INTRODUCTION Whether there is any one bacterium which may always be foundin decayed dentine, and which might therefore be entitled to the name of the bacterium of tooth decay, or whether there are various kinds which occur with considerable constancy, we are not able to say. It is now apparent, however, that various microorganisms are essential in the pathogenesis of dental caries. Orland (451) first demonstrated that selected streptococcal species, namely, entero-cocci, produced dental caries in germfree rats when fed a high-sucrose diet. Furthermore, indirect evidence that antibiotics suppressed ex-perimental dental cariesin rodents (396) strongly suggestedthe involvement of certain penicillin-susceptible bacteria in dental caries. Since that time, various investigations have been carried out to elucidate the causative relationship between specific oral bacterial species and dental caries. In 1960, some streptococcal strains, isolated from carious lesions of rats and hamsters, produced dental caries in" caries-re-sistant" rats and hamsters, respectively (157, 302).

Using a fluorescent-antibody technique, sev-eral streptococcal strains which shared immunological specificity with the cariogenic strepto-cocci derived from rats and hamsters were iso-lated from human carious lesions (275, 628, 629). These strains also produced severe dental caries in germfree animals. Since then, similar strep-tococcal species have been isolated from human carious lesions by several investigators (183, 202, 321, 322). Carlsson (54, 55) indicated that prop-erties of these cariogenic streptococci were sim-ilar to those originally isolated from human car-ious teeth by JK Clarke (81) in 1924 to which he had given the species name mutans. Thus, the rediscovery of Streptococcus mutans followed the original observation by about 36 years. S. mutans is now considered to play an im-portant role in the development of dental caries in animals and humans. Extensive research on this microorganism has been done during the last 10 years. Unfortunately, however, S. mutans is not given an independent species position in the newest edition of Bergey's Manual ofDeterminative Bacteriology (43). It willbe recognized from the evidence described below that S. mu-tans is the best-defined species among the oral streptococci.