Bacteria such as Escherichia coli live in environments such as the gut with many other types of microbes, and often develop communities of microbes cooperating and/or competing with each other for resources. But in order to cooperate or compete, bacteria must first be able to identify and discriminate between themselves and others. Sometimes microbes do this by exchanging membrane molecules, or secreting chemical signals that only partners can detect, or transferring plasmids or producing antimicrobial compounds that kill competitors.

In the current study, scientists discovered a strain of E. coli that carries around phages that help them distinguish other strains and compete with them. When this strain encounters another, the phages it carries attack and destroy cells of the other strain, while leaving the carrier strain mostly unharmed. This strategy is not without cost, though; the viral proteins take resources to produce, and when there's no competing strains around, the virus can attack its carrier to some extent.

Song S, Guo Y, Kim J-S, Wang X, Wood TK. 2019. Phages Mediate Bacterial Self-Recognition. Cell Reports 27:737-749.e4.