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She died of a fever

And no one could save her

And that was the end of sweet Molly Malone

But her ghost wheels her barrow

Through streets broad and narrow

Crying cockles and mussels alive, alive o!

—James Yorkston: Irish Ballad

The Enterobacteriaceae are a large and diverse family of Gram-negative rods, members of which are both free-living and part of the indigenous flora of humans and animals. A few are adapted strictly to humans. The Enterobacteriaceae grow rapidly under aerobic or anaerobic conditions and are metabolically active. They are by far the most common cause of urinary tract infections (UTIs), and a limited number of species are also important etiologic agents of diarrhea. Spread to the bloodstream causes Gram-negative endotoxic shock, a dreaded and often fatal complication. In 19th-century literature and song, dying “of a fever” usually meant typhoid fever (Salmonella ser. Typhi), which, because of its prolonged course and lack of localizing signs, unfortunates like Molly Malone seemed to be dying of fever alone.



The Enterobacteriaceae are among the largest bacteria, measuring 2 to 4 μm in length with parallel sides and rounded ends. Forms range from large coccobacilli to elongated, filamentous rods. The organisms do not form spores or demonstrate acid-fastness.

Rods are large

The cell wall, cell membrane, and internal structures are morphologically similar for all Enterobacteriaceae, and follow the cell plan described in Chapter 21 for Gram-negative bacteria. Components of the cell wall and surface, which are antigenic, have been extensively studied in some genera and form the basis of systems dividing species into serotypes. The outer membrane lipopolysaccharide (LPS) is called the O antigen. Its antigenic specificity is determined by variation in the sugars that form the long terminal polysaccharide side chains linked to the core polysaccharide and lipid A. Cell surface polysaccharides may form a well-defined capsule or an amorphous slime layer and are termed the K antigen (from the Danish Kapsel, capsule). Motile strains have protein peritrichous flagella, which extend well beyond the cell wall and are called the H antigen. Many Enterobacteriaceae have surface pili (fimbriae), which are antigenic proteins, but not part of formal typing systems.


K = polysaccharide capsule

H = flagellar protein

Enterobacteriaceae grow readily on simple media, often with only a single carbon energy source. Growth is rapid under both aerobic and anaerobic conditions, producing 2 to 5 mm colonies on agar media and diffuse turbidity in broth after 12 to 18 hours of incubation. All Enterobacteriaceae ferment glucose, reduce nitrates to nitrites, and are oxidase negative.

Facultative growth is rapid


Genus and species designations are based on phenotypic characteristics, such as patterns of carbohydrate fermentation, and amino acid breakdown. The ...

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