The defensive Spiroplasma

•Multiple strains of the heritable bacterium, Spiroplasma, protect insects against infection by natural enemies.•Defensive capabilities are broad, impacting parasitic nematodes, insects, and fungi.•A mechanism is implicated — toxic ribosome-inactivating proteins are common across Spiroplasma.•Model hosts and culturability position Spiroplasma as an exceptional model for the study of defensive symbiosis.