Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always – sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.
In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence – some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their host by somehow killing the developing wasp larvae.
H.defensa can be passed down from mother to daughter or even sexually transmitted. Infection rates go up dramatically when aphids are threatened by parasitic wasps. But not all strains are the same; some provide substantially more protection than others and Kerry Oliver from the University of Georgia has found out why.
H.defensa‘s is only defensive when it itself is infected by a virus – a bacteriophage called APSE (or “A.pisum secondary endosymbiont” in full). APSE produces toxins that are suspected to target the tissues of animals, such as those of invading wasp grubs. The phage infects the bacteria, which in turn infect the aphids – it’s this initial step that protects against the wasps.