|MadSci Network: Microbiology|
Bacteria which would be relevant to your experiment would be those which normally inhabit our mouths (oral cavity). Members of the genus Streptococcus would be a good choice. In particular, Streptococcus mutans as this bacteria is implicated in tooth decay. Another bacteria would be Bacteroides gingivalis, this bacterium causes the dental condition known as gingivitis. Another source of bacteria could be from your own mouth! If you have access to sterile "Q-Tip" or cotton swab you could swab the inside of your mouth and then streak the swab on a nutrient agar plate. Any single colonies of bacteria growing on the plate could be selected for your toothpaste inhibition study. This could be a neat addition to your study.
Regarding bacterial inhibitors in toothpaste: One ingredient that is always promoted in commercial ads is fluoride. However, fluoride is not an inhibitor per se' the fluoride strengthens your teeth to prevent/slow tooth decay (click here for a brief description of the role of fluoride in toothpaste). I could not really find any specific information on bacterial growth inhibitors (however checkout www.ada.org/newsrel/1295/nr-03.html. for a good summary on some ingredients found in toothpaste - click here). Some tooth-pastes may have mineral oxides which are bacterial growth inhibitors. For further help www.tooth.net has an "ask an expert site" you could try or perhaps one of the web sites for a major toothpaste producer, although I suspect they keep most of the ingredients secret for proprietary reasons. Keep in mind that tooth-pastes work indirectly to control bacterial growth by helping to remove trapped food particles that contribute to bacterial growth.
Regarding the experimental design: The experiment you propose is quite adequate. Plating your test bacteria on a plate and then placing sterile filter discs that were soaked in a tooth paste solution on the plate. Zones of inhibition will occur near the filter disc-the larger the zone the more active the test compound.
Hope this helps.
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