Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative, rod-shaped coccobacillus, a facultative anaerobic bacterium that can infect humans and animals.It causes the deadly disease named plague.. pestis is a nonmotile, rod-shaped, facultative anaerobe with bipolar staining (giving it a safety pin appearance). Similar to other Yersinia species, it tests negative for urease, lactose fermentation, and indole.