ABSTRACT
Background: The oral bacteria, mutans streptococci (MS), are an etiological agent of dental caries. Of MS, Streptococcus downei are rarely isolated bacteria. Aim: The aim of this study was to isolate and characterize S. downei from caries-active subjects. Materials and Methods: In all, 65 dental plaque samples were collected from dental caries-active subjects. All the isolates were further identified and characterized using 16S rDNA sequencing, biochemical tests, antibiogram, and minimum inhibitory concentration (MIC). Results: Five isolates have been identified as S. downei using 16S rDNA sequencing. Phylogenetic analysis showed that S. downei was closely related to S. sobrinus. The biotype traits of these five isolates were IV (n = 3), V (n = 1), and variants (n = 2). The study proposed one new biotype, classified as biotype VIII for the variant strain. The antibiogram tests revealed that all the strains of S. downei were susceptible to all the antibiotics used in the study with higher sensitivity to penicillin and ampicillin. The MIC of ampicillin and erythromycin against S. downei was 0.047 and 0.39 μg/mL, respectively. Conclusion: The study reports the prevalence of S. downei in caries-active subjects and recommends further investigations to determine its role in the disease.