on 23-Jun-2025 (Mon)

They are also ubiquitous in water, soil, and decaying vegetation

Though most enteric bacte- ria can be pathogenic in humans, over 95% of the medically important species are in the 13 genera we discuss shortly

All mem- bers of the family reduce nitrate to nitrite and ferment glucose anaerobically, though most of them grow better in aerobic environments. As previously noted, all of them are also oxi- dase negative

Members of the Enterobacteriaceae have an outer membrane that contains lipopolysaccharide composed of three antigenic components: • A core polysaccharide, shared by all of the enteric bacteria and called common antigen. • So-called O polysaccharide, which has various antigenic varieties among strains and species. • Lipid A, which, when released in a patient’s blood, can trigger fever, vasodilation, inflammation, shock, and DIC (see Figure 20.1). Because of the presence of lipid A, most of the enteric bacteria can cause serious disease and death

Other antigens of certain enteric bacteria include protein and polysaccharide capsular antigens called K antigens (but called Vi antigens in Salmonella) and flagellar proteins called H antigens. By controlling the genetic expression of K and H antigens, alternately producing and not producing the antigens, the bacteria survive by evading their host’s immune system.

Scientists use serological identification of antigens to distinguish among strains and spe- cies of enteric bacteria. For example, Escherichia coli O157:H7, a potentially deadly bacterium, is so designated because of its spe- cific O and H antigenic components

To culture members of the Enterobacteriaceae from clinical specimens, clinicians use selective and differential media such as eosin methylene blue (EMB) agar and MacConkey agar. In their role as selective media, these media inhibit the growth of many members of the normal microbiota, particularly Gram-positive bacteria, and allow the growth of enteric bacteria (FIGURE 20.10a); in their role as differential media, they enable laboratory technicians to dis- tinguish between enteric bacteria that can and cannot ferment lac- tose (FIGURE 20.10b)

Coliforms (kol′i-formz) are defined as aerobic or facultatively anaerobic, Gram-negative, rod-shaped bacteria that ferment lac- tose to form gas within 48 hours of being placed in a lactose broth at 35°C. Coliforms are found in the intestinal tracts of animals and humans, in soil, and on plants and decaying vegetation.

The most common of the oppor- tunistic coliform pathogens are bacteria in the genera Escherichia, Klebsiella, Serratia, Enterobacter, Hafnia, and Citrobacter