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Objective: To examine acid production of caries-associated strains of oral microorganisms and salivary microorganisms from sugar and sugar substitutes. Methods:Standard and clinical strains of Streptococcus mutans (S. mutans), Lactobacillus casei (L. casei) and Candida albicans were incubated in peptone-yeast-extract media containing 1% test sugar (sucrose, glucose, fructose) or sugar substitutes (xylitol, sorbitol, trehalulose and palatinose) at 37 ℃in 5% CO2 for 24-48 h. The pH of each culture was measured and microbial growth was determined as optical density at 660 nm. Paraffin-stimulated saliva collected from high caries-risk persons were added to media containing 10%test sugar or sugar substitutes. The pH of medium was measured at each time interval from 0-90 minutes. Results:All types of sugar and trehalulose could be fermented by all test microorganisms in pH lower than 5.5 except sucrose by standard strain of L. casei. All sugar and sugar substitutes supported growth of all organisms except xylitol for S. mutans. In the fermentation assay by salivary microorganisms, all sugar could be utilized and produced pH< 5.5 within 10 minutes of incubation and the pH drop was prolonged to until 90 minutes. Conversely, xylitol and palatinose were not fermented by microorganisms in saliva. Conclusions:All test microorganisms could ferment sucrose, glucose, fructose and trehalulose to pH lower than 5.5. Sugar alcohols and palatinose were not utilized well by organisms and may be used as sugar substitutes to reduce dental caries incidence. However, further studies particularly clinical investigations are required to evaluate the cariogenicity of these sugar substitutes.