In vitro organ culture assay of 042 (a) and 042aap (b). Bacterial cultures were incubated with colonic biopsies from normal children for 6 hours; then specimens were fixed, stained, and examined by scanning electron microscopy. White arrows (a) denote loose adherence of 042; gray arrows (b) show tight aggregates typical of 042aap. Bars, 10 μm.