Spiral organisms 2-6 μm long, with broad coils at 1 pm intervals, one of the infective agents of rat-bite fever. Seen by dark-ground illumination the spirillum is actively motile (darting moves are characteristic) due to terminal flagella. Romonovsky films of lymph gland juice, skin lesions and sometimes blood may show the parasite, and inoculation of rodents will reproduce the disease. Not so far artificially cultivated.
A species that infects rats and mice. It causes one form of rat-bite fever. It was formerly known as S. minor.