Fimbriae (also called pili), polar filaments radiating from the surface of the bacterium to a length of 0.5-1.5 micrometers and numbering 100-300 per cell, enable bacteria to colonize the epithelium of specific host organs. CFA/I fimbriae are rather rigid, thread-like filaments of 0.5-1 micrometer, with an apparent axial hole, and a diameter of 7 nanometers. A single CFA/I fimbria consists of about 100 identical protein subunits. CFA/I fimbriae are only expressed in the presence of the positive regulator CfaD. Belongs to the fimbrial CS1 protein family.