Unformatted text preview: One example of an oral bacterium is Streptococcus mutans. This organism has developed
a variety of ways to stick to teeth. One of these involves the production of an adhesive
dextran (polyglucose) capsule produced when sucrose is used as a nutrient. The sucrose is
degraded by the bacterium to glucose which is used to make the dextran. The dextran is
important because it coats teeth and is the progenitor of plaque. It traps the acid
fermentation products produced by Streptococcus mutans and other bacteria which
dissolves tooth enamel and causes cavities. Since sucrose is essential for dextran
production, a reduction of sucrose in the diet leads to a reduction in cavities. However,
there are many other bacteria in the mouth that are involved in tooth decay.
Saliva contains two antibacterial enzymes: lysozyme and lactoperoxidase. The latter
catalyzes a reaction between chloride ion and hydrogen peroxide which generates a toxic
oxygen species called singlet oxygen. Singlet oxygen is very reactive and can damage
bacteria. Saliva itself is not a rich growth medium for heterotrophs. Although it has a pH
of about 7, it contains only 0.5% dissolved solids half of them inorganic (chloride,
phosphate, bicarbonate, Ca, Mg). The half that is organic is made-up of proteins and
other enzymes for digesting food. There are normally only small amounts of amino acids,
sugars, vitamins etc. unless a meal has just been eaten. Because of the low concentrations
of nutrients and the constant flushing action (see above) the mouth supports “only” about
a few billion bacteria in total.
Pure culture and SSU rRNA studies have shown that the stomach harbours a diverse
collection of bacteria. This may seem surprising considering the bulk gastric contents have
a pH between 1-2 and indeed there are only about 10 viable organisms per mL of gastric
fluid. However, in the stomach mucosa the pH is around 5 so bacteria can grow there
attached to the stomach wall. Because of their tolerance of low pH, fermentative
heterotrophs are common in the stomach.
Beyond the bacteria that normally live in the stomach, all sorts of microorganisms
enter the stomach in food. Whether they are killed by the low pH depends on how long
they spend in the stomach before being passed into the small intestine (see below),
how the food affects the pH of the gastric contents and other factors (see Vibrio cholerae
and stomach acidity below).
An important stomach bacterium is the Helicobacter pylori a member of the Purple FOOD-304 group. Although this organism has a remarkable ability to resist acid pH (it survives at
pH 1 using the enzyme urease to generate ammonia), it will not grow at that pH. It can,
however, grow in the mucous lining of the stomach, where the pH is higher. The U.S.
Centers for Disease Control and Prevention (CDC) estimates that H. pylori colonizes the
stomachs of half the world's population. Most of the time, it does not cause any apparent
problem, but sometimes its presence has been linked to gastric ulcers and even can...
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- Fall '12