Habitat on Humanity

You have more bacterial cells in or on your body than you have human cells. These microbes are part of normal humans and are important to the health of humans. Usually when people hear about microbes, they hear about the microbes that make them sick. However, if you had no microbes inside of you, you would likely not feel very well since microbes provide people with important vitamins. Microbes are found in your mouth, in your guts, on your skin, under your arms, and on your feet.

When you eat sugar, you are not only feeding yourself, but you are also feeding the millions of microbes that call your mouth home. These microbes grow and stick to your teeth, forming plaque which can cause cavities and tooth decay.

Streptococcus
The most predominant microbes in your mouth are these sphere-shaped bacteria. They convert sugars and other carbohydrates into lactic acid. This acid then dissolves tooth enamel, eventually forming a cavity.

Streptococcus mutans
This bacterium is the major culprit in giving your teeth cavities. If you have ever had a dental cavity, you are not alone: this is probably the leading cause of infection. Cavities are not only a bummer to have, they are expensive to treat, costing about $20 billion per year in fillings in the United States!

Cavities

If you don't remove these bacteria, they can produce acids which dissolve the enamel on your teeth and produce cavities. Normally, the pH of your mouth is neutral, around pH 7. However, after you eat some sugar, bacteria in the plaque on your teeth produce acid, dropping the pH to about 5.5. People who do not eat sweets have about 90% fewer cavities.

Gum Disease

When you don't brush the plaque off your teeth, the plaque hardens. This hardened plaque is called "calculus", which only a dentist can remove. If left untreated the plaque and calculus growing near the gum line irritate the gums. This irritation causes a redness and soreness (inflammation). This inflammation is called gingivitis. The gum separates from the teeth and bacteria grow inside this space where they attack the bones. Gingivitis is the leading cause of tooth loss in older people.

Try This!

Eat a piece of candy. After about 5 minutes, notice what happens in your mouth. Do you notice a coating developing on your teeth? Some people say their mouth feels mossy. This sticky film is slime (dextran, a polysaccharide) that is produced by the bacterium Streptococcus mutans when sugar (sucrose) is present. This slime helps the bacteria stick to your teeth and to each other. Eventually, this slime on your teeth sticks to other bacteria and a plaque begins to form. Perhaps after you have performed this experiment, you would like to brush these bacteria off your teeth so they can't form a cavity.

Try This!

To see what happens to your teeth when they are exposed to sugar and acid, find a tooth and drop it into a glass of cola (read the label to make sure the cola has phosphoric acid.) (For the tooth, ask your tooth fairy, or a parent if you can't find your tooth fairy.) After one and two weeks, observe the tooth. How does it look?

Try This!

Now that you know how bacteria contribute to cavities, you can try to eliminate the problems they cause. Brushing and flossing prevent about 50% of cavities; eliminating sweets may reduce cavities by 90% and using fluorinated water reduces cavities by 60%. Regular visits to the dentist can also help you eliminate plaque and calculus.

The stomach has very few microbes in it because of its high acidity (low pH). (The stomach produces acid to digest food.) The lining of the stomach provides some refuge against the high acidity for some hardy bacteria.

One area of the stomach called the "antrum" produces no acids. In this part of the stomach, some microbes can live and may be responsible for giving people ulcers. To find out more about these ulcer-causing bacteria see: "Bacteria blamed for ulcers, cancer" in Microbes in the News.

Helicobacter pylori
This bacterium lives in a portion of the stomach called the "antrum."

Human guts are hosts to many billions of microbes. Each gram (about a thimble-full) from the large intestine contains up to ten trillion (10,000,000,000,000) microbes! Microbes provide nutrients to animals.

Microbes in the intestines perform many essential tasks. For example, microbes make several vitamins in the intestines. One of these is vitamin K, a vitamin that is otherwise lacking from human diets.

Matter passes through intestines in approximately 24 hours.

Some of the different microbes in the intestine are:

Bacteroides
This is one of the most abundant microbes in the intestine, and is found in higher numbers in people who eat meat than in vegetarians.

Lactobacillus acidophillus
This is a normal, helpful inhabitant of intestines. The billions of Lactobacilli in your intestines crowd out harmful microbes so they cannot grow. This is why many travelers take capsules filled with Lactobacillus, so they don't get traveler's diarrhea.

E. coli
This is the most well-studied organism. It is a normal resident of your intestine and provides you with vitamin K and some of the B vitamins.

Klebsiella
It is speculated that these bacteria can fix nitrogen into protein in the intestines of people who have too little protein in their diet.

Bifidobacterium
The intestines of newborn, milk-fed infants contain almost a pure culture of this bacterium.

Methanobacterium smithii
This bacterium produces methane in human guts. Normal humans fart about 10 - 15 times per day, passing a few hundred milliliters of gas per day. Boys and girls fart about the same amount. Though over 50 percent of this gas is nitrogen, some of the gas is microbially produced, including the gas methane, which is flammable and is also a powerful greenhouse gas. Carbohydrates that cannot be digested by humans, but can be digested by microbes, are the leading cause of farts. Methane is an odorless gas: smells come from dimethyl sulfide and methanethiol. Other gases include oxygen, hydrogen, carbon dioxide, and nitrogen.

Most skin is a dry, inhospitable place where few microbes like to live. The population of microbes on the back is only about 100 bacteria per square centimeter. However, on hot humid days sweaty skin becomes a much nicer place for microbes to live.

Propionibacterium acne
This bacterium is a normal inhabitant of skin. It produces propionic acid which prevents the growth of other, unwanted microbes.

Staphylococcus epidermidis
This sphere-shaped bacterium is resistant to the dry conditions often found on skin.