Colonization, persistence and virulence factors in Escherichia coli strains in the human intestinal microflora

Abstract

Escherichia coli is a member of the normal intestinal microflora of man and many animals. In this thesis, various aspects of the E. coli colonization pattern were studied, especially the role of so called virulence factors for persistence of E. coli in the gut microflora. First, a method was developed to distinguish individual E. coli strains based on RAPD (random amplified polymorphic DNA). Using this method, the E. coli colonization pattern during the first year of life was studied in 70 healthy Swedish infants. Colonization with E. coli occurred late and the turn-over rate of strains in the microflora was low. Only 61% were colonized by E. coli at two months of age. Each child carried 1.7 different E. coli strains on average over the first six months of life, which can be compared with the 8.5 strains previously found in Pakistani infants followed for the same time period. Secondly, a multiplex PCR method was applied to identify E. coli virulence factor genes. Commensal E. coli strains from Swedish infants, Swedish school-girls, and Pakistani infants were studied with respect to carriage of genes for type 1, P and S fimbriae, Dr hemagglutinin, K1 and K5 capsular antigens, aerobactin and hemolysin. Gene carriage rates were compared between resident and transient strains, i.e. strains persisting or only transiently appearing in the microflora, respectively. Genes for P fimbriae were significantly more common in resident than transient strains in all three commensal E. coli populations. In addition, the aerobactin gene was significantly more common in resident than transient E. coli strains from Swedish school-girls and Pakistani infants, and genes for the capsular antigens K1 and K5 were significantly more common in resident than transient E. coli strains from Swedish school-girls. Genes for type 1 fimbriae and hemolysin were more common in resident than transient strains from Swedish infants. Thus, several bacterial traits that enable E. coli to cause disease also seem to enhance their ability to persist in the commensal flora. Thirdly, a semi-quantitative reverse transcriptase PCR was developed to study the expression of E. coli virulence factors. E. coli strains carrying the genes for type 1 fimbriae, P fimbriae, or both, were studied during growth in broth, in human milk, or on agar plates. Strains carrying genes for both P and type 1 fimbriae expressed less P- and type 1-fimbrial mRNA during growth on agar than strains carrying only P- or only type 1-fimbrial genes, respectively. This could indicate cross-talk between adhesin operons. The method may be applied for studies of the expression of E. coli virulence factors in the intestine. In conclusion, the results show that E. coli is acquired late by Swedish infants today and that the turn-over rate of individual strains in the microflora is low, probably due to a limited circulation of faecal bacteria in the Swedish hospitals and homes. The results further suggest that certain virulence factors have evolved to increase the fitness of E. coli in the human colon. The expression of these virulence factors in vivo in the intestinal tract will be the subject of future studies.