Pasteurella multocida (“killer of many species”) is a nonmotile, gram-negative, facultative 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. The most common mode of transmission to humans is by direct inoculation by a bite or scratch. 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. Pasteurella has been cultured from the respiratory tract of healthy veterinary workers and animal handlers as well as from ill patients. Infections can occasionally occur in the absence of animal contact.
There are several species and subspecies of Pasteurella, with the most common ones causing human disease being P. multocida subsp. multocida, P. multocida subsp. septica, Pasteurella dagmatis, Pasteurella canis, and Pasteurella stomatis. These organisms can resemble Haemophilus and Neisseria species when visualized on Gram stain, grow well on sheep and chocolate agar, and appear as smooth, mucoid blue colonies.
Most of the virulence factors have been studied in animals. Pathogenesis of Pasteurella depends on the bacteria’s ability to adhere to the host’s respiratory epithelium, typically the tonsils, which can be mediated through fimbrae. Some species are capable of producing a leukotoxin that affects leukocytes and inhibits cellular immune responses. Differences between virulent strains of Pasteurella are identified according to capsular antigens A to F, which cause different animal diseases.