Enterobacteriaceae part 2

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 living in 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 Gramnegative
endotoxic shock, a dreaded and often fatal complication.

MORPHOLOGY AND STRUCTURE
The Enterobacteriaceae are among the largest bacteria, measuring 2 to 4 m in length and
0.4 to 0.6 m in width, 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.
The cell wall, cell membrane, and internal structures are morphologically similar for
all Enterobacteriaceae, and follow the cell plan described in Chapter 2 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 (Fig 21–1). The outer membrane lipopolysaccharide (LPS) is called the
O antigen. Its antigenic specificity is determined by the composition of 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 of the Enterobacteriaceae have surface pili, which are antigenic
proteins but not yet part of any formal typing scheme.