MEDICAL SYMBIONTS. “Some aphids also bear bacterial ‘secondary symbionts’, e.g. Serratia symbiotica, Hamiltonella defensa and Regiella insecticola, which can, variously, promote resistance to parasitoids and entomopathogenic fungi, aphid usage of certain plants and thermal tolerance.”
http://rspb.royalsocietypublishing.org/content/275/1634/565.short