Development of resistant oral viridans streptococci after administration of prophylactic antibiotics: time management in the dental treatment of patients susceptible to infective endocarditis.

The American Heart Association recommends prophylactic administration of penicillin before each dental session to patients susceptible to infective endocarditis. Such preventive treatment, however, may trigger the transient appearance of penicillin-resistant bacterial strains. In order to investigate the behavior of oral streptococci, 29 healthy volunteers who did not harbor penicillin-resistant viridans streptococci received 4 gm of phenoxymethyl penicillin orally over a period of 10 hours. This amount constituted the sole dose of antibiotics administered in the entire experiment. Daily specimens of oral flora were obtained for 14 successive days from each participant and incubated aerobically with a penicillin-saturated disk for 24 hours. Viridans streptococci were considered resistant when bacterial colonies grew adjacent to the disk for 1 day or more. The study population was divided into high- and low-resistance groups, according to the individual antibiograms. Resistant viridans streptococci were already detected at 6 hours after penicillin ingestion in nine (31%) of the subjects. Six months later, oral specimens were taken from ten randomly selected participants; these specimens served as a control. The difference in bacterial resistance between the high- and low-resistance groups was significant for the duration of 9 days, as was that between the high-resistance and control groups (p less than 0.05 in both cases). In order to minimize the odds that penicillin-resistant bacterial strains will develop in patients susceptible to infective endocarditis, elective dental treatments in these persons should be scheduled in intervals of not less than 10 days.