Abstract: Objective: to investigate effect of calcium ions concentration on lactic acid production (mode of fermentation) from streptococcus mutans at excess glucose media (cariogenic environment). Materials and methods: Streptococcus mutans Clarke (ATCC 25175)was investigated in-vitro under glucose excess instances with different concentrations of glucose, at different calcium ion concentrations and also at different bacterial cell concentrations.The supernatants of the bacteria were analyzed using HPLC for lactic and acetic acids. Results: it was found that calcium ions can increase lactic acid or decrease it, and can encourage either mixed acid fermentation or homofermentation according to their proportion to glucose molecules and bacterial cell number.Discussion:to interpret these findings C-type lectins or C-type lectin-like molecules were suggested to be found on bacterial cell wall. These supposed molecules bind glucose molecules and calcium ions. Consequently when glucose molecules are introduced to the bacteria with high amounts they bind to these C-type lectins or the C-type lectin-like molecules using extracellular free calcium ions which become bonded to them. As a result of that,concentration of the free calcium ions decreases.This promotes lactic acid production as a major end product in an attempt to chelate calcium ions (from the supposed C-type lectins or the C-type lectin-like molecules or from tooth surface causing demineralization; caries) and restore the level of the free calcium ions extracellularly which restores mixed acid fermentation. Conclusion: mixed acid fermentation can be achieved at glucose excess medium (; Crabtree effect can be prevented) if calcium ion concentration was high enough in proportion to glucose molecules and number of bacterial cells. As well as pyruvate dehydrogenase enzyme can be activated in glucose excess medium when appropriate calcium ion concentration is provided. So, we mayuse S. mutans as a model to understand the mode of respiration in cancer cells (Warburg and Crabtree effects) which is very important in knowing carcinogenesis which may be due to the same mechanisms.