After oral administration of a live, non-pathogenic E. coli strain the strain colonized effectively the intestine in full-term and preterm infants and persisted for many weeks and months showing, that it is capable to establish itself as a resident strain in the gut. The presence of E. coli stimulated significantly antibody production in the gut and saliva of colonized infants. An early induction of secretory IgA production is important particularly in formula-fed infants, where it partly replaces the lacking immunoglobulin supplied with human milk. In full-term and premature infants the early presence of the non-pathogenic E. coli strain in the intestine decreased significantly the presence of pathogenic bacterial strains in the intestine but also other mucosal surfaces of the body, decreased the number of nosocomial infections, mortality rate in connection with infection, the need for antibiotics, and replaced successfully pathogenic strains in carriers. Colonization after birth influenced the immune reactions: after 10 years occurrence of repeated infections and allergic diseases in premature infants was significantly lower in colonized infants than in controls. After twenty years the difference was still significant in the percentage of allergic diseases in full-term infants.

        Key words: oral colonization with the E. coli strain, neonates, immunity, nosocomial infection, long-term action