The T rojan H orse of the microbiological arms race: phage‐encoded toxins as a defence against eukaryotic predators

Summary Phage‐encoded S higa toxin ( S tx) acts as a bacterial defence against the eukaryotic predator T etrahymena . To function as an effective bacterial anti‐predator defence, S tx must kill a broad spectrum of predators. Consistent with that assertion, we show here that bacterially encoded S tx efficiently kills the bacteriovore A canthamoeba castellanii in co‐culture. We also show that, in addition to S tx, the phage‐encoded exotoxin, diphtheria toxin ( D tx) expressed by C orynebacterium diphtheriae also can function as part of an anti‐predator strategy; it kills A canthamoeba in co‐culture. Interestingly, only exotoxins produced by bacteria internalized by the A canthamoeba predator are cytolethal; the presence of purified D tx or S tx in culture medium has no effect on predator viability. This finding is consistent with our results indicating that intoxication of A canthamoeba by these exotoxins does not require a receptor. Thus bacteria, in the disguise of a food source, function as a ‘ T rojan H orse’, carrying genes encoding an exotoxin into target organisms. This ‘ T rojan H orse’ mechanism of exotoxin delivery into predator cells allows intoxication of predators that lack a cell surface receptor for the particular toxin, allowing bacteria‐bearing exotoxins to kill a broader spectrum of predators, increasing the fitness of the otherwise ‘defenceless’ prey bacteria.