الفهرس 
Inhibitory effect some of irradiated and un- irradiated herbal extracts on dental caries induced by Streptococcus mutans
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Abstract
Dental caries remains the main cause of tooth decay. It is initiated by acid producing bacteria, especially Streptococcus mutans, colonizing the dental surface and causing carious lesions in the presence of ferementable carbohydrates. S. mutans can form and sustain polysaccharide biofilm commonly known as dental plaque; it produces glucosyltransferases that form water insoluble glucans from dietary sugars. The glucans promote the adherence of S. mutans and other oral bacteria to tooth surface. So the key for dental health is the regular and effective plaque control. The objective of the study was to investigate the effect exerted by some natural herbal extracts on S. mutans (ATCC 25175). The antimicrobial activity of lemon, cinnamon, clove, olive and camphor oils in addition to the alcoholic extracts of turmeric, ginger, green tea, coffee, licorice, guava leaves, anise and caraway on S. mutans was assessed by disc diffusion method and revealed inhibitory activity by all the tested extracts and oils except for clove, camphor oils and caraway alcoholic extract. Minimum inhibitory concentration (MIC) of extracts and oils; exhibiting antimicrobial activity, was determined using agar dilution method and ranged from 0.12 {u2013} 5.75 mg/ml. The effect of sub-MIC concentration of the extracts and oils on S. mutans biofilm formation was investigated and showed a percentage inhibition ranging from 6.25 {u2013} 31.25%. Licorice extract exhibited highest biofilm inhibition percentage, and scanning electron microscopic examination of S. mutans biofilm of licorice-treated sample revealed deformed scattered cells with no clumping in a polysaccharide matrix. MIC values of irradiated extracts at doses of 0.5, 1, 1.5 and 2 KGy irradiated extracts and oils were determined; a decrease in the activity of herbal extracts and oils was observed following irradiation. Growth curves of both control and 1/4 MIC licorice-treated samples showed typical sigmoid curves with no change in the growth pattern, ruling out the effect of 1/4 MIC of licorice alcoholic extract on the viability of the bacteria. The effect of 1/4 MIC of licorice alcoholic extract on the gene expression of glucosyltransferases (gtfB, gtfC) was determined and showed down regulation in the expression of target genes. GtfB gene was down regulated by 207.49 folds and gtfC gene was down regulated by 265.3 folds. These results suggest the use of licorice extract in pharmaceutical products to suppress dental plaque biofilm