A 63-year-old man is admitted with fever, hypotension and rapidly progressive cellulitis of the forearm. He had been licked on an excoriated hand by his daughter's dog two days earlier. He underwent splenectomy after trauma 20 years ago and drinks heavily. The skin is mottled, and blood cultures later show slender Gram-negative rods that grow slowly on routine media. Which one of the following is the most likely causative organism?