Bacterial Responses to Protist Predation: Cheating, facilitation, cooperation and phage transfer in mixed microbial communities
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Shiga toxin (Stx) is encoded by temperate bacteriophages. Stx is only produced and released by bacteria during phage lytic growth. Phage-encoded Stx released by E.coli kills bacterivorous predators, thereby providing Stx + E.coli with a growth advantage over Stx - cells. Our results indicate that the released toxin behaves as the public good and the fitness benefit of predator killing, for the most part, is communal as it is not limited to the Stx producing bacteria. Moreover, since bacterial death accompanies Stx release therefore Stx producers are classified as self-sacrificing cooperators. If the released Stx phage encounters phage susceptible bacteria it uses these phage susceptible strains as surrogate for toxin and phage amplification. The lytic growth of these phage in susceptible, non-producing bacteria helps maintain cooperative behavior in the bacterial population. We find that Tetrahymena thermophila has two roles in our experimental microcosms 1) it induces the prophage and 2) it coingests and accumulates various bacteria and concentrate free phage in its phagocytic vesicles. The ingested phage retain their ability to infect phage susceptible bacterial strains. Consequently, we found that Stx-encoding temperate phage are transferred from one bacterial host to another with higher frequency within the ciliate phagosome. This finding suggests that T. thermophila aids in phage transfer by increasing their multiplicity of infection as they concentrate phage and accumulate bacteria within the same phagocytic vesicles.