Expulsion mechanism of xylitol 5-phosphate in Streptococcus mutans

The expulsion mechanism of xylitol 5-phosphate in Streptococcus mutans ATCC 25175 was studied using resting cells incubated in the presence of 14C-xylitol. The expulsion appeared to be a two-step process: xylitol 5-phosphate was first hydrolyzed to xylitol and inorganic phosphate, and the xylitol was subsequently expelled from the cells. The dephosphorylation step appeared to be energy-requiring and it was most likely associated with a phosphatase which was active on xylitol 5-phosphate. Two to three successive cultivations of the cells in the presence of 6% xylitol increased this enzyme activity 4.3-fold. These results are in accordance with the presence of an energy-dependent xylitol 5-phosphate cycle in S. mutans, which is regulated by exogenous xylitol.