Figure 8. Axial sections through the surgically removed apical portion of the root with a therapy-resistant apical periodontitis. Note the light microscopically visible cluster of bacteria (BA in a) in the root canal. Serial semi-thin sections (b to e), taken at various distances from the section plane of (a), reveal the emerging and gradually widening profiles of an accessory root canal (AC) that is clogged with bacteria (BA). Magnifications: (a) 52x, (b-e) 62x. (From Nair et al., 1990a.)