Light micrograph of chains of Gram-stained rat bite fever bacteria (Streptobacillus moniliformis). This Gram-negative filamentous rod-shaped bacterium is common in wild and pet rats, where it causes no symptoms. It can be transferred to humans by rat bites or scratches, but also from infected water and close contact with, or handling of, rats. Infection causes fever, a rash of the hands and feet, and joint pain. Treatment is with antibiotics.