276 P. H. KEYES AND H. V. JORDAN 



environment. If inorganic ions form part of this milieu, which ones 

 and why? 



There are many interesting questions related to the specific re- 

 quirements of microorganisms and how they invade. Along these 

 lines, the mechanisms whereby fluoride exerts its protective effect 

 might again be examined in terms of what the inability to invade 

 might mean to the organisms involved. The two effects most often 

 advanced to explain the action of fluoride are a decreased solubility 

 of the apatite mineral and an antienzymatic effect which inhibits 

 glycolysis. Recent observations in hamsters have shown that caries 

 can be effectively inhibited by adding fluoride to the drinking water 

 at the time of third molar eruption, i.e., during the very early erup- 

 tive and posteruptive period. Although lesions which started before 

 the addition of the fluoride ion continued to progress slowly and 

 known cariogenic streptococci were recovered from the mouth in 

 high numbers (Keyes et al., 1962), the third molars were exceed- 

 ingly well protected. Also, this tooth acquired a long-lasting re- 

 sistance, as indicated by its ability to remain intact long after with- 

 drawal of the drug. 



Apparently the colonization of pathogenic organisms was not pre- 

 vented sufiiciently to explain the lowered level of caries activity. It 

 would be of interest to know whether the organisms were behaving 

 as usual. Is it possible that the reduced solubility of the apatite 

 (Gray et al., 1962; Zipkin et al., 1962) and its altered chemical com- 

 position (Brudevold, 1962«, 1962^?; Nikiforuk et al, 1962) prevent 

 the pathogenic bacteria from obtaining essential requirements at 

 a critical time? For example, the amount of salivary CO2 available 

 at the interface between cell membrane and crystal and deep in the 

 lesion is unknown, but it may not be enough for organisms faced 

 with the problem of rapid metabolic activity, especially since 

 salivary CO2 probably does not diffuse readily through the acid 

 plaque and lesion. The reduced solubility of enamel mineral and 

 replacement of hydroxyl ions could conceivably reduce the availa- 

 bility of buffers necessary for neutralization of the acids produced 

 during metabolism. 



One is able to think of many possibilities in regard to what the 

 effect of a change in the availability of inorganic ions can mean to 



