Figure 5. A microbial biofilm at the root-tip of a human tooth with secondary acute apical periodontitis of endodontic origin. The mixed bacterial flora consists of numerous dividing cocci, rods (lower inset), filaments (FI), and spirochetes (S, upper inset). Rods often reveal a Gram-negative cell wall (GW, lower inset). C, cementum; D, dentin. Magnifications: 2680x; upper inset x 19,200; lower inset x 36,400. (Adapted from Nair, 1987.)