Chapter Overview

Gram-negative bacilli are a large, diverse group of bacteria, widely dispersed in the environment and on the bodies of humans and animals. A number of them are agents of human and animal diseases.
The source of their pathogenesis is derived from the outer membrane of the cell wall, which contains endotoxin and an assortment of enzymes and other virulence factors.
Pseudomonas species are prominent residents of soil and water with serious ecological and medical impact. Ps. aeruginosa is an opportunistic pathogen in lung, skin, and surgical infections.
Respiratory and systemic infections are caused by Bordetella pertussis, the cause of whooping cough, and Legionella pneumophila, the cause of Legionnaires disease.
Zoonotic infectious agents in this category are Brucella, the cause of brucellosis, which is spread to humans from cattle and pigs; and Francisella tularensis, the cause of tularemia, an infection of wild rabbits, rodents, carnivores, and arthropod vectors.
The largest subgroup represented is the Family Enterobacteriaceae, also called the enterics, which are residents of the large intestine of humans and animals.
Enterics may infect as either true pathogens or opportunists. Infections range from severe diarrheal and toxigenic illness to urinary tract and wound infections. The group accounts for over 40% of nosocomial infections.
Escherichia coli is the most important coliform, acting as both a true pathogen and an opportunist. It causes severe invasive and toxic diseases such as infant diarrhea and toxemia, as well as urinary tract and surgical infections.
A prominent enteric pathogen is the genus Salmonella, the cause of typhoid fever, enteric fever, and food poisoning. Typhoid fever is strictly a human disease that is spread by contaminated food and water. Other salmonelloses are zoonotic in origin, transmitted by food products obtained from infected poultry and cattle.
Members of the genus Shigella originate from humans and are highly transmissible in food that has been contaminated by human feces. Shigella toxins damage the large intestine lining and give rise to shigellosis or bacillary dysentery.
Yersinia pestis causes the plague, a zoonosis carried by rodents and acquired from fleas and inhaled bacteria. Bubonic plague is a lymphatic form, and pneumonic plague affects the lungs.
Haemophilus influenzae is another cause of meningitis, an invasive infection of the central nervous system that may lead to serious complications.
Gram-negative bacillary infections for which there is useful vaccination are pertussis, brucellosis, tularemia, typhoid fever, plague, and Haemophilus meningitis.

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