope for their work, but observe foci of viruses on dense mats of cells, and use the contents of these foci.
What is a focal point?
A focus is formed when a bacterium infected with bacteriophage is found on a developing mat of bacteria (Petri dish). When the bacteriophage finishes multiplying, the bacterium explodes, and the bacteriophages released from it infect all neighboring bacteria. These also explode after a while and infect their neighbors, and so on. The continuation of the process would have caused all the bacteria in the plate to explode, but bacteriophages can reproduce only within dividing bacteria - and when these stop dividing, the bacteriophage's infection cycles stop.
As a result, a focus is formed - a clear circle, devoid of bacteria, on the cloudy carpet of dense bacteria. Each such focus started with a single bacteriophage that infected a single bacterium. Each center contains millions of bacteriophages - all descendants of the same first virus, which are not visible to the eye, but can be collected and continued work.
The bacteria eaters
Even before that, in Mexico, in 1910, the Canadian-French bacteriologist Felix d'Herelle, who was studying a deadly disease of locusts, encountered a similar phenomenon - the appearance of clear foci on the bacterial mats he isolated from the insects. But it wasn't until 1917, when he moved to the Pasteur Institute in Paris and tried to develop a vaccine against the Shigella bacteria that caused diarrhea (dysentery) among the French soldiers in World War I, and came across the phenomenon, that Dahrel realized that before him were viruses that attack bacteria, and he was the one who invented the name "bacteriophages" for them - eater The bacteria (phagus in Greek means "food"), and coined the name "plaques" for the centers free of bacteria.
Dahrle was the first to come up with the idea of using bacteriophages as a means of treating diseases caused by bacteria. In 1919, Dehrle's group was able to successfully treat several dysentery patients with a suspension of viruses, which they prepared by adding the contents of a clear focus to a test tube containing a suspension of growing bacteria. The researchers first drank the suspension themselves, to verify its safety. Dharle and his colleagues also conducted successful trials of treating cholera and dengue patients with viruses in India.
Another group that began to successfully treat bacterial infections, especially surgical wounds that were contaminated with staphylococci, was the group of Richard Bruynoghe and Joseph Maisin. The direction looks promising - in the 20s, two large companies, L'Oreal in France and Eli Lilly in the United States, produced a total of a dozen commercial preparations of different bacteriophages.
What stopped the development of the field, at least in what is known today as "the West", was the discovery of antibiotic substances. The antibiotic substances were portrayed in the eyes of many as the miracle drug that would solve the problem of bacterial diseases, and the need for bacteriophages as an antibacterial drug was pushed aside. At the same time, bacteriophages began to be used as a main research tool in molecular biology, and Dharle is considered one of its founders.
In the Soviet Union, its political metastases and descendants continued to develop and use bacteriophages to cure diseases. Two institutes led the research in the field of human care: the Hirszfeld Institute in Poland, founded in 1952, and the Eliava Institute in Georgia, of which Felix Dahrel was one of the founders in 1923, and would have settled there if the founder and director, the bacteriologist, had not been banned Georgi Eliava (Eliava), in 1937 by Stalin's regime.
Better than antibiotics
Following the appearance of antibiotic-resistant strains of many pathogenic bacteria, the West also began to return to the viral option. The viral option has an advantage over the (fading) antibiotic option also in that the viral damage is very unique and it is possible to target only the pathogenic bacteria, while the antibiotics damage in a unique way, and therefore also harm the bacteria of the natural flora, which are of great importance to the patient's health. Damage to these bacteria may cause a secondary bacterial disease, for example the severe intestinal inflammation (pseudomembranous colitis) caused by the bacterium Clostridium difficile, which thrives as a result of the damage to the normal bacterial flora of the intestines.
The treatment of viruses has another advantage: when the target bacteria develop resistance to the bacteriophage (usually as a result of a mutation in the coat protein used as a receptor for the virus), it is possible to locate in a relatively short time another virus, or a mutant of the same virus, that will kill the bacteria. Finding new antibiotics is a longer and more expensive process.
The disadvantage of using bacteriophages is, among other things, the difficulty of damaging bacteria that are intracellular parasites, for example chlamydia and rickettsiae. Since these bacteria are inside the host's cells, it is not yet clear how they will succeed in bringing the bacteriophages to them.
The bacteriophage as a DNA injector
The bacteriophages, which are similar in most of their properties to the animal viruses (viruses that attack animals), have to overcome a special problem. The cells of most bacteria are surrounded by a hard wall that does not allow viruses to penetrate. To overcome the problem, some bacteriophages are equipped with a special structure: a hollow tube is attached to the protein box that wraps the genetic material of the virus, wrapped in a protein tail that can contract. After the virus adheres to a special protein in the bacterium's envelope, the protein tail shrinks, and then the tube penetrates, like the needle of a syringe, through the wall and the membrane into the bacterial cell. Through the tube, the hereditary material is transferred into the bacterium - the envelope of the virus remains outside the cell, as you can see in the figure.
A variety of bacteriophages and a method for identifying bacteria
Later in the study, many bacteriophages were found, and it turned out that most of them are unique to the host, and even to certain strains of the same host, and only a minority of them can infect different hosts. The unique viruses are usually referred to by the name of their host along with some nickname. Coliphage and coliphage T4, for example, are viruses of Escherichia coli. Staphylophages are viruses that infect staphylococci.
The uniqueness of the infection allows the laboratories, which hold a large variety of bacteriophages, to identify and differentiate between the different strains involved in epidemics and outbreaks of bacterial diseases.
The bacteriophage contribution to bacterial violence
So far we have known the bacteriophages that always kill the bacteria that host them. However, some bacteriophages have another feature - they can remain in the host cell in a dormant state, known as a prophage, while they are actually part of the bacterial genome. But when the bacterium is damaged, they can "wake up" and switch to the "lethal" (helitic) state, multiply, escape from the bacterium and look for other hosts. It turns out that such bacteriophages, in the prophage state, sometimes carry with them genes that are expressed in the bacteria.
There are bacteria whose prophages make up a significant part (20-10%) of their genome. The pathogenic strain of E.coli, O157:H7, for example, contains 18 different prophages, which make up about 16% of the genome. Some of these genes give bacteria the ability to cause disease: bacterial toxins derived from prophage genes give Clostridium botulinum the ability to cause botulism, and Corynebacterium diphtheriae the ability to cause diphtheria.
Even to the bacterium Streptococcus pyogenes, which causes strep throat and sometimes more serious diseases, a prophage is sometimes added that codes for the toxin that causes the characteristic rash of scarlet fever. Dysentery is also caused by strains containing toxin-encoding prophages of E. coli and Shigella. Protein attachments originating from prophages allow Streptococcus mitis (known to us as the cause of caries) to attach to the blood platelets and through them reach the pericardium and cause inflammation there. A transmission system originating from the prophage is responsible for the cell penetration ability of Salmonella strains.
In conclusion
Bacteriophages are parasites of bacteria, which usually cause their death. In the current period, towards the end of the antibiotic era, it is possible that bacteriophages will replace the antibiotic substances as a treatment for patients with bacterial diseases. Viruses in a "dormant" state have a significant genetic contribution to the host bacterium, and some give it new properties that contribute to its ability to cause disease.
And the main thing is that we will be healthy.
Dr. Dror Bar-Nir teaches microbiology and cell biology at the Open University
