Bacteria exploit viruses to distinguish their friends from their enemies

The bacterial cells that normally colonize our entrails can be distinguished from other bacterial species by using what is traditionally considered their enemy: a virus. Researchers report April 16 in the newspaper Cell reports that some bacteria use viruses that have infected them (that is to say, phages) to recognize themselves and thus show a better physical shape, repelling competitors who do not have this adaptation.

"This is the first evidence that cells can distinguish themselves from their competitors by the use of a virus," said Thomas Wood, of Pennsylvania State University, a main authors of the study. "The implications are that we should re-evaluate the relationship between a virus and its host cell, as a viral infection sometimes has advantages."

The idea for the study began when Wood and his team noticed a demarcation line that was formed between different strains of Escherichia coli K-12, but not between identical clones, while they swam to others. To study the underlying mechanisms, Wood, Pennsylvania State University's first author Sooyeon Song and co-lead author of the study, Xiaoxue Wang of the Chinese Academy of Sciences, reviewed the behavior of full swim E. coli Kindergarten to Grade 12 library with 4,296 inactive entries for a single gene.

They discovered that the demarcation line had completely disappeared for a single mutation affecting a gene necessary for the replication of certain phages. The results suggest that phage-related proteins are responsible for bacterial self-recognition.

In keeping with this idea, the researchers found that the demarcation line had also been removed E. coli Strain K-12 lacks all nine cryptic prophages – bacteriophage genomes that have integrated with bacterial chromosomes but do not form active phagic particles or break ("lyse") their host cells. In particular, additional experiments revealed that cryptic prophage CPS-53 and one of its proteins, YfdM, were needed for the demarcation line.

Since CPS-53 is essentially inactive in terms of cell lysis and phage particle production, the researchers suspected the formation of the demarcation line by cell lysis caused by another active phage. They found that exposure of bacterial cells containing an active lytic phage called SW1 to YfdM stimulated the production of phage particles and caused lysis of cells, mainly those lacking SW1. Higher concentrations of YfdM or phage particles produced thicker demarcation lines between bacterial cells. The results suggest that SW1 controls the formation of the demarcation line using one of the host's cryptic prophage proteins, YfdM of CPS-53, to spread.

"E. coli uses the tools of his old enemy, stuck in his chromosome, to work with this new SW1 virus, which stays mostly outside the cell, basically surfing the cell, "says Wood. The bacterial cell is therefore both using a new virus, SW1, against its competitors and using a protein from a virus that has attacked millions of years. "

This strategy was clearly beneficial for the host cells, which repelled other strains lacking SW1 and showed a growth advantage when stimulated by phage particles of other strains. "A new virus, SW1, and a former viral protein, YfdM, are being used as tools by the cell when it is looking for food – and all the bacteria are usually hungry," Wood said. "The basic idea is that the cell carrying the SW1 virus is not killed as much as the one that has not seen the SW1 virus before.So, the cell that carries the SW1 virus is no longer in the form of the cell to which the virus is lacking. "

This newly discovered mechanism of self-recognition allows bacteria to form social groups, cooperating with relatives while antagonizing non-relatives in behaviors that may be important for food, virulence, protection, quorum formation of biofilms. "Bacteria are often thought to live individually, but they can actually feed in groups," says Wood. "To be able to act as a group, they must be able to distinguish themselves from other bacteria.In a type of social activity, when they communicate, the bacterial cells secrete chemical signals to communicate." We now show that the cells use viruses to distinguish themselves from related bacteria. "

In future studies, researchers plan to study how SW1 avoids attacking its host cells and killing mainly bacteria that lack the virus. In the end, understanding how cells compete might be useful for synthetic biology applications that mimic nature and use bacteria in groups. "In addition, if we better understand how viruses choose the cells to attack, we may be able to use these viruses to fight bacterial infections."