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The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera (Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others). Some enteric organisms, such as Escherichia coli, are part of the normal microbiota and incidentally cause disease, but others, the salmonellae and shigellae, are regularly pathogenic for humans. The Enterobacteriaceae are facultative anaerobes or aerobes, ferment a wide range of carbohydrates, possess a complex antigenic structure, and produce a variety of toxins and other virulence factors. Enterobacteriaceae, enteric gram-negative rods, and enteric bacteria are the terms used in this chapter, but these bacteria may also be called coliforms.

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The Enterobacteriaceae are the most common group of gram-negative rods cultured in clinical laboratories and along with staphylococci and streptococci are among the most common bacteria that cause disease. The taxonomy of the Enterobacteriaceae is complex and rapidly changing since the introduction of techniques that measure evolutionary distance, such as nucleic acid hybridization and nucleic acid sequencing. According to the National Library of Medicine's Internet Taxonomy database (available at http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/www.tax.cgl?id=543) 51, genera have been defined; however, the clinically significant Enterobacteriaceae comprise 20–25 species, and other species are encountered infrequently. In this chapter, taxonomic refinements will be minimized, and the names commonly used in the medical literature are generally used. A comprehensive approach to identification of the Enterobacteriaceae is presented in Chapters 35, 36, and 37 of Versalovic et al, 2011.

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Members of the family Enterobacteriaceae have the following characteristics: They are gram-negative rods, either motile with peritrichous flagella or nonmotile; grow on peptone or meat extract media without the addition of sodium chloride or other supplements; grow well on MacConkey agar; grow aerobically and anaerobically (are facultative anaerobes); ferment rather than oxidize glucose, often with gas production; are catalase positive and oxidase negative (except for Plesiomonas) and reduce nitrate to nitrite; and have a 39–59% G + C DNA content. Examples of biochemical tests used to differentiate the species of Enterobacteriaceae are presented in Table 15-1. There are many others in addition to the ones listed. In the United States, commercially prepared kits or automated systems are used to a large extent for this purpose. The implementation of matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF MS) for identification of culture isolates may soon replace the more traditional panels of biochemicals currently in use in most clinical microbiology laboratories. This new technology seems to work quite well for identification of most of the common Enterobacteriaceae encountered in clinical material except for Shigella species. This technology is unable to differentiate Shigella from E coli.

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Table 15–1 Examples of Biochemical Reactions of Selected Enteric Gram-Negative Rodsa

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