Social Bacteriophages

Abstract

Despite their simplicity, viruses can display social-like interactions such as cooperation, communication, and cheating. Focusing on bacteriophages, here we review features including viral product sharing, cooperative evasion of antiviral defenses, prudent host exploitation, superinfection exclusion, and inter-phage peptide-mediated signaling. We argue that, in order to achieve a better understanding of these processes, their mechanisms of action need to be considered in the context of social evolution theory, paying special attention to key population-level factors such as genetic relatedness and spatial structure.

Keywords: bacteriophage; cooperation; social evolution; sociovirology; virus–virus interactions.

Figure 1

Two examples of altruistic-cheater virus interactions. Both defective-helper systems and Prisoner´s dilemma exhibit frequency-dependent selection, but they differ in the arrangement of fitness values, leading to different population dynamics. Defective viruses cannot reach fixation because they ultimately depend on helper variants. In contrast, in Prisoner´s dilemma, the cheater fully outcompetes the altruistic variant. Arrows indicate the direction of selection.

Figure 2

Anti-CRISPR proteins as an intracellular public good. A first (altruistic) virus expresses Acr proteins, but these are not sufficient to inactivate the CRISPR system, leading to degradation of the viral genome and abortive infection. However, the cell remains in a transient immunosuppressed state due to the action of the Acr proteins, which allows a second (potentially identical) virus to overcome CRISPR and successfully complete the infection.

Figure 3

Hypothesized depolymerase-mediated phage-phage interactions. Soluble depolymerases should be shareable extracellular products allowing for synergistic or antagonistic interactions among phages. (Top) A depolymerase produced by the red phage digests the exopolysaccharide (EPS) capsule, exposing phage receptors attached to the cell membrane, but also exposing the blue receptor, which promotes entry of the blue phage. (Bottom) A depolymerase produced by the red phage digests the EPS capsule, exposing its receptor, but also releasing the blue receptor, which was embedded in the EPS capsule, thus blocking entrance of the blue phage.

Figure 4

Two possible evolutionary interpretations of superinfection exclusion (SIE). Superinfection exclusion could be considered as a mechanism for avoiding competition for intracellular sources. In this scenario, superinfection exclusion would directly increase the fitness of the actor virus, which would capitalize on cellular resources at the expense of the excluded virus. Alternatively, superinfection exclusion could be considered as a cooperative mechanism whereby the actor virus would pay the cost of promoting the spread of the excluded virus (indicated by dotted arrows) to uninfected neighbor cells, which would improve overall resource exploitation.