The gram-negative bacilli comprise a large group of nonspore- forming bacteria adapted to a wide range of habitats and niches. The genera in this category are highly diverse in metabolism and pathogenicity. Because the group is so large and complex, and many of its members are not medically important, we have included only those that are human pathogens.

A large number of representative genera are inhabitants of the large intestine (enteric); some are zoonotic; some are adapted to the human respiratory tract; and still others live in soil and water. It is useful to distinguish between the frank (true) pathogens (Salmonella, Yersinia pestis, Bordetella, and Brucella, for example), which are infectious to the general population, and the opportunists (Pseudomonas and coliforms), which are resident flora that cause infection in people with weakened host defenses.