FIG. 1.

Pap phase variation in uropathogenic E. coli. Pap17 pilus phase variation of uropathogenic E. coli strain C1212 was visualized with anti-Pap17 antibodies labeled with 10-nm colloidal gold particles. The bacterium at the left is in the ON-phase state for Pap17 expression, whereas the two bacteria at the right are in the OFF phase. Note that these two bacteria express unmarked Pap21 pili, which are also under phase variation control but are not marked with the anti-Pap17 antiserum. In addition to the Pap pili (diameter of about 7 nm), flagella (diameter of about 20 nm) can also be seen.