Pasteurella multocida (“killer of many species”) is a gram-negative, pleomorphic coccobacillus best known for its association with soft-tissue infections after animal bites. However, this organism is also capable of causing invasive and life-threatening infections.

Pasteurella multocida is found worldwide. It commonly colonizes the upper respiratory tract of many animals, most notably cats (70% to 90%) and dogs (50% to 66%). Human infection is usually related to animal exposure. Direct inoculation by a bite or scratch is the most common mode of transmission of P. multocida to humans. Inoculation can also occur by nontraumatic animal contact, such as when a wound is licked by an animal. The second mode of transmission is by colonization of the human respiratory tract occurring with exposure to animals such as nuzzling or grooming of pets. The organism has been cultured from the respiratory tract of healthy veterinary workers and animal handlers as well as from ill patients. Infections can also occasionally occur with no history of animal contact.

There are several species and subspecies of Pasteurella, but the most common ones causing human disease are P. multocida, Pasteurella dagmatis, Pasteurella canis, and Pasteurella stomatis. These organisms are nonmotile, gram-negative facultative anaerobes that on Gram stain can resemble Haemophilus and Neisseria species. The organism grows well on sheep and chocolate agar and appears as watery mucoid blue colonies.