Type 1 fimbriation and its phase switching in diarrheagenic Escherichia coli strains

Type 1 fimbriae can be expressed by most Escherichia coli strains and media te mannose-sensitive (MS) adherence to mammalian epithelial cells. However, the role of type 1 fimbriae in enteric pathogenesis has been unclear. Expr ession of type 1 fimbriae in E, coli is phase variable and is associated wi th the inversion of a short DNA element (fim switch). Forty-six strains of diarrheagenic E. call were examined for the expression of type L fimbriae. Only four of these strains were originally type 1 fimbriated. Seventeen str ains, originally nonfimbriated, expressed type 1 fimbriae in association wi th off-to-on inversion of the fim switch, after serial passages in static c ulture. The switching frequencies of these strains, from fimbriate to nonfi mbriate, were greater than that of the laboratory strain E. coli K-12. None of the 16 strains of serovar O157:H7 or O157:H- expressed type 1 fimbriae after serial passages in static culture. The nucleotide sequence analysis o f the fim switch region revealed that all of the O157:H7 and O157:H- strain s had a 16-bp deletion in the invertible element, and the pm switch was Loc ked in the "off" orientation. The results suggest that expression of type 1 fimbriae may be regulated differently in different E. call pathogens causi ng enteric infections.