Antigenic Structure of Enterobacteriaceae
المؤلف:
Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia
المصدر:
Jawetz, Melnick, & Adelberg’s Medical Microbiology
الجزء والصفحة:
28e , p237-238
2025-08-31
406
Enterobacteriaceae have a complex antigenic structure. They are classified by more than 150 different heat-stable somatic O (lipopolysaccharide) antigens, more than 100 heat-labile K (capsular) antigens, and more than 50 H (flagellar) antigens (Figure 1B). In Salmonella serotype Typhi, the capsular antigens are called Vi antigens. The antigenic classification of Enterobacteriaceae often indicates the presence of each specific antigen; for example, the antigenic formula of an E. coli may be O55:K5:H21.
Fig1. A: Gram-stain of E. coli. Original magnification ×1000. (Courtesy of H Reyes.) B: Antigenic structure of Enterobacteriaceae.
O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide. Some O-specific polysaccharides contain unique sugars. O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination. Antibodies to O antigens are predominantly IgM.
Although each genus of Enterobacteriaceae is associated with specific O groups, a single organism may carry several O antigens. Thus, most shigellae share one or more O antigens with E. coli; however, E. coli may also cross-react with some Providencia, Klebsiella, and Salmonella species. Occasionally, O antigens may be associated with specific human diseases (eg, specific O types of E. coli are found in diarrhea and in urinary tract infections).
K antigens are external to O antigens on some but not all Enterobacteriaceae. Some are polysaccharides, including the K antigens of E. coli; others are proteins. K antigens may interfere with agglutination by O antisera, and they may be associated with virulence (eg, E. coli strains producing K1 antigen are prominent in neonatal meningitis, and K anti gens of E. coli cause attachment of the bacteria to epithelial cells before gastrointestinal or urinary tract invasion).
Members of the genus Klebsiella form large capsules consisting of polysaccharides (K antigens) covering the somatic (O or H) antigens and can be identified by capsular swelling tests with specific antisera. Human infections of the respiratory tract are caused particularly by capsular types 1 and 2 and those of the urinary tract by types 8, 9, 10, and 24.
H antigens are located on flagella and are denatured or removed by heat or alcohol. They are preserved by treating motile bacterial variants with formalin. Such H anti gens agglutinate with anti-H antibodies, mainly IgG. The determinants in H antigens are a function of the amino acid sequence in flagellar protein (flagellin). Within a single serotype, flagellar antigens may be present in either or both of two forms, called phase 1 (conventionally designated by lowercase letters) and phase 2 (conventionally designated by Arabic numerals), as shown in Table 1. The organism tends to change from one phase to the other; this is called phase variation. H antigens on the bacterial surface may interfere with agglutination by anti-O antibody.
Table1. Representative Antigenic Formulas of Salmonellae
There are many examples of overlapping antigenic structures between Enterobacteriaceae and other bacteria. Most Enterobacteriaceae share the O14 antigen of E. coli. The type 2 capsular polysaccharide of Klebsiella is very similar to the poly saccharide of type 2 pneumococci. Some K antigens cross-react with capsular polysaccharides of Haemophilus influenzae or Neisseria meningitidis. Thus, E. coli O75:K100:H5 can induce antibodies that react with H. influenzae type b.
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