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Some Streptococcus mutans strains change shape from bacillary to coccal or ellipsoid form in response to the ratio of bicarbonate to potassium or of borate to potassium in growth media. So that insight into determinants of shape of these streptococci could be gained, and future genetic studies facilitated, the shapes of a series of transformable and nontransformable strains of S. mutans were studied and attempts made to isolate a mutant of augmented transformability. Several strains were mutagenized by ethylmethane sulfonate and mutants with altered colonial and cellular morphologies isolated. Cell shapes were studied by Gram stain and Nomarski interference microscopy, and by scanning and transmission electron microscopy. Diverse shape-altered mutants were isolated from seven transformable and two nontransformable strains of S. mutans. Among these, length-to-width ratios ranged from > 10 to about 0.25. Regulation of timing of cell division, septum formation, or septum completion events may have been altered in these mutants. While most mutants substantially or completely lost transformability, mutant LT11 had transformation efficiency of 1.3 x 10(-4) to 2.3 x 10(-3), more than two to three orders of magnitude greater than its parental UA159 and the well-known transformable strain GS5(HK), respectively. There was no evidence of production of competence factor by LT11. Competence of LT11 was maintained for at least six months upon storage at -70 degrees C, facilitating its use for genetic studies. While the morphologies of several shape-altered mutants were no longer responsive to changes of the bicarbonate/potassium, unlike those of their parentals, the morphology of LT11 persisted in its response to this condition.(ABSTRACT TRUNCATED AT 250 WORDS)

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).

Morphological changes of S. mutans NCTC 10449S associated with growth in modified Jordan medium and FMC medium (Terleckyj et al., Infect. Immun. 11:649-655, 1975) were studied by scanning electron microscopy. The cells were bacillary in Jordan medium, but coccoid and of unequal size in FMC. Transfer of the cells from Jordan medium to FMC and vice versa reversed their shapes, as did salt exchange between these media. Morphological changes could not be ascribed to either medium pH, concentration of P, or Na+/K+ ratio. However, they were growth dependent, since the changes did not occur when the cells were suspended in salt components alone or in media supplemented with protein synthesis inhibitors. Only a high bicarbonate/K+ ratio, as in FMC, produced spherical cells, whereas cells remained bacillary in medium with a low bicarbonate/K+ ratio, as in Jordan medium. Manipulating this ratio in other media resulted in similar shape changes. Thus, the shape of S. mutans 10449S can be dictated by the ratio of bicarbonate to K+ in the growth medium.