•272 p. H. KEYES AND H. V. JORDAN 



plaques found on hamster teeth have been associated with typical 

 caries. Apparently plaques do not form unless specific types of organ- 

 isms are present. 



In hamsters, cariogenic plaque formation does not appear to be 

 simply the result of bacterial growth in food residues coincidently 

 retained on the teeth, although in certain instances this may be the 

 case. The plaque which so rapidly forms in either naturally infected 

 or inoculated animals appears to be the product of an interaction 

 between bacteria and the diet. There seem to be specific conditions 

 which determine whether colonization occurs or not. This is to say 

 that, experimentally, the formation of the plaque itself is not the 

 result of one set of factors but of the simultaneous interaction of 

 two sets, or three if one adds those contributed by the enamel. 



Question of Bacterial Specificity 



It seems important to clarify the etiologic role of the acidogenic 

 microbiota in caries initiation in vivo. On the basis of what is gen- 

 erally recognized about the specific nature of the biochemical, nutri- 

 tional, and environmental factors which determine implantation and 

 propagation of microorganisms, it does not seem unreasonable to 

 postulate a degree of specificity to caries initiation. This does not 

 imply that ever)^ lesion would have exactly the same microbiological 

 component; indeed, some observations in rats and hamsters suggest 

 that this may not be so, but probably the point will require gnoto- 

 biotic technics to determine conclusively. Nevertheless, the position 

 of streptococci in an environment with high concentrations of carbo- 

 hydrate would seem to be more delicate than that of lactobacilli. 

 The fact that streptococci are not so aciduria suggests that these 

 organisms would thrive better in a locus in which their ovm acid 

 by-products could be readily neutralized, as well as those from other 

 neighboring organisms. 



A question not too frequently asked is, "What does the carious 

 lesion, i.e. the invasion of the enamel and dentin, mean to the organ- 

 isms involved?" Certainly from numerous histopathological studies 

 caries appears to be a highly invasive infection. Figure 6 shows sev- 

 eral examples of bacterial invasion in hamster molars. Is this phe- 



