Iron represses the expression of CFA/I fimbriae of enterotoxigenic E. coli

Experiments were designed to study the effect of iron on the expression of CFA/I fimbriae by enterotoxigenic E. coli (ETEC). Addition of 0.05 mM ferrous sulfate to growth media decreased CFA/I antigen and fimbrial production by the CFA/I-positive ETEC strain H-10407 as measured by quantitative ELISA and hemagglutination assay. The repressive effect was reversed by the addition of the iron chelators, sodium citrate or dipyridyl. With a CFA/I subunit gene promoter-lacZ fusion, it was found that the activity of the subunit gene promoter was significantly higher in the presence of iron chelators than in medium containing iron in the fur+ strain DHB24. This difference was not observed in the fur mutant strain SBC24, suggesting that the global E. coli metalloregulatory protein Fur (ferric uptake regulation) is involved in the repression. The repressor may bind to the promoter of the CFA/I subunit gene since several potential Fur-binding sites were identified in the promoter area.