Lactate dehydrogenase-deficient mutants of serotype g Streptococcus mutans

Three LDH-deficient mutants of a serotype g strain of Streptococcus mutans were produced essentially as described by Hillman (1978): They were produced using ethylmethane sulfonate, and isolated using triphenyltetrazolium-glucose agar. Similar attempts to obtain mutants from a serotype c strain were unsuccessful. The g mutant did not revert during 12 weeks of daily transfer in broth, and, when grown in glucose-containing broth, they reached pH values of 4.9 to 5.0, compared with 4.4 for the parent strain. The efficiency of conversion of glucose to cell mass (Yg) was at least as great with the mutants as with the parent strain. The LDH activities of the mutants were less than 1% of that of the parent strain. Like Hillman's (1978) mutants, ethanol, acetic acid, and acetoin were the major products resulting from the metabolism of glucose. Although at pH 7.0 the mutants grew more slowly than did the parent, at pH 5 and pH 6 one of the mutants grew as rapidly as did the parent. The stability, serotype, and ability of these mutants to grow at low pH suggest their potential usefulness for replacement therapy.