Plaque samples were obtained from 13 children receiving long-term therapy with benzathine penicillin for the prevention of rheumatic fever recurrences, 31 children receiving oral sulfadiazine for the same purpose, and 29 untreated siblings. The therapies were found to have no effect upon the proportions of Streptococcus mutans or lactobacilli in dental plaque, upon the percentage of children harboring the organisms, nor upon the susceptibility of the organisms to penicillin and sulfadiazine. Of the S. mutans strains tested, 97% had a minimnal inhibitory concentration of penicillin G of less than 48 ng/ml and, of the lactobacillus strains tested, 96.8% had a minimal inhibitory concentration of less than 1,600 ng/ml. All strains of both organisms were profoundly resistant to sulfadiazine.Children receiving 200,000 U of oral penicillin daily for rheumatic fever prophylaxis have been reported to experience reductions in the incidence of dental caries of up to 56% as compared with their untreated siblings (13, 18). Handelman and Hawes have reported that the oral penicillin therapy had no effect on the number of salivary lactobacilli (12) and that the penicillin resistances of the total flora, the streptococci, and the staphylococci in the salivas of the antibiotic users were higher than those in the untreated siblings (11). They did not study the effect of oral penicillin on Streptococcus mutans; consequently, it is not known whether the caries-inhibiting effect of that agent (13) resulted from its action on that organism. Furthermore, they did not examine the dental plaque flora, which is more intimately associated with the development of the carious lesion than is the salivary flora.

Investigations were conducted to determine the in vitro effects of low levels of penicillin and sulfadiazine on the growth, plaque formation, and agglutination ofStreptococcus mutans and on the synthesis and activity of enzymes synthesizing extracellular polymers. The concentrations tested were equivalent to those expected in the saliva of subjects receiving oral therapy with the agents. Penicillin at 0.5 ng/ml and sulfadiazine at 1 ug/ml substantially inhibited in vitro plaque formation. At these concentrations, sulfadiazine but not penicillin also inhibited growth of the organism. Neither antimicrobial agent affected the agglutination ofS. mutans with dextran or the synthesis or activity of enzymes synthesizing extracellular polymers. The effect of sulfadiazine on plaque formation was attributed, at least in part, to the inhibitory action of that agent on S. mutans growth.Streptococcus mutans has been strongly implicated as a causative agent in dental caries both because of its ability to cause caries in animals and because of its association with caries in humans (5). A property of S. mutans that is believed to contribute to its cariogenicity is its ability to form sticky extracellular polyglucans from sucrose, thereby enabling the organisms to adhere to the teeth (7). Thus, in theory, agents which could interfere with the ability of S. mutans to form adherent deposits on teeth might reduce caries experience in humans.In earlier studies, children receiving 200,000 U of penicillin G orally per day for the prophylaxis of rheumatic fever showed up to 56% fewer new carious lesions than their untreated siblings (8,9). The present in vitro study sought to determine whether penicillin G, at the concentrations that would be expected in the salivas of such individuals, affects activities of S. mutans that might be related to the cariogenicity of that organism. The activities examined were growth, formation of adherent plaques on steel wires, agglutination, and the formation and activity of enzymes capable of producing extracellular polymers from sucrose.Because sulfadiazine (1 g/day) is also currently used in the prophylaxis of rheumatic fever, parallel in vitro studies were carried out to determine whether that agent might also affect the activities of S. mutans. At the pres-' Present address: Faculty of Dentistry, McGill University, P. 0. box 6070, Montreal, Quebec H3C 3G1, Canada. ent time there is no information on whether sulfadiazine therapy has any effect on caries in humans. As with penicillin, the concentrations examined were those expected to occur in the salivas of treated subjects. MATERIALS AND METHODSMicroorganisms. Type strains ofS. mutans tested included OMZ-61 (serotype a), BHT (serotype b), JC-2, Ingbritt (serotype c), SL-1, 6715 (serotype d), and LM-7 (serotype e) (2, 13, 14). Fresh isolates were obtained from children who were untreated siblings of outpatients at the Toronto Hospital for Sick Children.Selection of concentrations of antimicrobial agents. The antimicrobial agents were tested at a ran...