A new biotyping method for Streptococcus mutans with the API ZYM system

Objective: To test a new system for the biotyping of Streptococcus mutans, based on the measurement of enzyme activity, and to investigate the relationship between biotype and in vitro susceptibility to seven clinically useful antibiotics. Methods: In total, 160 oral isolates of S. mutanswere classified into different biotypes with the APINM test for enzyme activity, excluding results that were positive or negative in >80% of the strains. The susceptibility of all 160 strains to amoxycillin, cefazolin, erythromycin, clindamycin, vancomycin, teicoplanin and imipenem was tested by dilution in a solid medium. Statistical analysis of susceptibility (mean minimum inhibitory concentrations (MICs)) was based on chisquared tests. Results: Eight different biotypes (1-8) were identified on the basis of three kinds of enzyme activity: valine aryl amidase, acid phosphatase and a-galactosidase. Biotype 5 was found to be the most common. The mean MIC values showed strains belonging to biotype 4 to be the most susceptible to amoxycillin, cefazolin and erythromycin, whereas biotype 1 was the least susceptible to teicoplanin. Conclusions: The proposed biotyping method, which is relatively fast and simple to perform, provided reproducible results, and may contribute to clinically effective treatment of S. mutans infections.