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The Effects of Xylitol on Bacterial Growth
Ryan Cusick
Research and Thesis 493
Advisor: Dr. Douglas Oba
Abstract
The purpose of this work was to determine if xylitol inhibited the in vitro
growth of the oral bacteria Streptococcus mutans, Lactobacillus lactis,
Lactobacillus delbrunckii and Streptococcus lactis. Two 15% xylitol
solutions were made by dissolving 0.15 g xylitol in 1 mL of filtered saliva
or distilled water. Saturated disks were placed on the surface of Tryptic
Soy Milk agar that had been confluently plated with four plates of each
bacterium. The controls included distilled water, filtered saliva, and the
commercial antibiotic, tetracycline. The cultures were incubated in a
carbon dioxide environment. Zones of inhibition were observed only on
the tetracycline control. Xylitol solutions of 15% did not inhibit growth in
the bacteria.
INTRODUCTION
Dental hygiene and the prevention of cavities is a global concern. Tooth decay,
which is the destruction of tooth enamel, is triggered when bacteria that live on teeth
digest carbohydrates left on the surface of a tooth. Oral bacteria produce acids that
destroy the enamel of teeth; over time this destruction of enamel results in caries,
commonly know as cavities. A biofilm of different bacteria provide an environment that
is conducive to caries formation (Briner et al. 1986).
The prevention of dental decay includes fluoride therapy, fissure sealants, dietary
counseling and oral hygiene measures such as brushing and flossing (Lewis 1995).
Treatments involving remineralization of tooth enamel have been introduced. Since
damaged tooth enamel facilitates caries formation, increasing remineralization may result
in cavity prevention (Reynolds 1997).
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Some manufacturers of chewing gum have promoted their products as a method
to prevent tooth decay. Trident gum containing Recaldent is purported to deliver
calcium beneath the surface of the tooth (Reynolds 1997). Chewing gum also enhances
the amount of salivary flow in the mouth (Makinen 1972). Xylitol, a five-carbon
molecule related to xylose is an alcohol found in certain gums such as Clen Dent.
Xylitol induces synthesis of salivary enzymes (Knuuttila and Makinen 1975) such as
lactoperoxidase, which breaks down plaque and reduces attachment ability of
Streptococcus mutans and other oral bacteria (Makinen et al. 1976). Streptococcus
mutans plays a crucial role in the initial attachment of the bacterial biofilm to the surface
of the teeth. Most microorganisms, including S. mutans, cannot metabolize xylitol,
however, a few microorganisms have been found to develop the ability to digest the
substrate over a period of months (Knuuttila and Makinen 1975). Because S. mutans
cannot ferment xylitol, acid production is reduced preventing oral pH levels from
dropping (Trahan 1995).
Chewing gum containing xylitol prevents caries formation (Hildebrant and
Sparks 2000). Kandelman and Gagnon (1990) showed that increasing the percentage of
xylitol in gum from 15% - 65% did not affect caries inhibition. Although the percent of
xylitol in the gums did not affect the observed inhibition of caries, the frequency of the
chewing did (Hildebrant and Sparks 2000). The mechanical process of mastication breaks
down plaque, thus physically inhibiting caries formation (Kandelman and Gagnon 1990).
A brief summary of established roles xylitol plays in inhibiting caries are the
following: first, plaque is broken down because the pH level is maintained and S. mutans
cannot thrive, because bacterial attachment to the tooth surface is decreased. Second,
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xylitol causes a salivary response, rinsing teeth of bacteria and further reducing bacterial
attachment. Third, it induces enzyme synthesis, increasing plaque breakdown. Most
microorganisms cannot metabolize xylitol preventing the acidic end product that erodes
tooth enamel. Finally, the bacterium grows in the presence of different carbohydrates,
but growth is retarded by xylitol. As a main carbohydrate source, xylitol eliminates
bacterial growth (Trahan 1995).
Although xylitol eliminates bacterial growth it has not been investigated whether
xylitol’s inhibition of dental bacteria is due to anti-microbial activity. When xylitol is the
only available carbohydrate bacterial growth is not seen. However, when xylitol is one of
several carbohydrates, the bacterium grows but the growth is retarded by xylitol (Trahan
1995). The purpose of this work was to determine if xylitol demonstrated anti-bacterial
activity of the in vitro growth of the oral bacteria S. mutans, Lactobacillus lactus,
Lactobacillus delbrunckii and Streptococcus lactis.
Materials and Methods
Two xylitol solutions were made by dissolving 0.15 g xylitol in 1 mL of filtered
saliva or distilled water. These solutions were aseptically micropipetted onto and
absorbed by a sterile paper disk. Each disk was placed on the surface of Tryptic Soy
Milk agar, which had been confluently plated with either S. mutans, L. lactis, L.
delbrunckii or S. lactis. Four plates of each bacterium were prepared. The controls
included distilled water, filtered saliva, and the commercial antibiotic Tetracycline. The
cultures were incubated for 36 hours at 37 º C in a carbon dioxide environment.
of inhibition were measured.
Results
Zones
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Solutions of 15% xylitol (w/v) in distilled water or filtered saliva did not cause
zones of inhibition in any one of the four bacteria plated. The tetracycline control caused
the only observed zones of inhibition. Each zone of inhibition plus or minus the standard
deviation is given below in Table 1.
Table 1.
The growth inhibition, in mm, of bacteria by the presence of xylitol
and controls, the mean +/- standard deviation.
Bacteria
plates
Filtered
Saliva
Distilled
Water
15% Xylitol
in distilled
water
15% Xylitol in
filtered saliva
S. mutans
L. delbrunkii
L. lactis
S. lactis
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Tetracycline
25 +/- 4.7
35+/-0.58
36 +/- 8.8
26 +/- 4.7
Conclusion
The purpose of this study was to determine if xylitol inhibited the in vitro growth
of various oral bacteria. The results indicate that xylitol did not inhibit the growth of
bacteria in a carbon dioxide environment on Tryptic Soy Milk agar. Although previous
studies have established that S. mutans will not grow in the presence of xylitol as a
carbohydrate source, this study shows that inhibition is not due to anti-microbial activity.
This study determined that xylitol solutions of 15% did not inhibit bacterial growth in a
carbon dioxide environment on Tryptic Soy Milk agar. Therefore, the main mode of
inhibition by xylitol appears to be the replacement of the carbohydrate source. Any
further research with xylitol should focus on the replacement value of the crystalline
alcohol as a carbohydrate source for oral bacteria.
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Acknowledgements
I would like to thank Dr. Oba for all his help, the biology faculty for their insights
and corrections, Jennifer Barnes for help in the lab, and those who have helped revising
the paper.
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