Influence of some bacterial and host factors on colonization and invasiveness of Escherichia coli K1 in neonatal rats.

Of 209 healthy infants examined, 44 (21.1%) carried Escherichia coli K1 in their feces. Of these 44 isolates, 36 (81.8%) were attributed to 10 different known clonal groups of E. coli K1 and 4 isolates represented unknown types. The influence of mannose-resistant (MR) adhesins, aerobactin production, and resistance to serum on colonization and invasiveness of E. coli K1 in orally infected inbred LEW baby rats was investigated. Strains expressing MR adhesins had significantly higher colonization and invasion rates than non-MR strains did. Mixed-infection experiments of LEW rats revealed interactions between different types of E. coli K1 strains affecting colonization and invasion rats. P-fimbriated strains appeared to have a selective advantage for colonization. The bacteremic potentials of different E. coli K1 strains could not be associated with their resistance to sera from LEW rats free of members of the family Enterobacteriaceae. No differences in virulence between fecal E. coli K1 isolates and clinical isolates from diseased humans were found. An influence of the major histocompatibility complex on host susceptibility to invasive E. coli K1 was indicated by comparing the parental LEW rat strain with different congenic LEW strains (RT1).