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Introduction to Bacteria

Assortment of the bacteria Escherichia coli

Assortment of the bacteria Escherichia coli (Mattosaurus [Public domain], Wikimedia Commons)

Assortment of the bacteria Escherichia coli

Assortment of the bacteria Escherichia coli (Mattosaurus [Public domain], Wikimedia Commons)

Let's Talk Science

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What exactly are bacteria? What types of bacteria are there? And are they really as bad as people think they are?

Bacteria have been around for a very long time. In fact, they are the oldest known forms of life on Earth! The earliest fossils are of prokaryotes. That’s a group of organisms that bacteria are part of. These fossils date back to over 3.5 billion years ago

Bacteria have evolved into a wide variety of different types since then. They have also adapted to a range of different environments. They can live inside the human body, at the North Pole, and even at the bottom of the ocean!

Did you know?

There are more bacteria living in your mouth than there are people who have ever lived on Earth!

Size of Bacteria

Bacteria are single-celled organisms. This means that each bacterium is made up of only one cell. This is very different from humans, whose bodies are made up of trillions of cells

Bacterial cells are much smaller than human cells. Bacterial cells can measure from about 1 to 10 μm long. Most of them are only about 1 to 2 μm in diameter. 1 μm, or micrometre, is 1 000 times smaller than a millimetre. That is very tiny! It’s much smaller than the human red blood cell, which is (on average) about 7 μm in diameter.

Relative sizes of cells and their components
Diagram showing the relative sizes of some very small things including bacteria, which are typically around 1 to 2 μm in diameter (Source: Michigan Nanotechnology Institute for Medicine and Biological Sciences).

Even though they are small, bacteria have many different parts to their cells.

Bacterial Structure

The common structures that most bacteria have include: 

Structure of a typical bacterium
Structure of a typical bacterium. The numbered parts are: (1) pilus, (2) plasmid, (3) ribosome, (4) cytoplasm, (5) cytoplasmic membrane, (6) cell wall, (7) capsule, (8) nucleoid, and (9) flagellum (Source: LadyofHats [Public domain] via Wikimedia Commons).
  1. Pili - hair-like structures that help bacteria attach to surfaces and other bacteria 
  2. Plasmids - genetic material (DNA)
  3. Ribosomes - structures that make proteins
  4. Cytoplasm - a gel-like material in which the ribosomes and genetic material are suspended 
  5. Cytoplasmic Membrane - a thin layer of phospholipids and proteins that controls the movement of nutrients in and out of the cell
  6. Cell Wall - a rigid wall that gives the cell its structure and protects the plasma membrane
  7. Capsule - a third layer that helps prevent the bacteria from drying out or being engulfed by larger microorganisms (only present in some types of bacteria)
  8. Nucleoid - a mass of genetic material (DNA)
  9. Flagellum - structure that helps the bacteria move around and sense their environment 

One of the most important structures of a bacterial cell is the cell wall. Bacterial cell walls have quite a few functions. They protect bacteria from bursting. They also help bacteria maintain their shape. Finally, they control molecules going in and out of the cell. Scientists can use the thickness of the cell wall to identify and categorize different types of bacteria.

Classification of Bacteria

There are millions of different types of bacteria in the world. That’s why it’s important to have a way to classify them. Scientists usually classify bacteria based on two characteristics: 

  • The thickness of the bacteria’s cell wall 
  • The shape of the bacteria

Scientists use a technique called Gram staining to determine the thickness of bacterial cell walls. Scientists stain bacteria with a dye called crystal violet. A thick cell wall retains the violet colour. A thin cell wall does not. 

  • Gram Positive bacteria have thick cell walls. When dyed, they appear blue or purple.
  • Gram Negative bacteria have thin cell walls. When dyed, they appear pink or red.

Bacteria Shapes

Bacteria can be assigned to three major groups based on shape. These include bacteria that are spherical (cocci), rod-shaped (bacilli) and spirals and others. 

Bacteria can be spherical, rod-shaped, spiral-shaped, and other shapes
Bacteria can be grouped according to their shape. Bacteria are typically either spherical, rod-shaped or have a spiral shape (© 2019 Let’s Talk Science based on an image by ttsz via iStockphoto).

Is Bacteria Always Bad?

We often think of bacteria as being ‘bad’. That’s because many bacteria are pathogenic, which means that they can make us sick. There are lots of diseases and conditions caused by bacteria. Examples of diseases or conditions that are caused by pathogenic bacteria include: 

  • tetanus
  • typhoid fever
  • tuberculosis
  • strep throat
  • anthrax
  • various forms of food poisoning
Electron microscope images of Streptococcus pneumoniae, Salmonella typhimurium and Treponema pallidum
Electron microscope images of: Streptococcus pneumoniae which causes pneumonia (left), Salmonella typhimurium which causes food poisoning (centre) and Treponema pallidum, which causes syphilis (right). (Sources: CDC/Dr. Richard Facklam [Public domain] via Wikimedia Commons, Photo: Volker Brinkmann, Max Planck Institute for Infection Biology, Berlin, Germany [CC BY 2.5] via Wikimedia Commons, and CDC/Dr. David Cox [Public domain] via Wikimedia Commons).

Did you know?

Some types of bacteria can cause Necrotizing fasciitis, more commonly known as flesh-eating disease.

We often hear about pathogenic bacteria. But only a very small fraction of the bacteria in the world cause us harm. In fact, many bacteria are very helpful! For example, probiotic bacteria in our digestive system help protect us from other bacteria. 

Bacteria is all around you - even in your kitchen! Lots of food and beverages are made from bacteria. Here are some examples:

Yogurt, sourdough bread, chocolate, kimchi, sauerkraut and many types of cheese
Assortment of food made using bacteria. Clockwise from top left: Yogurt, sourdough bread, chocolate, kimchi, sauerkraut and many types of cheese (Sources are all from Wikimedia Commons: [CC BY-SA 2.0], Vanleuven [CC BY-SA 4.0], Roozitaa [CC BY-SA 3.0], Nagyman, a flickr user [CC BY-SA 2.0], Qwerty Binary [CC BY-SA 3.0] and Thegoodlifefrance [CC BY-SA 4.0]). 

Bacteria are a diverse and interesting group of living things. People may think of bacteria as things that make us sick, but they do a lot of good for us as well!


Arizona State University. (2014, July 3). Microbes: The good, the bad, the ugly.

Bailey, R. (2019, August 20). Bacteria shapes. ThoughtCo.

Bruckner, M. Z. (n.d.). Gram staining. Carlton College.

Davidson, M. W. (2015, November 13). Bacteria cell structure. Florida State University.

Microbiology Online. (n.d.). Bacteria. Microbiology Society.

Microscope Master. (n.d.). Unicellular organisms.

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