Species-specific PCR method for identification of Streptococcus downei

Aims:  To establish a rapid method to differentiate Streptococcus downei and S. sobrinus by multiplex PCR. Methods and Results:  A PCR primer pair specific to S. downei was designed on the basis of the nucleotide sequence of the dextranase gene of S. downei NCTC 11391T. The primer pair specifically detected S. downei, but none of the other mutans streptococci (16 strains of six species). The PCR procedure was capable of detecting 1 pg of genomic DNA purified from S. downei NCTC 11391 and as few as 14 CFU of S. downei cells. The mixture of primer pairs specific to each S. downei (this study) and S. sobrinus (Igarashi et al. 2000) detected only the strains of these two species among all the mutans streptococcal strains, and concomitantly differentiated the two species by species-specific amplicons of different lengths. Conclusions:  The present PCR method is highly specific to S. downei and is useful for detection and identification of S. downei. Significance and Impact of the Study:  Multiplex PCR using dextranase gene primers is a useful method for simultaneous detection and differentiation of S. downei and S. sobrinus.