Xylitol and strep mutans

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Xylitol and strep mutans
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Xylitol

• Xylitol is a five-carbon sugar alcohol
• Small amounts occur naturally in various fruits
  and berries
• It has also been called "birch sugar", as it can
  be produced from xylan derived from birch wood
  chips.
• It is equal in sweetness to sucrose, with 1 g
  yielding 4.06 kcal, and it is commonly used as a
  sweetener.

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Xylitol

• Xylitol is a normal intermediate of human
  metabolism, and several grams of it is produced
  daily by the liver .
• Exogenous xylitol is metabolised to glucose and
  glucogen or pyruvate and lactate mainly in the
  liver .
• Slow intestinal absorption of xylitol may result
  in osmotic diarrhoea when the daily dose exceeds
  20-40 g in adults,
• Excessive long-term consumption of xylitol by
  voluntary subjects at daily doses of 60-70 g for
  two years did not lead to any adverse events.

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Xylitol

• Many bacteria are unable to utilize xylitol as an
  energy source, and its presence is harmful to
  some bacteria despite the availability of an
  alternative energy source such as glucose

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Sugar metabolism of oral bacteria

• Bacteria in the oral cavity are transiently
  exposed to different sugars
• Live under constantly alternating "feast and
  famine" conditions.
• The rapid utilization of available sugars is
  possible through the phosphotransferase systems
  (PTS), which transport various sugars in
  bacterial cells and phosphorylate them
• The PTS consists of enzyme components, a number
  of which are specific to sugars.

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PTS system
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PTS

• In addition to sugar metabolism, the PTS is a
  complex protein kinase system regulating many
  metabolic processes and gene expression in many
  Gram-positive and Gram-negative bacteria
• PTSs function is significant for oral
  streptococci such as Streptococcus mutans (S.
  mutans),which are dependent on sugars as an
  energy source .

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Xylitol vs S. mutans
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Meat and Potatoes

• The mechanism of action of xylitol on bacteria
  has been studied in detail only in S. mutans
• xylitol-induces growth reduction is inhibited in
  the presence of fructose but not in the presence
  of other sugars
• This suggesting that the effect is mediated
  through a fructose-dependent system.
• Other research has shown that xylitol is
  transported into the S. mutans bacteria and
  phosphorylated through a constitutive fructose
  PTS .

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Meat and Potatoes

• S mutans are not able to utilize the xylitol
  phosphate (X5P)as an energy source so it is
  kicked out
• the expulsion of xylitol from the bacterial cell
  results in an energy-consuming futile xylitol
  cycle which, together with a harmful
  intracellular accumulation of xylitol phosphate,
  results in inhibition of the growth of S. mutans
• Iteresting point
• If the S. mutans lacks any constitutive fructose
  PTS activity, the bacteria become insensitive to
  xylitol .
• Since the PTS in bacteria regulates many
  metabolic processes and the expression of various
  genes it is likely that in addition to growth
  retardation, xylitol may also disturb the
  metabolic processes in the remaining viable
  bacteria.

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Whats going on inside cell

• Xylitol affects polysaccharide synthesis in S.
  mutans, resulting in decreased bacterial
  adherence.
• The ultrastructure of viable S. mutans bacteria
  is damaged after exposure to even small
  concentrations of xylitol
• Finally protein synthesis is also disturbed,
  which implies that xylitol acts as a strong
  metabolic inhibitor for this species.

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Another adherance problem

• Since bacteria adhere to host cells through
  carbohydrate-binding proteins extracellular
  xylitol may disturb the binding process by acting
  as a receptor analogue for the host cell, which
  could result in decreased adherence.
• Xylitol in a 6 concentration is capable of
  reducing the adherence of S. mutans.

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Lets look at that acid

• Inhibits rate of acid production from glucose and
  changes profile of acidic end products to
  formate-acetate domnance rather than lactic acid.
• Xylitol decreases the intracellular level of
  fructose 1,6 bisphosphate and increases the
  accumulation of intracellular xylitol 5-phosphate
  (X5P) toxic.

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Article 1

• Xylitol inhibition of anaerobic acid producton by
  streptococcus mutans at variouse pH levels
• Miyasawa H, Iwami Y
• Oral microbiology immunology 200318215-219

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Purpose and Intro

• Study the effect of PH on xylitol inhibition of
  acid production by S mutans and its biochemical
  mechanism
• It has been supposed that xylitol is incorporated
  into the bacterial cells as xylitol 5-phosphante
  (X5P) and that the X5P inhibits the enzyme
  activity of sugar metabolism, resulting in the
  inhibition of both bacterial growth and acid
  production

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Intro continued

• When dietary sugars are taken into the oral
  cavity, the sugars are degraded into acids by
  dental plaque and bacteria, and subsequently the
  plaque pH decreases rapidly from 7 to about 4.
• This study was done under anaerobic conditions
  since the interior of dental plaque is strictly
  anaerobic and the glycolysis of dental plaque
  bacteria under anaerobic conditions differs from
  that under aerobic conditions.

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Methods

• Different S mutan stains used and inoculated In
  brain heart infusion broth under strict anaerobic
  conditions and incubated
• Different type of anaerobic chambers with
  different pH
• S mutans placed in Glucose suspension and Glucose
  Xylitol suspension.
• Reaction was started and rate of acid production
  by cells monitored at pH 7.0 to 5.0 by an
  automatic pH titrator.

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Analysis of acidic end products

• Lactic, acetic, formic, and pyruvic acids were
  quantified with carboxilylic acid analyzer.

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Results

• Inhibitory effect of xylitol on acid production
  at different pH levels
• The acid production by S. mutans from glucose was
  decreased in the presence of xylitol at pH 7.0 to
  5.5.
• At 5.0 xylitol inhibition diminshed but was
  statistically significant.
• Addition of xylitol changed the profile of acidic
  end products.

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Results

• In absence of xylitol at pH 7.0 about 50 of
  total amount of acidic end products of S. mutans
  was lactic acid.
• With xylitol presant the proportion of lactic
  acid decreased while that of formic and acetic
  acids increased.
• The proportion of lactic acid increased as the
  reaction of pH was lowered

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Results of intermediates during glycolysis

• When S mutans metabolize glucose lots of fuctose
  1,6 bisphosphate (FBP) and little glucose
  6-phosphate (G6P) and fructose 6-phosphate
  detected.
• When xylitol present large increase in X5P
  detected in the cells.
• Both decrease in FBP and accumulation of X5P in
  presence of xylitol more evident at pH7.0 that
  5.0.

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Results

• Effect of xylitol on profile of glycolytic
  inermediates

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Results

• The FBP at pH 7.0 decreaseed by 78.6 and pH at
  5.0 level decreased by 54.4 the amount of X5P
  accumulated at pH 7.0 was about twice as high as
  that at pH 5.0.

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So What?
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• Lactic acid is the major end-product of
  glycolysis by Streptococcus mutans under
  conditions of sugar excess or low environmental
  pH.

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Mechanisms of action of xylitol

• Decreased acid production
• Reduces plaque by suppressing formation of
  adhesive macromolecules, especially glucans
• Selects for less adhesive mutans streptococci
• Toxic to cells

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Conclusion

• Xylitol possesses a unique property distinct from
  the other caries-preventive sweeteners.
• This sugar alcohol cannot be metabolized to acids
  but is taken up by Streptococcus mutans and
  accumulated as a toxic sugar-phosphate in the
  cells, resulting in growth inhibition.
• Decreases adhearance of plaque
• Decreases producton of lactic acid production

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Genetics

• Construction of a Strep. mutans isogenic mutant
  carrying the gbpClacZ fusion gene indicated
  that gbpC expression of cells repeatedly cultured
  in the presence of xylitol was elevated 20-fold.
• DNA transfer experiments indicated that this
  phenotypic change did not appear to be due to a
  mutation.
• These cells also exhibited decreased adhesion to
  glass surfaces when grown in the presence of
  sucrose. (Sato Y, Yamamoto Y, Kizaki H)

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Genetics

• Cells repeatedly cultured in the presence of
  xylitol evolved into those exhibiting an elevated
  dextran-dependent aggregation phenotype. This
  phenotype was found to result from expression of
  the gbpC gene, which was previously reported to
  be expressed only under certain stress
  conditions.

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