Bacteria: Classification and Structure

How do bacteria live, reproduce and interact with the rest of our world?  


The study of microbiology is the study of microscopic, unicellular organisms which includes both prokaryotes and viruses (as well as protista and fungi). This unit will focus primarily on bacteria and virus microbes, things that are too small to be seen with the naked eye. Many microbes may also be called “germs” or pathogens that cause disease. We will focus on the following areas for both viruses and bacteria:

  1. structure and function
  2. metabolism
  3. reproductive processes
  4. biological importance

We will also discuss a couple of related “particles” similar to viruses that impact living things: viroids and prions.

Classification: Archaebacteria vs. Eubacteria

Recall from the previous module that all bacteria have been divided into 2 different Domains:

  1. Domain Bacteria, Kingdom Eubacteria
  2. Domain Archaea, Kingdom Archaebacteria


Scientists believe Archaea are more closely related to the eukaryotes than the bacteria are, based on gene sequences. Both of the bacterial kingdoms are composed of prokaryotes. Prokaryotes lack an enclosed nucleus and membrane-bound organelles. All prokaryotes have cell walls. Let’s compare the two Kingdoms:


  • Members of the eubacteria are small and ubiquitous (found in almost any environment) except for extreme cold. Their cell walls contain peptidoglycan, a polymer of sugars and amino acids, which protects the cell from injury and determines its shape.
  • Examples of eubacteria and/or some diseases that they cause include: E. coliStreptococcus, Lyme disease, Staphylococcus, Tetanus, Typhoid fever, Leprosy, Tuberculosis, Salmonella, and tooth decay.
  • Currently, estimates of the total number of species of bacteria range from about 10 million to a billion, but these estimates are tentative, and may be off by many orders of magnitude.


  • Members of the Archaebacteria have cell walls without peptidoglycan.
  • They live in extremely harsh conditions such as hot springs (thermophiles), methane containing swamps (methanogens), as well as salty (halophiles) or acidic environments (acidophiles).
  • There are about 300,000 known species of archaebacteria

Bacterial Structure and Function

Most bacteria have similar basic cellular structures. Below is a picture of a typical bacterium. Bacteria may or may not have all of the structures.


Although it seems that unicellular bacteria could not possibly be diverse, they most certainly are! Bacteria vary in a number of characteristics:

  1. shape
  2. cell wall construction
  3. movement
  4. how they obtain/use energy