Bacterial association with host mucosal surfaces involves a large number of steps. Successful negotiation of each of these requires -- or is at least facilitated by -- the development of a distinct set of characteristics (virulence factors) by the bacterium. The major steps include: (a) chemotactic attraction of motile bacteria to the surface of the mucus gel, (b) penetration of and trapping within the mucus gel (which may be passive or can be promoted actively by bacterial motility and chemotaxis), (c) adhesion to receptors in the mucus gel or to mucosa-associated layers of the indigenous microflora, (d) adhesion to epithelial cell surfaces, and (e) multiplication of the mucosa-associated bacteria. Each reaction is further modified -- or reversed entirely -- by substances such as taxins, inhibitors of adhesion, and substrates for bacterial growth that are present in the mucosal microenvironment. Association with the mucosa is often important for bacterial colonization but can also lead to more effective elimination of the bacterium by the host. Bacteria lacking one or several of these virulence factors may still be successful colonizers if they show exceptionally high competence in relation to others. Examples are the strong adhesion to epithelial cells by Escherichia coli strains bearing the K88 antigen (such strains need not be motile in order to be pathogenic) or the active chemotactic association with mucus gel by cholera vibrios (some of which do not appear to adhere strongly to epithelial cells). Consequently a single in vitro assay for "adhesion" can be expected to correlate with bacterial pathogenicity only when the assay is based on the same specific mechanism(s) which the bacterium under study actually uses for mucosal association in vivo.