The Enterobacteriaceae family contains a large and diverse number of gram-negative organisms that includes over 70 different genera, occurring as gram-negative bacilli and coccobacilli. Some species of Enterobacteriaceae inhabit the human gastrointestinal (GI) tract as normal microbiota (eg, Escherichia coli, Klebsiella species, Proteus species, etc.), while others are found in humans only at the time of infection (eg, Shigella species, Salmonella species, Yersinia species).
The clinically relevant members of the Enterobacteriaceae family are responsible for causing a wide spectrum of disease, including the capability of producing serious virulence factors (eg, endotoxin, enterotoxins) that can lead to systemic, life-threatening infections. For those species that colonize humans, opportunistic infection occurs when the patient's own endogenous strain of bacteria enters a normally sterile site (eg, urinary tract infection caused by E. coli that enters the bladder via the urethra). For species that do not normally inhabit the human GI tract, infection occurs when the patient ingests pathogen-contaminated food or water (eg, food poisoning caused by Salmonella after eating contaminated food).