Reduced susceptibility to penicillins is commonly observed in oral streptococci from neutropenic patients, but little is known on the susceptibility to penicillins of the commensal oral flora of the healthy population.
The aims of this study were to evaluate the presence of oral streptococci with reduced susceptibility to amoxicillin in healthy patients undergoing tooth extraction and to follow the evolution of this susceptibility after amoxicillin therapy.
Methods:
Thirty three healthy patients undergoing tooth extraction were randomly assigned in a double blind control fashion to a 7-day amoxicillin treatment or a 3-day amoxicillin+4-day placebo treatment (day 0). The patients treated with any antibiotic during the last 45 days were excluded. The tooth was extracted at day 2 and the post-operative follow-up was done at day 9. Oral flora was collected at day 0 and 9 and cultivated on several selective Columbia blood agar containing 0mg/L, 0,5mg/L, 2mg/L or 16mg/L of amoxicillin. The MICs were determined on streptococci by E-tests. Intermediate (I) and resistant (R) strains were identified according to the antibiogramme commitee of the French Society for Microbiology.
Results:
At day 0, amoxicillin-I and –R streptococci were isolated in 31 and 2 patients respectively. In most cases, the amoxicillin-I strains represented less than 5% of the total streptococcal flora.
At day 9, amoxicillin-I streptococci were isolated from all patients and amoxicillin–R strains from 7 patients. The proportion of the I-strains increased greatly between day 0 and 9, and the amoxicillin-I strains became the dominant flora in 8 patients at day 9. No significant differences were observed among the two groups of patients.
Conclusion:
These results demonstrate that amoxicillin treatment rapidly selects streptococci with reduced susceptibility to amoxicillin in the commensal flora of the oral cavity.