What Are Bacteria?

Contributor: Felicia Sabur. Lesson ID: 11645

Where do germ cells like to sit in the lunchroom? In the "back-teria." Many jokes are bad, but not all bacteria are bad for you! They are very tiny, and you are covered with them, so get to know them!


Life Science

learning style
personality style
Grade Level
High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

Audio: Image - Button Play
Image - Lession Started Image - Button Start

Bacteria may seem larger in comparison to viruses, but these tiny unicellular microbes are smaller than a red blood cell. They are everywhere and all around us! How many bacterial cells do you think you have on your body for every one non-bacterial cell that is on you?

relative sizes of microbes

Image by CNX OpenStax, via Wikimedia Commons, is licensed under the CC BY 4.0 license.

There are about ten bacteria cells for every one non-bacterial cell on your body.

Before you continue with this lesson, if you have not completed, or need to review, the previous four Related Lessons in this Viruses and Bacteria series, find them in the right-hand sidebar.

Bacteria are single-celled organisms that do not have a nucleus. The word "bacteria" is actually the plural form of bacterium. Many bacteria are actually good for you, like the good bacteria in your gut and cheese. Bacteria are classified in two different kingdoms.

The archaebacteria are the first kingdom of bacteria and they like extreme conditions. They can usually be found in harsh environments where little else can live. These bacteria are divided into four groups:

  • Methanogens are bacteria that live in oxygen-free environments and produce methane gas as a waste product of their "digestion," or process of making energy.
  • Halophiles are bacteria that live in super-salty environments like the Dead Sea.
  • Thermophiles are bacteria that live at extremely hot temperatures in sulfur springs.
  • Psychrophiles are bacteria that live at unusually cold temperatures.

For a more in-depth look at this kingdom and to see some examples, read Archaeabacteria Definition, by Biology Dictionary. When you are done, answer the questions in your Viruses and Bacteria Unit Workbook on Page Eight.

The Eubacteria are the second kingdom of bacteria, and they live in mild environments. Many are beneficial to the environment around us. Eubacteria are classified by their shape and energy use. These are some examples of how eubacteria are classified by energy use:

  • Saprophytes are bacteria that get their energy from dead organic matter like decaying leaves and plants. This type of bacteria is a heterotroph, which means it can't make its own food.
  • Photoautotrophs are bacteria that are capable of making their own food from inorganic substances using light as an energy source. An inorganic substance is something that is not made of living matter.
  • Chemosynthetic are bacteria that use inorganic materials, such as sulfur and nitrogen, as a source of energy and convert them into organic substances. Some of these types of bacteria play an important role in converting nitrogen in the air into forms that plants can readily use.

Image - Video

For an more in-depth look at eubacteria, watch this video. Fill out Lesson Five, What Are Bacteria?, in the Viruses and Bacteria Unit Workbook found in Downloadable Resources in the right-hand sidebar, and answer the questions that correspond with the video. Feel free to pause and rewind the video as necessary.

Classifications of eubacteria by BiologyMonk:

Image - Video


Discuss what you viewed in the video with your parent or teacher.

  • Which class of bacteria do you think is more common?

Archaebacteria and Eubacteria are two kingdoms of life in the classification system. Both classes of bacteria are unique. Eubacteria are more common than archaebacteria because they like milder environments.

Continue on to the Got It? section to continue filling out your worksheet.

Image - Button Next