Education Services / L’équipe des services d’éducation
11 March 2014

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Did you know that the number of bacteria living in your mouth is larger than the number of people who have ever lived on earth? Bacteria have been around for a long, long time; in fact, the earliest fossils of prokaryotes (of which bacteria are a major sub-section) are the oldest known forms of life on Earth, dating back over 3.5 billion years. Over this extensive period of time bacteria have evolved into a wide variety of different types and have adapted to a variety of different environments - including the inside of your mouth and your intestines!

Bacterial Structure

Bacteria are single-celled organisms. This means that each organism is made up of only one cell. This is very different from humans and other animals that are made up of trillions of cells. Bacterial cells are much smaller than human cells. They usually measure 3 000 nm (nm is the short form for nanometre which is 1/1 000 000th of a millimetre) in diameter; a human blood cell is 10 000 nm. Even though they are small, bacteria have many different parts to their cells.

Figure 1: Structure of a typical bacterium
Figure 1: Structure of a typical bacterium. Image source: Wikimedia Commons.

Figure 1 shows the structure of a typical bacterium. Bacteria have structures to help them attach to other bacteria and host surfaces (pili) (1); genetic material (DNA) in the form of the nucleoid (8) and plasmids (2); structures to make proteins (ribosomes) (3); a gel-like cytoplasm in which the ribosomes and genetic material are located (4); a set of membranes (plasma membrane (5), cell wall (6) and capsule (7)) that helps control the movement of nutrients and wastes into and out of the cell; and structures to help the bacteria move and sense their environment (flagellum) (9).

One of the most important structures of a bacterial cell is the cell wall. Bacterial cell walls protect bacteria from bursting and help to give bacteria their shape. The cell wall also helps to control entry of molecules into and out of the cell. Most bacteria have either a thin cell wall or a thick cell wall. The thickness of cell walls helps scientists to identify different types of bacteria and put them into categories.

Classification of Bacteria

There are millions of different types of bacteria in the world, living in many different environments. Because there are so many different types, it is important to have a system to identify bacteria. Bacteria are usually classified based on two characteristics: their shape (see Table 1) and the thickness of their cell walls, which scientists can figure out using a technique called Gram staining. With Gram staining, scientists use different dyes to tell if bacteria have thick cell walls or thin cell walls. Bacteria with thick cell walls appear blue or purple when they are dyed. These are known as Gram Positive bacteria. On the other hand, bacteria with thin cell walls appear pink or red when they are dyed. These are known as Gram Negative bacteria.

Table 1: Types of Bacteria

Bacteria Shape






Staphylococcus epidermidis
This type of bacteria lives in human and other animal skin and does not commonly cause disease.

Figure 2: Staphylococcus epidermis. Image source: Wikimedia Commons.




Escherichia coli (E. coli)
This type of bacteria lives in the lower intestines of birds and mammals. Some strains of E. coli can cause serious diseases and even death.

Figure 3: Escherichia coli. Image source: Wikimedia Commons.


Helix-shaped (shaped like a spiral)

Campylobacter jejuni
This type of bacteria is commonly associated with poultry (e.g., chickens and turkeys) and lives in the digestive tracts of many types of birds. It is one of the most common causes of food poisoning.       

Figure 4: Campylobacter jejuni. Image source: Wikimedia Commons.



Vibrio vulnificus
This type of bacteria lives in marine environments such as estuaries and coastal areas. Vibrio vulnificus can cause infection after eating seafood, especially raw or undercooked oysters.

Figure 5: Vibrio vulnificus. Image source: Wikimedia Commons.

‘Good’ Bacteria and ‘Bad’ Bacteria

Bacillus anthracis
Figure 6: Bacillus anthracis. Image source:
Wikimedia Commons.
We often think of bacteria as being ‘bad’ because many bacteria are pathogenic, which means that they cause disease. Examples of diseases or conditions that are caused by pathogenic bacteria include tetanus, typhoid fever, tuberculosis, strep throat, anthrax (see Figure 6) and food poisoning. A number of bacteria can even cause Necrotizing fasciitis, which is more commonly known as flesh-eating disease.

Even though we often hear about pathogenic bacteria, only a very small fraction of the bacteria in the world cause us harm. Actually, many bacteria are very helpful; for example, we have bacteria that live in our digestive system which help to prevent harmful bacteria from growing there and making us sick. These bacteria are called pro­biotic because they promote good health. Bacteria are also used to make certain foods such as vinegar (with acetic acid bacteria), sauerkraut (with lactic acid bacteria) and yogurt (see Figure 7). Your favourite sourdough bread (see figure 8) from the bakery is also made using bacteria!

Figure 7: Frozen yogurt, commonly made with Lactobacillus bulgaricus and Streptococcus thermophilus cultures. Image source: Wikimedia Commons.
Figure 8: Sourdough breads are made with naturally occurring lactobacilli and yeasts. Image source: Wikimedia Commons.

Thank you to the Let’s Talk Science Challenge volunteer writers who provided content in this backgrounder.

Education Services / L’équipe des services d’éducation

This content is provided through Let's Talk Science's Education Services team.

Ce contenu est fourni par l'équipe des services d'éducation de Parlons sciences.

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