Calcification of a cariogenic Streptococcus and of Corynebacterium (Bacterionema) matruchotii

The main aim of this investigation was to challenge the idea that cariogenic streptococci do not calcify. Calcium uptake of calcification of Streptococcus mutans C180-2, proven to be an acidogenic and cariogenic strain, was compared with calcium uptake and calcification of Corynebacterium (Bacterionema) matruchotii, known as a ready calcifier. Bacteria were grown on Brain Heart Infusion Agar (BHIA) and on well-buffered semi-synthetic E-agar, both containing 1.4 mmol/L calcium, 2 g/L glucose, initial pH 7.4. Calcium uptake from BHIA by C. matruchotii (25 mmol Ca/kg wet bacterial cell mass), but not by S. mutans, was found. Grown as a plaque-like lawn on E-agar, the S. mutans cell mass concentrated calcium to 63 +/- 11 mmol/kg compared with 145 +/- 61 mmol/kg in C. matruchotii. X-ray diffraction confirmed the presence of crystalline apatite in the bacterial cell masses. Electron microscopy revealed crystals and mineralized deposits in both organisms. Heavy calcifications in some cells of S. mutans were seen. Calcification was partly inhibited by magnesium ion and by methanehydroxybisphosphonate. S. sobrinus 6715, as well as freshly isolated S. mutans and S. sobrinus from patients, concentrated very large quantities of calcium, up to 500-fold from the medium, when maintained for several weeks on E-agar of initial pH 7.6. Our observations widen the view on acidogenic bacteria as mineralization agents and support the notion that members of the mutans group of streptococci may be involved in events that trigger heavy intracellular calcifications and, possibly, dental calculus formation.