Bacterial adherence to animal cell surfaces is of interest because of its relation to pathogenicity and the insight it provides into determinants of intercellular recognition. The attachment of various strains of Escherichia coli and Salmonella spp. to epithelial cells and phagocytes is inhibited by
D-mannose, and the adherence of other bacteria is inhibited by
sugars such as L-
fucose and
D-galactose, suggesting that
sugar-mediated adherence is widespread. This intercellular recognition is thought to be mediated by
sugar residues (e.g.
D-mannose) on the surface of animal cells, to which bacteria attach by a
sugar-binding substance on their surface. The nature of the receptors on the animal cells is unknown. There is evidence that E. coli produces
lectin-like substances specific for
D-mannose, by which it binds to the cells. The most common form of these
lectin-like substances appears to be the bacterial pili, which can be reversibly dissociated into their
protein subunits. The
lectin can also be in the form of bacterial flagella or tightly attached to the outer membrane of the bacteria.
Mannose-specific attachment may assist bacteria in colonizing and invading their hosts:
methyl alpha-D-mannoside (but not
methyl alpha-D-glucoside) significantly reduced
infection of the urinary tract of mice by virulent strains of E. coli. Once bacteria penetrate the host their ability to binding
sugars on phagocytes may impair their virulence by facilitating phagocytosis. Further studies of the
sugar-mediated bacterial adherence by organisms growing in vivo and the structural identification of the host cell receptors may lead to the design of more effective adherence inhibitors that may help to prevent certain
bacterial infections.