K99 surface antigen of Escherichia coli: purification and partial characterization.

K99, a presumed colonizing factor of enterotoxigenic Escherichia coli of calf origin, has been purified. K99 was removed from K99+ bacteria by salt extraction and subsequently purified by ammonium sulfate precipitation and column chromatography on diethylaminoethyl-Sephadex. The purified material was homogenous in size, having an s20,w of 13 to 15 S. It was composed of two subunits: a major component with a molecular weight of 22,500 and a minor component of 29,500. When observed in the electron microscope, K99 appeared to be rod-shaped, with a strong tendency for self-aggregation. At concentrations where aggregation was minimized, individual rods were observed with diameters of 8.4 nm and mean lengths of 130 nm. Based on the subunit structure, exterior location, and rod-like shape of K99, it is concluded that K99 is a pilus or pilus-like structure. Chemically, K99 is composed primarily of protein and has an isoelectric point of greater than 10. Purified K99 did not hemagglutinate guinea pig erythrocytes.