• Living organisms can be divided into four main groups known as kingdoms: bacteria, fungi, plants and animals. 
• Bacteria are microscopically small unicellular organisms. They can be found almost everywhere: in the water, on the ground, in our skin, and even in our digestive systems.
• Some bacteria are parasites, whereas others are decomposers. Some bacteria are capable of photosynthesis, which means that they are autotrophic organisms.
• Some bacteria are harmful or even dangerous, as they can cause illnesses and diseases (the picture on the right shows E. coli bacteria, which cause diarrhea in humans). 
• Bacteria reproduce quickly and in a straightforward manner: a bacterium cell can divide every 10 minutes! 
•  All bacteria are not harmful. For example, the human digestive system requires certain beneficial bacteria to function properly.
• Bacteria cells do not have nuclei, which means that they are prokaryotic organisms. They are the oldest group of living organisms in the world.

• no nuclei (=prokaryotic)
• unicellular
• microscopically small
• the first living organisms on planet Earth
• reproduce asexually by dividing
• can survive harsh conditions
• found almost everywhere on Earth
• both autotrophic and heterotrophic organisms