The Genus Helicobacter
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The Genus Helicobacter JAY V. SOLNICK, JANI L. O’ROURKE, PETER VANDAMME AND ADRIAN LEE
Historical Background Spiral-shaped bacteria were first seen in human gastric mucosa early in the twentieth century (Krienitz, 1906) and were subsequently described by several investigators (reviewed in Marshall, 1989). Similar organisms were also seen in the gastric mucosa of dogs, cats and a variety of other animals (Rappin, 1881; Bizzozero, 1892; Salomon, 1896; Lim, 1920; Doenges, 1939). The significance of these organisms was alternately debated and forgotten until Marshall and Warren isolated a Gram-negative, microaerophilic bacterium from human gastric biopsies and proposed that the organism may be the cause of chronic gastritis and peptic ulcer disease (Marshall and Warren, 1984a). The bacterium was thought originally to be a member of the genus Campylobacter and named “Campylobacter pyloridis,” later corrected to “Campylobacter pylori.” Subsequent 16S rRNA sequence analysis showed that the distance between the true campylobacters and C. pylori was sufficient to exclude it from the Campylobacter genus (Romaniuk et al., 1987), and it was renamed “Helicobacter pylori” (Goodwin et al., 1989a), the first member of the new genus Helicobacter.
Description of the Helicobacter Genus The Helicobacter genus consists of a group of organisms that colonize the mucus layer covering the epithelial surface of the gastrointestinal tract of humans and a variety of animal species. There are currently 6 validated Helicobacter species isolated from gastric tissue (Tables 1 and 3) and 16 validated enterohepatic species (Tables 2 and 3). Some Helicobacter species may be commonly (Helicobacter aurati) or occasionally (Helicobacter bilis and Helicobacter muridarum) isolated from both gastric and enterohepatic sites. There is, in addition, a growing list of candidate and unvalidated species (Tables 4 and 5).
Cellular Morphology and Ultrastructure In general, Helicobacter spp. are spiral/helicalshaped, although variant forms include short or tapered rods, and they are motile by means of flagella (Fig. 1). These characteristics are thought to be advantageous for survival in their niche of gastrointestinal mucus. Ultrastructural studies reveal bacteria with a typical Gram-negative cell wall structure, i.e., outer and inner membranes separated by a 30-nm periplasmic space and a dense cytoplasm containing nucleoid material and ribosomes (Fig. 2). Electron-lucent areas and a “polar membrane,” an additional electrondense band 6–8 nm thick located 20 nm below the plasma membrane, are often seen at the terminal region of organisms (Lee and O’Rourke, 1993a; Figs. 2A, C–E). A similar membrane has been seen in other spiral-shaped bacteria, where it has been found to be an assembly of ATPase molecules and, as such, has been implicated in energy production for motility or cell wall synthesis (Murray and Birch-Andersen, 1963; Ferris et al., 1984; Brock and Murray, 1988). Such a membrane was not seen in H. mustela; instead,
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