Introduction: There are more bacteria on Earth than anything else. There are more these organisms than there are trees, leaves on trees, or even blades of grass. In fact, it is estimated that there are 20 bacteria for each cell in the human body. There are between 10 - 100 trillion cells, bringing the total bacteria to 2000 trillion! 

When people think of bacteria, they often think of their negative actions, such as causing diseases. However, most bacteria are harmless and many are even beneficial. Many products used by people are derived from bacteria. The tanginess in yogurt, the holes in Swiss cheese, and many different medicines all come from bacteria. 

Bacteria are unicellular organisms that are in the domain Bacteria. They are prokaryotic, which means their cells do not have a membrane-bound nucleus. They are also lacking many of the membrane-bound organelles found in eukaryotic cells. Bacteria usually come in one of three distinct shapes, which helps scientists to identify them. bacilli are rod-shaped, cocci are spherical, and spirilli are twisted spirals.

This diagram shows a generalized image of a bacillus bacterium.

Directions for this Lesson: Answer the practice questions and then watch the video to learn more about the importances of bacteria.