Resistance in oral streptococci after repetition of a single-dose amoxycillin prophylactic regimen.

Ten healthy volunteers received 3 g amoxycillin each week for up to four weeks. One volunteer carried streptococci resistant to amoxycillin in small numbers before the first dose was administered. After one administration of amoxycillin, this volunteer and one other yielded streptococci resistant to 1 mg/l amoxycillin on direct culture of dental plaque; after two administrations, resistant streptococci were detected in one further volunteer; and a total of six volunteers yielded resistant streptococci after the fourth week of amoxycillin administration. Repeated use of the single-dose prophylactic regimen encourages the emergence of streptococci considerably more resistant to amoxycillin than those generally encountered in the oral cavity, but the clinical significance of these results is uncertain as the peak serum concentrations of amoxycillin achieved after a 3 g oral dose for prophylaxis exceed the minimum bactericidal concentrations of amoxycillin for these strains.