Bacteria are the most abundant living things on Earth. The total number of bacteria in the world is an astounding     million trillion trillion! Bacteria live almost everywhere, including the air, ocean, soil, and intestines of animals.     They are even found in rocks deep below Earth's surface. Any surface that has not been sterilized is likely to be covered with bacteria.     Bacteria are also the most diverse organisms on Earth. Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species are classified on the basis of various traits.... Show more

Bacteria are the most abundant living things on Earth. The total number of bacteria in the world is an astounding    
million trillion trillion! Bacteria live almost everywhere, including the air, ocean, soil, and intestines of animals.    
They are even found in rocks deep below Earth's surface. Any surface that has not been sterilized is likely to be covered with bacteria.    
Bacteria are also the most diverse organisms on Earth.

Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species are classified on the basis of various traits. For example, they may be classified by the shape of their cells or how they react to gram stain.    
- Bacteria come in several different shapes: bacillus (rod shaped), coccus (sphere shaped), and spirillus (spiral shaped). The different shapes can be seen by examining bacteria under a light microscope.    
- Different types of bacteria stain a different color when gram stain is applied to them. Bacteria that stain purple are called gram-positive bacteria. They have a thick cell wall without an outer membrane. Bacteria that stain red are called gram-negative bacteria. They have a thin cell wall with an outer membrane. These differences in structure explain why they react differently to gram stain.

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