Resistance in oral streptococci after repeated two-dose amoxycillin prophylaxis

Twelve normal volunteers received two doses of 3 g amoxycillin at weekly intervals on up to five occasions. Amoxycillin-resistant oral streptococci were not isolated from any subject beforehand, but they had appeared in all eleven subjects (who could be included in the analysis) by the end of the investigation, and in one subject after one administration of double dose amoxycillin. Resistant streptococci were undetectable in all volunteers 13 weeks after their last dose of amoxycillin. All resistant streptococci identified were Streptococcus sanguis (non-dextran-producing), many were highly resistant to amoxycillin and some were also moderately resistant to erythromycin. The minimum bactericidal concentrations for the most resistant streptococcal strains isolated were less than the peak serum amoxycillin concentrations expected following a 3 g oral dose of amoxycillin. In the majority of volunteers the counts of oral resistant streptococci had greatly declined by about 6 weeks following the last administration of a double dose of amoxycillin. If further dental treatment liable to cause bacteraemia is required within six weeks of double dose amoxycillin prophylaxis an alternative regimen should be sought.