1. You have more bacterial cells in or on your body than you have human cells.
These microorganisms are the “normal humans flora” and are important to the health of humans.

2. Teeth Millions of bacteria inhabit your mouth. Streptococcus
Cocci means sphere-shaped bacteria. Streptococci produce lactic acid which attacks tooth enamel, eventually forming a cavity.
Streptococcus mutans
One of the most common bacterial species in your mouth, S. mutans is probably the leading cause of infection.

3. Stomach The stomach is very acidic and thus, has very few microbes.
However, one area of the stomach called the "antrum" produces no acids. H. pylori, the pathogen that causes ulcers, lives in this part of the stomach.
Because we now know that ulcers are a bacterial infection, they can often be treated and cured by antibiotics.
Helicobacter pylori
This bacterium lives in a portion of the stomach called the "antrum."

4. Large Intestine
You depend on the bacteria in your intestines to perform many essential tasks. For example, the bacteria in your gut make vitamin K and biotin. Both are vitamins that are otherwise lacking in human diets.

5. Some of the different microbes in the intestine are:
E. coli
Even though some strains of E. coli produce harmful toxins, this bacterium is a normal and necessary resident of the human intestine. E. coli provides 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.

6. 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.

7. Bacteroides
One of the most abundant microorganisms in the intestine, this species is found in higher numbers in people who eat meat than in vegetarians.
Lactobacillus acidophillus
Lactobacilli help protect your intestines by preventing the growth of other harmful microorganisms.

8. Skin 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.