FIG. 3.

Autoaggregation of O-antigen-negative mutants. Wild-type E. coli O157:H7 strain ATCC 43895 (43895) and strain 8624 (8624) and the corresponding O-antigen mutants (43895Δper and 8624 mutant) were grown overnight with shaking in LB broth. Cells taken directly from the shaker were observed microscopically by phase-contrast microscopy (magnification, ×1,000) to examine clumping (A), or cultures were monitored visually for sedimentation after they stood undisturbed for 3 h at room temperature (B). Cell aggregation is indicated by arrows.