Enterobacteriaceae

General: Composed of numerous genera of Gram- bacilli, about 9 of which are of common clinical significance. Typical flora of the gastrointestinal tract, but may cause both GI and extra-GI (esp. urinary, respiratory, wound, & bloodstream) infections. Include both motile and nonmotile species, but never form spores.

Clinical:

• Normal:
• Abnormal:

Resistance: Some species carry chromosomally encoded beta-lactamase, which must be induced in order to be expressed, unlike with plasmid encoded beta-lactamases; i.e., it's possible that usual susceptibility testing will erroneously categorize them as susceptible.

Morphology: Gram- bacilli or coccobacilli, usually short and plump, ranging from 0.5-2.0 x 2-4um.

Growth characteristics: Grow well after 18-24hrs at 35C under aerobic conditions on routine blood agar as well as differential MacConkey agar or selective differential Hektoen or xylose-lysine-deoxycholate agar. Usually medium to large, dry or mucoid/glistening, gray colonies, and some are beta-hemolytic or motile; see specific genera or species as some have more distinct morphology. Ferment glucose and other carbohydrates, are oxidase-, catalase+, and reduce nitrate to nitrite. Commonly identified with indole, methyl red, Voges-Proskauer, and citrate screen, also known as the IMViC tests.

Common/important pathogens: By genus:

• Escherichia sp.
• Enterobacter sp.
• Klebsiella sp.
• Pantoea sp.
• Proteus sp.
• Citrobacter sp.
• Serratia sp.
• Salmonella sp.
• Shigella sp.
• Yersinia sp.