it is the cumulative result of consecutive cycles of demineralization and remineralization at the interface between the biofilm and the tooth surface . upon this acid challenge , the hydroxyapatite crystals are dissolved from the subsurface . a number of etiologic factors , e.g. , the presence of cariogenic microorganisms , regular consumption of refined carbohydrates , and poor oral hygiene , contribute to the development of dental caries . dental caries occurs as a result of imbalance between the remineralization and demineralization processes as the interface between the enamel surface and dental biofilm . some favorable properties of sugar ( sucrose ) are its availability and , most importantly , its cost and public perception , which made it(sucrose ) acceptable by the public . replacing sugar ( sucrose ) with a suitable substitute to combat dental caries is an option that is wide open . recent , encouraging studies suggest antimicrobial properties as well as lower cariogenicity of some sugar substitutes such as xylitol . in the beverage field , it is clear that artificial sweeteners formed new products that were additional to the sugar - sweetened products and not competitive with them . saccharin can substitute for the taste of sugar ; lactose or sorbitol for the weight or bulk of sugar ; xylitol for both taste and bulk . similarly , the point remains that the per capita consumption of sugar has been stable for decades , in spite of the use of saccharin , cyclamate , and now aspartame . as xylitol is almost nonfermentable by plaque bacteria , it can inhibit the proliferation , growth , and accumulation of oral bacteria and reduce dental plaque adhesion . long - term consumption of xylitol has been associated with reduced growth and activity of streptococcus mutans and lower numbers of these microorganisms in both dental plaque and saliva . these beneficial effects of xylitol render it a good option for the prevention of dental caries . while some researchers have confirmed the efficacy of this substance in preventing caries , others have rejected such benefits . twetman compared the effects of various xylitol and sorbitol concentrations on s. mutans counts in normal individuals , subjects with fixed braces , and 24-year - old children , respectively . we compared the effects of xylitol and glucose - containing chewing gums among adults with poor  moderate oral hygiene , and their findings were similar to ours . considering the significance of dental caries prevention and the antibacterial effects of xylitol , the present study sought to compare the effects of chewing xylitol - containing gums and conventional gums on s. mutans in the saliva of subjects with moderate and poor oral hygiene . in this experimental research , 32 individuals ( 16 dental students and 16 patients visiting dental clinics ) who were aged 1835 years were recruited . the participants oral hygiene was categorized as moderate or poor based on the caries risk assessment table . individuals were not included if they had any systemic diseases , history of head and neck radiography , or sensitivity to xylitol . the study protocol was approved by the ethics committee of shahed university , and the participants signed informed consent forms . xylitol chewing gums ( orion food vina co. ltd , lai thieu townlet , thuan an district , vietnam ) containing 70% xylitol , isomalt , gum arabic , and calcium lactate were purchased from a gum wholesaler in tehran , iran [ figure 1 ] . pk gums ( 1.6 g , wrigley , vadapalani , chennai , india ) containing sucrose , glucose , gum , mint extract , pigments , and titanium dioxide were also purchased . all subjects were instructed to take three gums a day after main meals ( breakfast , lunch , and dinner ) and chew each gum for 15 min . xylitol chewing gums ( orion food vina co. ltd , lai thieu townlet , thuan an district , vietnam vietnam ) a crossover design was used in the current research . the two groups with moderate and poor oral hygiene ( n = 16 ) were each divided into two groups of eight to receive either pk gums or xylitol gums during the first 10-day period . after a 10-day washout period , each group received the other type of gum for another 10 days . saliva samples were collected and tested before and after each stage . it should be noted that 2 participants were excluded from the study due to the administration of antibiotics during the second stage . the samples were cultured using blood agar powders ( ceneda co. , germany ) . at each stage , the culture medium was prepared by mixing 40 gr blood agar powder with 1 l distilled water . the mixture was then boiled and the resultant clear solution was autoclaved at a temperature of 121c and a pressure of 15 atm for 15 min . after cooling the sterilized solution down to 4550c , 50 ml defibrinated sheep blood was added to the medium . the culture medium was finally transferred to 8-cm plates ( farazbin co. , tehran , iran ) and the plates were maintained at 4c until use . mitis salivarius - bacitracin agar ( msba ) , ( ceneda co. , germany ) was applied to differentiate between colonies suspected to be s. mutans and other colonies formed on blood agar plates . in order to prepare msba plates , a mixture of 25 medium powder and 250 ml sterile distilled water was boiled to obtain a clear solution . at the same time , 0.25 g potassium tellurite ( measured by a sensitive laboratory scale ) was mixed with 25 ml sterile distilled water and the mixture was autoclaved ( at 121c and 15 atm ) with the medium solution for 15 min . after sterilization , the containers holding the medium and potassium tellurite were cooled down to 4550c . the potassium tellurite solution and 0.25 g bacitracin the resultant medium was finally moved to 8-cm plates ( farazbin co. , iran ) and kept at 4c until required for the experiments . sampling and microbial culturing were performed at baseline , after the first and second 10-day periods of gum consumption , and after the washout period . at the time of sampling , the participants were asked rinse their mouths with water immediately after getting up in the morning ( before eating breakfast or brushing their teeth ) . afterward , 0.5 ml samples of unstimulated saliva were collected in sterile capped tubes ( greiner bio - one , frickenhausen , germany ) and immediately transferred to the microbiology laboratory of imam ali clinic ( tehran , iran ) . in the laboratory , an inoculating loop was flamed and sterilized . then , saliva samples were transferred to the blood agar medium using the loop [ dilution factor : 10 colony - forming units ( cfu)/ml ] . after placing the plates in a candle jar ( containing 5% carbon dioxide ) , , colonies suspected to be s. mutans were placed on petri dishes and gram staining was performed to confirm the presence of s. mutans . a sterile loop was used to move the grown colonies of streptococcus from the blood agar medium to the msba . the msba was then placed in a candle jar and incubated at 37c for 24 h. colonies of s. mutans ( dark blue colonies 23 mm in size ) were then observed on the culture medium . when the presence of s. mutans was confirmed , the number of s. mutans colonies formed on the blood agar medium was counted . the number of cfus was identified by morphology , counted in a stereomicroscope , and expressed as cfu ml . all statistical analyses were conducted using spss for windows ( spss inc . , chicago , il , usa ) . a total of 32 individuals aged 1835 years were allocated to two groups of 16 based on their oral hygiene status ( poor or moderate ) . the study consisted of two stages . saliva samples were collected and tested before and after each stage . in the group with moderate oral hygiene , the colony count at baseline ranged between 4  10 cfu / ml and 9  10 cfu / ml . these values were reduced to 26  10 cfu / ml after the first stage of the study . further reductions in the mean number of colonies were observed after the second stage ( 7  10 cfu / ml at baseline versus 4  10 cfu / ml after the second stage ) . in the group with poor oral hygiene , colony counts at baseline ( 2250  10 cfu / ml ) were reduced after the first stage of the study ( 1238  10 cfu / ml ) . the mean value measured after the second stage was also lower than the baseline value ( 19  10 cfu / ml vs 27  10 cfu / ml ) . the mean baseline colony count in this group ( 19  10 cfu / ml ) declined after xylitol chewing gums three times a day for 10 days ( 11  10 cfu / ml ) . the mean number of colonies was also considerably lower after the second 10 days of xylitol chewing gums than after the washout period ( 10  10 cfu / ml vs 17  10 cfu / ml ) . the mean colony count in this group was 18  10 cfu / ml , which was lowered to 15  10 cfu / ml following 10 days of chewing pk gums three times a day . the value after the washout period ( 16  10 cfu / ml ) was also reduced to 13  10 cfu / ml after another 10 days of pk chewing gums . as the mean reduction was higher in the xylitol group than in the pk group ( 7.5  10 cfu / ml vs 3  10 cfu / ml ) , the xylitol - containing gum was more effective . in the group with moderate oral hygiene , paired t - test revealed a significant difference in the mean count of s. mutans colonies before and after xylitol gum consumption ( p = 0.000002 ) . the same test also suggested a significant difference between the numbers of s. mutans colonies after the 10-day washout period and after the second 10 days of chewing gums ( p < 0.05 ) . in the group with poor oral hygiene , significant differences were detected between the mean s. mutans colony counts before and after 10 days of xylitol chewing gums ( p < 0.01 ) and before and after the second stage of chewing gums ( p < 0.05 ) . similarly , significant differences in the mean colony counts before and after the first stage of pk gum consumption were confirmed in subjects with both poor and moderate oral hygiene ( p < 0.01 ) . likewise , the mean colony counts before and after the first stage of pk gum consumption were significantly different in participants with both poor and moderate oral hygiene ( p < 0.05 ) [ table 1 ] . xylitol , a nonfermentable polyol , is a natural sweetener that does not promote tooth decay . it increases salivary flow , which in turn stimulates the remineralization process and inhibits bacterial growth and metabolism in dental plaques . despite these properties , the efficacy of xylitol as the present study compared the effects of xylitol - containing and conventional chewing gums on salivary levels of s. mutans . our results indicated the effectiveness of xylitol in inhibiting s. mutans growth among people with poor  moderate oral hygiene . the acid produced following the fermentation of sugar and sugar alcohols by s. mutans can destroy tooth enamel and result in dental caries . furthermore , xylitol can inhibit the growth of s. mutans by creating an alkaline environment in the mouth . the formation of such complexes in the mouth will promote the remineralization of tooth enamels that have previously lost their minerals . although makinen et al . reported similar results , our results are more reliable due to the use of a crossover design . twetman and stecksen - blick compared the effects of various xylitol and sorbitol concentrations on s. mutans counts in normal individuals , subjects with fixed braces , and 24-year - old children , respectively . while we compared the effects of xylitol and glucose - containing chewing gums among adults with poor  moderate oral hygiene , their findings were similar to ours . in a randomized clinical trial , ritter evaluated 2180-year - old individuals in terms of occlusal and smooth surface caries over a 33-month period . the results confirmed the beneficial role of xylitol in preventing root caries in adults with active decay . we , however , adopted a crossover approach and recorded bacterial counts in two different stages . despite differences in methodology , our study and that of ritter 's yielded similar results . nevertheless , our findings might be more accurate because we calculated colony counts after the chewing of both types of gums . in contrast to our findings , duane followed preschool children for 9 months and 21 months and reported lozenges , xylitol / maltitol , and erythritol / maltitol to be ineffective in reducing dental caries . this difference can be attributed to the type of intervention and the age group of the participants ( 1835 years in the present research ) . in addition , we used a crossover design to determine microbial counts after the chewing of gums . the frequency of chewing gums , rather than the exact concentration of xylitol , seems to be the important determinant of reduction in salivary levels of s. mutans and tooth decay prevention . in fact , chewing gums should be used at least three times a day to reduce dental caries by 4050% . long - term application of xylitol is believed to inhibit the growth and activity of s. mutans and prevent dental caries . in the current study , chewing gums ( either xylitol or pk ) could more effectively decrease salivary levels of s. mutans in subjects with poor oral hygiene ( without usual hygienic measures ) than in those with moderate oral hygiene ( with usual hygienic measures ) . in the absence of all usual hygienic measures in the group with poor oral hygiene , it is also noteworthy that in both groups , xylitol ( which did not contain sucrose ) was more effective than pk ( containing sucrose ) . this finding can be justified by the intrinsic capabilities of sugar substitutes to decrease salivary levels of s. mutans and control the acidity of saliva . as all subjects were both cases and controls , our results regarding the effects of the intervention ( xylitol or pk ) were more reliable than other , similar studies . to the best of our knowledge , no previous research has ever used this design . moreover , previous research has generally used higher frequencies of chewing gums or longer periods of follow - up . considering the significance of lowering salivary levels of s. mutans in individuals with poor oral hygiene , we compared the effects of xylitol - containing and conventional chewing gums in people with poor and moderate oral hygiene . we observed both types of gums , especially xylitol , to effectively reduce salivary levels of s. mutans in the two groups . the major limitation of the present study was the lack of exact supervision in the appropriate use of chewing gums . considering the effects of xylitol in decreasing dental plaques , further studies are recommended to assess the effects of this polyol on periodontal diseases . researchers are also suggested to evaluate the effects of xylitol - containing mouthwashes , toothpastes , and candies on oral bacteria and periodontal diseases . based on the results of this study , sucrose - free ( xylitol ) chewing gums when chewed 3 times a day for 15 min over a period of 10 days were more effective than sucrose - containing chewing gums in reducing salivary levels of s. mutans , especially in subjects with poor oral hygiene . using these chewing gums can thus be recommended as an effective substitute for mechanical methods in individuals with poor oral hygiene or mental and/or physical disabilities .
objective : dental caries is among the most common chronic diseases in humans . 
 streptococcus mutans is generally responsible for most cases of dental caries . 
 the present study sought to compare the effects of xylitol - containing and conventional chewing gums on salivary levels of s. mutans.materials and methods : this study adopted a crossover design . 
 two type of chewing gums ( one containing 70% xylitol and approved by the iranian dental association , and another containing sucrose ) were purchased . 
 the participants were 32 individuals aged 1835 years whose oral hygiene was categorized as moderate or poor based on a caries risk assessment table . 
 salivary levels of s. mutans were measured at baseline , after the first and second phases of chewing gums , and after the washout period . 
 the measurements were performed on blood agar and mitis salivarius - bacitracin agar ( msba ) . 
 pairwise comparisons were then used to analyze the collected data.results:salivary levels of s. mutans in both groups were significantly higher during the two stages of chewing gum than in the washout period or baseline . 
 moreover , comparisons between the two types of gums suggested that chewing xylitol - containing gums led to greater reductions in s. mutans counts . 
 this effect was more apparent in subjects with poor oral hygiene than in those with moderate oral hygiene.conclusions:xylitol-containing chewing gums are more effective than conventional gums in reducing salivary levels of s. mutans in individuals with poor  moderate oral hygiene .