Mannitol and sorbitol catabolism in Streptococcus mutans

Mannitol- or sorbitol-adapted Streptococcus mutans NCTC 10449 contained high levels of mannitol-1-phosphate dehydrogenase and sorbitol-6-phosphate dehydrogenase activity. Neither of these activities was present at significant levels in cells grown on glucose or any of several other carbon sources. Moreover, when glucose was added to medium containing mannitol or sorbitol, the cellular level of both mannitol-1-phosphate dehydrogenase and sorbitol-6-phosphate dehydrogenase was reduced to that found in glucose-adapted cells. The two hexitol phosphate dehydrogenase activities were due to distinct enzymes, which could be separated by polyacrylamide gel electrophoresis and by Sephadex G-200 chromatography. Both enzymes were specific for nicotinamide adenine dinucleotide and neither was active with the corresponding non-phosphorylated hexitol substrates. The reaction product for both enzymes appeared to be fructose-6-phosphate. The available data indicate that Strep. mutans NCTC 10449 ferments both mannitol and sorbitol by a pathway that involves phosphorylation of the substrates prior to their oxidation by distinct, inducible enzymes to the common glycolytic intermediate, fructose-6-phosphate. This pathway for hexitol catabolism also appears to be operative in other strains of Strep. mutans.