Role of type 1 and S fimbriae in the pathogenesis of Escherichia coli O18:K1 bacteremia and meningitis in the infant rat.

The role of fimbriae in the pathogenesis of Escherichia coli infection was studied in the infant rat model. Rat pups were challenged intraperitoneally at the age of 5 days with E. coli K1 (strain IH3080, O18:K1:H7) and three different subpopulations (type 1, type S, or nonfimbriated) of it. All bacterial subpopulations were able to produce peritonitis, bacteremia, and meningitis. However, the type 1 fraction was the least virulent and the type S fraction was the most virulent, as judged by the bacterial counts in body fluids and by the mortality rates of the pups. Fimbrial phase variation to mainly the type-S-fimbriated forms was observed in all body fluids. An initially type-S-fimbriated inoculum remained predominantly type S fimbriated in the peritoneal fluid and blood. In the cerebrospinal fluid, however, about 50% of the bacteria were type S fimbriated and 50% were nonfimbriated 1 h after challenge with the type-S-fimbriated subpopulation; at later times the share of type-S-fimbriated bacteria also increased in the cerebrospinal fluid.