Bacterial viruses have been shown to be able to read chemical signals from other species indicating how many bacteria are still available for capture, and use this information to decide whether to enter dormancy or actively replicate. However, this "eavesdropping" was only useful for the viruses that produce these chemical signals themselves, not for those that read them. Since bacterial viruses are being explored as a means of combating antibiotic-resistant infections, the ability to influence the behavior of these viruses will improve the effectiveness of such measures. The study was published in the journal Cell.

HOW CAN DIFFERENT LIVING ORGANISMS COMMUNICATE WITH EACH OTHER?

Animals often use others' warnings of danger: for example, scaly-sided ...

Such signals often influence competition between individuals: nightingales can adjust their song to that of rivals for females to increase their chances of reproduction. And male elephant seals have been shown to recognize the song of their rivals and decide to retreat if they know it belongs to larger individuals. Such relationships also persist at the microscopic level: different species of bacteria and fungi influence each other's growth through chemical signals. But a British-American team of scientists led by specialists from the University of Exeter decided to investigate how viruses—organisms that lack a cellular structure and don't even produce their own signaling compounds—interact with each other.

WHAT HAS BEEN FOUND OUT ABOUT THE INFLUENCE OF VIRUSES ON EACH OTHER?

Scientists studied bacterial viruses called bacteriophages. The researchers focused on the transition between two behavioral patterns that bacteriophages can exhibit when they enter bacteria: bacterial lysis, which is rapid bacteriophage replication with host cell death and viral propagation, and lysogeny, which is the process of killing the host cell. Some bacteriophages are capable of inducing bacteria to produce short amino acid sequences called signal peptides. High concentrations of these signal peptides indicate to members of their species a shortage of available host cells and the benefit of lysogeny, while low concentrations indicate advantages of lysis, as there are many potential hosts nearby.

To determine how signal peptides from different bacteriophage species influence them, scientists studied free-living bacteria of the genus Bacillus, which frequently harbor bacteriophages in nature. By analyzing the bacterial DNA harboring lysogenic forms of bacteriophages, the scientists discovered that more than half contained only one bacteriophage capable of sending chemical signals. However, different bacteriophage species still frequently encountered each other in nature. Two species were found in 35 percent of bacterial genomes, three and four species were found in 4.3 and 3.9 percent, respectively, and one bacterium contained eight bacteriophage species.

To test the ability of bacteriophages to respond to signal peptides from another species, the scientists infected the bacterium B. subtilis with a virus incapable of producing its own peptides and then added peptides from eight different bacteriophage species to the colony. In response to the peptides from this virus and four other peptides from related species, the bacteriophage initiated lysogeny, despite the surrounding bacteria being available for infection. In subsequent experiments, the scientists confirmed that some bacteriophages are capable of responding even to signal peptides from related species.

WHY DOES THIS COMMUNICATION BETWEEN VIRUSES HARM SOME OF THEM?

Signal peptides in viruses likely evolved to conserve resources, as prematurely killing a bacterium would also harm the virus, which cannot survive without it. Therefore, when bacteriophages of the same species communicate via these peptides, it increases the likelihood of species survival. However, when another bacteriophage species relies on these signals, it may miss the opportunity to invade new bacteria, for example, those of a species that does not infect the bacteriophage sending the signal. Thus, the virus sending the signal peptides may even suppress competitors.