Bacteria

 It is perhaps a surprise to you all, but the human body contains 10 times more bacteria than it contains human cells.

Although some bacteria do cause disease, many kinds of bacteria in the human body  prevent disease. 

Bacteria, single-celled organisms, lack a true nucleus, a feature that distinguishes them from plant and animal cells.  

In plants and animals the nucleus carries genetic material in the form of DNA. Bacteria also have DNA but it floats within the cell, usually in a loop or coil. A tough but resilient protective shell surrounds the bacterial cell.

Bacteria have remarkable abilities to adapt to extreme environments and thrive in parts of the Earth that are inhospitable to other forms of life.They existed long before any other form of life, when  this planet was a very hostile place, due to temperature, lack of oxygen and radiation.

It is estimated that 500 to 1000 species of bacteria live in the human gut and a roughly similar number on the skin, where they form a protective barrier on the outside. Many of the bacteria in the digestive tract are able to break down certain nutrients such as carbohydrates that humans otherwise could not digest. They also synthesize vitamins such as folic acid, vitamin K and biotin.

The majority of these commensal bacteria are anaerobes, they survive in an environment with no oxygen. Bacteria of the normal flora can act as opportunistic pathogens at times of lowered immunity.