Which bacterium discovered by Pasteur causes fowl cholera?

The bacterium discovered by Pasteur that causes fowl cholera is Pasteurella multocida.

The disease, also called avian cholera or avian pasteurellosis, affects chickens and many other birds, including turkeys, ducks, geese, and wild waterfowl. Pasteur received samples from the veterinary scientist Henry Toussaint and isolated the organism in 1880. The genus name Pasteurella later honored Pasteur.

Pasteurella multocida is a Gram-negative, nonmotile coccobacillus. It is not the same organism as Vibrio cholerae, which causes human cholera, and it is unrelated to Bacillus anthracis, the cause of anthrax. Clostridium tetani causes tetanus.

The discovery became important beyond identifying a poultry disease. Pasteur’s experiments with weakened cultures of the bacterium helped establish the principle of attenuation: an infectious organism could be weakened and used to stimulate protection. That work helped launch the modern science of laboratory-produced vaccines.