BACTERIAL ETIOLOGY OF DENTAL CARIES.-The microbiota indigenous to the oral cavity of man is complex, and consequently it has been difficult to elucidate the role of these organisms in dental diseases. However, there is little doubt that bacteria are the etiologic agents of dental caries. The main evidence which supports this view may be summarized as follows:

1. In vitro experiments have indicated that bacteria indigenous to the oral cavity can decalcify enamel and dentin.': 2.12, 33.42, 44 2. Unerupted teeth, both of man and experimental animals, do not develop caries. This is most evident in the hamster, where extensive caries can develop in erupted first and second molars, while unerupted third molars remain caries-free until they enter into the oral cavity and are exposed to the oral microbiota." 3. Antibiotics, such as penicillin, which act specifically upon bacteria, are effective in reducing the incidence and severity of caries when administered to experimental ani-mals. P: 40, 46 Antibiotics have also been reported to reduce caries in man." 4. Rats raised in a germ-free state fail to develop caries, even when maintained on a cariogenic diet. Control animals, reared conventionally, develop extensive decay when fed the same diet. These experiments, first performed by Orland et at. in 195431 and confirmed by Fitzgerald, Jordan, and Stanley in 1960, 8 provide direct evidence that caries does not occur in the absence of bacteria in the rat. The applicability of these findings to human caries cannot be definitely established, but at the present time there is little reason to believe that the etiology of caries differs among species. The means by which bacteria are able to destroy the mineral and organic matrices of enamel and dentin have been extensively studied with the hope of elucidating the caries mechanism. These have recently been reviewed by Macdonald in his Chemistry and Prevention of Dental Cariest" and consequently will not be discussed in detail here. The available evidence suggests that caries is initiated by decalcification of enamel, most likely caused by acids resulting from the microbial metabolism of carbohydrates. The most direct evidence indicating that acid plays a major, if not the sole, role in caries formation is based on experiments with germ-free animals. Orland et at. in 195532 successfully produced caries in gnotobiotic rats with an enterococcus in combination with either an anaerobic or a proteolytic rod, demonstrating that caries could be produced in animals with a known, limited flora. However, the biochemical capabilities of the organisms employed in this study were not well characterized, and consequently little could be deduced regarding the caries mechanism. Recently, Fitzgerald, Jordan, and Stanley" succeeded in producing caries in gnotobiotic rats with a single streptococcus strain. The organism was a typical homofermentative streptococcus, which evidently had little or no proteolytic capacity, for it failed to hydrolyze gelatin or collagen or to peptonize casein. Thus caries was produced exclusively by an acidogenic, non-proteo-