What are bacteria?

Bacteria are very small living organisms that can only be seen using a microscope. They are single-celled, but they form groups of multicellular colonies. Because they are living organisms, they need nutrients, water, and energy to survive. Despite their small size, bacteria are very important to life on Earth, and there are about 5 x 1030 bacteria on the planet—that’s 5,000,000,000,000,000,000,000,000,000,000 bacteria!

Bacteria can be good or bad. Good bacteria are almost everywhere, and they are an important part of big processes like the water cycle as well as smaller processes in our own bodies, like digesting food. The bacteria in your drinking water are most likely all good bacteria! Bad bacteria are less common, but they can cause people, other animals, and plants to become sick. Different types of bacteria have different characteristics. These can include things like shape and color, and they can help us to determine the type of bacteria we’re looking at.

Key Termslogo3

  • Organism: a living thing made up of cells
    • Examples: animals, plants, bacteria
  • Cell: the most basic unit of life
  • Single-celled: a microscopic organism made up of only one cell
    • Examples: bacteria
  • Multicellular: a life form made up of more than one cell


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