Bacteriophages trapped together in a bacterial cell have to decide whether to enter into an irreconcilable fight with each other or begin to cooperate.
Bacteriophages are viruses that infect bacterial cells. In general, their structural plan is the same as that of other viruses – it is a nucleic acid (most often DNA) with the virus genes written in it, enclosed in a protein shell-capsid.
Bacteriophages on E. coli. (Photo by Dennis Kunkel Microscopy, Inc. / Visuals Unlimited / Corbis.) Bacteriophages under an electron microscope. (Photo by Dr. Harold Fisher/Visuals Unlimited/Corbis.) ‹ › View full size
The appearance of bacteriophages is quite unique: their particles consist of a head, which looks like a regular geometric figure and contains the nucleic acid, and a tail, with the help of which the phage injects its nucleic acid into the bacterium. Some phages also have threads on their tails that look like legs, which help them stay on the surface of the bacterial cell. From the outside, they look like some kind of sinister mechanism, and when they talk about viruses, they most often imagine phages, with their tails and “legs.”
In an infected cell, bacteriophages behave in the same way as ordinary viruses: their nucleic acid switches over all the cell’s resources, forcing the bacterium to synthesize viral proteins and copy viral genetic material; the proteins and DNA of the virus are assembled into full-fledged viral particles, which fill the cell to capacity, and it eventually dies. Another way is also possible: the phage genome is integrated into the DNA of the bacterium and remains there for some time, passing in this form from generation to generation. Then, when it seems convenient to the phage, it is activated and starts “stamping” new copies of itself.
Different viruses usually specialize in different hosts, and bacteriophages are no exception. But it often happens that two phages of the same variety penetrate into the same cell. What happens in this case? Do they compete, trying to displace each other, or do they cooperate? Researchers from Texas A&M University say it can be either way, depending on the conditions in which the bacteriophages have to live.
Experiments were carried out with Escherichia coli and bacteriophage lambda. To allow tracking of two different phages and their descendants, they were labeled with four fluorescent tags. Two of them belonged to proteins of the viral envelope, and if protein tags appeared in the cell, this meant that the phages were hastily collecting ready-made particles.
The other two marks became visible if the phages turned on the genes responsible for preserving the viral genome in bacterial DNA. It turned out that when it came to assembling particles, the phages began to directly compete with each other – in the end, only one remained in the cell. On the contrary, if viruses decided to “sleep” in cellular DNA, they began to cooperate, helping each other integrate into someone else’s genome.
That is, the phages, who found themselves together in the same cell, had to decide how to live further. The decision largely depended on external conditions: if there were a lot of bacteria around, if the environment around was favorable for the growth of a bacterial colony, then the viruses entered into a race, trying to suppress each other by the number of descendants. The advantage was given to the one who managed to synthesize more DNA copies – viral DNA, as we said above, draws upon itself the cellular resources necessary for its own doubling, so the one whose DNA turned out to be more deprived of resources not only the host cell, but also the competitor . (It is important here who penetrates the cage when – the one who did it first will have a certain head start).
If there were few cells for reproduction, and if the cell itself lived poorly (so that it could not quickly create a lot of viral DNA), the viruses went to sleep together. Moreover, they really helped each other, as was evident in experiments with mutant bacteriophages: if two viruses with bad mutations in different genes responsible for integration into someone else’s genome entered a cell, then for successful implementation they used each other’s remaining normal proteins – in other words , solved the problem together. Being together, the mutant phages inserted themselves into the bacterial genome more efficiently than if they had entered the cell individually.
Such joint actions, as the authors of the work in Nature Communications write, increase the diversity of the viral genome: when integrated into bacterial DNA, their genes are mixed, so that in the future, when it comes to the synthesis of viral particles, such viruses that collaborated in the past can produce individuals with a new genetic variants. In short, mutual aid in times of need increased the genetic diversity of viruses, and diversity is always a good thing because it increases the chances of survival.
In the future, the results obtained may help in the creation of a new generation of antibacterial drugs. As you know, microbes quickly develop resistance to antibiotics, and therefore there is now often talk about taking a fundamentally different route – for example, starting to use bacteriophages as an anti-infective agent, and the more we know about the relationship of phages with each other and with bacteria , the more effective such drugs will be.