Salmonella are gram-negative, rod-shaped bacilli that can cause salmonellosis, a diarrheal illness in humans. Gram-negative bacteria usually have a cell wall composed of a thin layer of peptidoglycan, covered by a membrane. There are over 2,300 subtypes of the Salmonella enterica bacterium, including serovars enterititis, Salmonella Agbeni, and typhimurium. The bacteria live in the gut of infected humans and animals.