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Fig. 4. Section through cuticle and attached bacterial film removed from over early stage caries. Note parallel arrangement of organisms (1)extending from cuticle (2) outward toward surface  the pile (3). Acids produced at the surface are carried as if by a wick or sponge through the material down to the cuticle.

Fig. 5. Bacterial film material from over early stage caries lesion torn apart, stained, mounted in 50 per cent glycerin and photographed by transmitted light, shows filamentous nature of material.

Nature of the Bacterial Film Over the Early Stage Lesions

A film or pad of soft bacterial material of variable thickness is present on the surface of the tooth at all areas where it is protected from removal by functional or other friction. It is thickest where it is best protected. Microscopic examination of appropriate preparations of this material ("soft tartar") shows it to be composed entirely of bacteria, usually of many different kinds. (Figure 5). One characteristic of such bacterial material over early stage caries lesions (and elsewhere in most cases) is that it consists mostly of long rod and filamentous forms, one end of which is attached to the cuticle on the tooth. The rods and filaments extend outward, more or less parallel to each other, toward the surface of the pile or pad. At the surface there are the growing ends and fruiting heads of the long forms of which the deeper part of the film is composed (Figures 7, 8, 9); and among these, large numbers of other bacteria of many different kinds.

Production of Acids At Caries Locations

Food, as it is masticated, is thoroughly and heavily inoculated with many different kinds of bacteria in the saliva, derived from all the different locations within the mouth. The bacteria in such heavily inoculated food material lodged and retained upon the constantly present bacterial film pad at favorable locations about teeth, multiply and, through the action of their enzymes, break down the material which serves as their culture media. Many bacteria, when growing in the presence of favorable carbohydrates, produce acids.

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