Bacteria and viruses that kill bacteria

Like animals and plants, bacteria sometimes encounter predators. These are viruses called phages or simply phages that infect and destroy them. It was discovered and highlighted in 1915 by the Englishman Frederick Torte (1877-1950) and then in 1917 by Canadian Felix de Heerel (1873-1949). These “bacteria-eating” viruses were first observed under an electron microscope in 1940.

Microbiologists classify phages into two main categories. The first includes lytic phages that penetrate inside the bacteria and hijack the cellular machinery to reproduce their genetic material. When they multiply in a bacterial cell, it explodes and dies, releasing hundreds of new phages that can infect other bacteria. Bacteriophage T4 is the perfect example of a lytic phage.

Lysogenic or temperate phages such as Lambda Phage are the other large family of phages. They go inside the bacteria, but do not destroy them. They use it “hostage” and give it special properties, always for their own benefit.

For a long time, researchers believed that most phages infect only a limited range of bacteria, or even a single species of certain phages. Therefore, for this characteristic, phages were used long before the discovery of bacteria to treat infectious diseases. This phage treatment is not authorized in France, but is widely applied in many Eastern European countries.

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A discovery that calls into question everything

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Illustration of how a phage injects its DNA (genome) into a bacterial cell.

The latest study, conducted at the Technical University of Zurich, questions the specificity of phages to one type of bacteria. The Swiss team of scientists has discovered that some phages, such as those that attack bacteria of the genus Staphylococcus, can infect many bacterial species.

They also discovered a new property of phages. It kills bacteria, at least for lytic sterilants, and can also transfer antibiotic resistance genes from one bacterium to another. Therefore, this method of transferring resistance genes from one bacterium to another through a phage was studied by a team of researchers on Staphylococcus.

When phages attack bacteria, they inject their genetic material into the bacterial cell which then finds itself “forced” to reproduce the viral genome in several copies. Sometimes genes from bacteria, such as resistance genes, find themselves inadvertently inserted into the viral genome and are also transcribed. When the bacteria explode, they kick out new viruses whose genome contains this resistance gene. If one of these new viruses infects new bacteria, it transmits the resistance gene.

Staphylococci are shell-shaped bacteria and Gram-positive type including fifty species such as Staphylococcus aureus, Staphylococcus intermedius, Staphylococcus pseudintermedius, and Staphylococcus delphini. These bacteria are very common and colonize both humans and animals. It is also prevalent in the fresh waters of lakes and ponds, especially when water replenishment is low.

The most famous of these is certainly Staphylococcus aureus which is better known by the French name Staphylococcus aureus. It is naturally found on our skin or in our noses. In some cases, it can turn into a dangerous pathogen that is resistant to antibiotics. It is, among other things, responsible for many nosocomial infections, which are transmitted during a hospital stay or in a care center.

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Germs from sewage treatment plants

For their research, scientists found 94 types of natural phages of staphylococcus in the water of sewage treatment plants. They tested these phages on 117 strains of bacteria to try to understand the pattern of infection. Among its strains, there were 29 staphylococcal species with or without antibiotic resistance.

They realized that phage infects an average of four different types of bacteria. This multispecies infection allows staphylococcus to exchange genetic material with more than 17 other bacteria of the same family.

On the other hand, the frequency of resistance gene transfer between bacteria via phages remains ambiguous. From one phage to another, the rate of introduction of a bacteria resistance gene into a phage can range from 1 per 100 viral particles to 1 per 10 million. This depends, among other things, on the life cycle of the virus and the enzymes used to package the genetic material.

However, this result should not be considered negative. The fact that phages can infect several types of bacteria makes it possible to use them to treat more infectious diseases. However, care must be taken when selecting phages that have a well-known and well-controlled propagation mechanism.

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