Factors influencing cell shape in the mutans group of streptococci.

Electron and light microscopic and growth studies of representatives of the diverse species of mutans streptococci revealed the cells to be either bacillary or coccoid in shape. Some strains changed from bacillary to coccoid if the HCO3-/K+ ratio of the media was increased and from coccoid to bacillary if the ratio was decreased. Doubling times of rods and cocci were the same despite an HCO3-/K+ ratio change between 0.008 and 2.84. For strain 10449S, no tested anions or cations substituted for HCO3- or K+ to produce this effect, except for B4O7(2-). Strain 10449S grown at a high B4O7(2-)/K+ ratio became ellipsoid, and this phenomenon was associated with slower doubling times. Up to three incomplete septa could be observed in one rod, but no more than one incomplete septum could be observed in either ellipsoid or spherical cells. Interseptal distances were greatest in rods, shorter in spheres, and shortest in ellipses. All of the above differences were statistically significant (P less than 0.001).