What is Bacteria? - Definition & Types

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  • 0:00 What is Bacteria?
  • 2:05 Classification
  • 3:20 How We Group Bacteria
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

Germs...bugs...prokaryotes...monerans. Whatever you call them, bacteria outnumber every other kind of life on this planet. In this lesson, we'll take a look at these tiny creatures to determine exactly what they are, as well as the methods we use to try and classify them.

What is Bacteria?

A bacterium is a single-celled (unicellular) microorganism that does not have a nucleus or any other membrane-bound organelles. Bacteria are sometimes called 'prokaryotes.' In Greek, 'prokaryote' literally means 'before the nut' (where 'the nut' is the nucleus.)

Bacteria adapt to become well-suited to their environments, and therefore come in many shapes and forms. However, they all have a few parts in common.

Anatomy of a typical bacterium.
General image for bacterium anatomy

1. Capsule: A protective, often slimy, coating, often of sugars, that helps to protect the bacterium. It also makes bacteria virulent. This means the bacteria is more likely to cause disease, since it aids the cell in survival against attack. For example, the bacteria may survive an attack from the human body's immune system.

2. Cell wall: In bacteria, the cell wall is usually made of peptidoglycan, a protein and sugar compound. This structure gives the cell some rigidity and protection.

3. Cell membrane: As in most cells, the bacterium's plasma membrane acts by coordinating the passage of molecules into and out of the cell.

4. Cytoplasm: Again, as in many cells, the cytoplasm serves as a medium through which molecules are transported, as well as a system to maintain conditions (like temperature and pH) that are best for the cell.

5. Ribosomes: The main site for the bacterium's protein synthesis.

6. Nucleosome: A basic unit of chromatin, which won't be covered in this lesson.

7. Nucleoid: This is the region where the bacterium's DNA is located. Again, it's not the same as a nucleus because it's not surrounded by a membrane.

8. Flagellum: In many bacteria, a flagellum is present, and is the means by which the cell moves around.


Because bacteria are so diverse in both form and habitat, biologists have struggled with their classification, (also called their 'taxonomy.') For many years, bacteria were called 'monerans,' and placed in a kingdom of the same name. Although you still might hear bacteria referred to as monerans, it's not a term currently accepted among biologists.

Life is now classified into three domains; a domain is a taxonomic level that is higher than a kingdom, and based on an organism's DNA. Bacteria is one of these domains, while the others are eukarya, composed of organisms that have nucleated cells, and archaea, composed of unicellular prokaryotes, many of which have evolved to exploit extreme environments. Organisms in the archaea domain were originally considered a type of bacteria - known as archaebacteria - but scientists have since reclassified them. In this image, groups of archaea are indicated by green lines, and bacteria are in blue. Red lines indicate eukaryotes.

The three domains of life. Groups of archaebacteria are indicated by green lines, and eubacteria in blue. Red lines indicate eukaryotes.
Simplified tree of life - Domains, based on DNA analysis.

How we group bacteria

Scientists try to make sense of the sheer number and diversity of bacteria by grouping them in various ways. Here are just a few ways that are accepted in the scientific and medical communities:

  • Shape: Bacteria are usually one of three shapes. Cocci are round; bacilli are rod shaped; spirilla (also called spirochetes) are spiral.

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