Bacteria: Everything You Need To Know!

Bacteria are microscopic organisms that can be found virtually everywhere! Within nature you can find different types of bacteria that vary in size, shape and arrangement!

In this article, we are going to explain the different types of bacteria, including the different shapes and sizes that we know of.

Bacteria: Everything You Need To Know!

So, if you are looking to learn more about bacteria, then you are in the right place!

Definition Of Bacteria

Bacteria are unicellular microorganisms which have cell walls yet lack organelles and an organized nucleus. They are classified under the domain Prokaryota and like eukaryotes, lack membrane-bound organelles.

The majority of bacteria that we know exist are microscopic, however, there have been studies that show large bacteria – some that can be seen with the naked eye alone.

There are many different shapes and sizes of bacteria, each of which displays a large diversity of cell morphology. 

Size Of Bacteria

The average size of most bacteria is between 0.2 and 2.0 um. Because bacteria are so small, they can thrive in microenvironments such as intertidal marine sediments and more, because other microorganisms are absent.

This means that bacteria can exploit the environment and thrive.

Bacteria rely on hosts to survive. Their nutrition comes from said hosts, and so the small size of the organisms means they are beneficial for parasitism and oligotrophy.

Some bacteria survive in environments with low levels of nutrients – these are known as oligotrophic bacteria.

The high surface area to volume ratio that bacteria have means that they can take up as many nutrients as they need to survive. During this process, they can continue to grow and even reproduce.

The primary unit of measurement in bacteriology is the micron (micrometer). 1 micrometer (um) is one-thousandth of a millimeter – so pretty tiny!

The human eye has a resolution limit between 100 and 200 um (around the size of a human hair), so the majority of bacteria species cannot be seen with the naked, human eye.


Many microbiology books indicate that the average size of bacteria is between o.2 and 2.0 micrometer in diameter. This is about the diameter of the bacteria and not the length of the organisms.

Bacteria also vary in length to one another, as well as in diameter.


The smallest bacteria are known as nanobacterium and there has been debate surrounding classification due to how small they are.

They are shown to range between 0.2 and 0.5 um and there have been some studies that show some bacteria as small as 0.1um.

These bacteria are characterized by their slow growth as well as being gram-negative.

Some examples of this bacteria include mycoplasmas which are about .25um, Haemophilus inflenzae which are between 0.2 and 0.3um, and Mycoplasma gallicepticum which range between 0.2 and 0.3 nanometers (in diameter). 

Some of these small bacteria, such as Mycoplasma, can exist independently, even though they do not have a cell wall. They are often found in hosts such as humans, plants and insects, etc. 


Some of the largest bacteria ever discovered are the Thiomargarita namibiensis which may grow as large as 0.75mm in diameter, meaning they can be seen with the naked eye.

Other large bacteria include Schaudinnum butschlii which measure between 4 and 5um in diameter.

Shape And Arrangement Of Bacteria

Shape And Arrangement Of Bacteria

Bacteria are single-celled organisms that are small in size and lack a cell wall. This means that morphologically speaking, they lack membrane-bound organelles.

In addition, these organisms have a capsule that protects the inner contents of the cell. This is where the nucleoid, ribosome, plasmid, and cytoplasm would be found. 

The above characteristics can characterize bacteria however, each type of bacteria is different in shape and size.

This means that different types of bacteria can be classified based on their shape. This includes the following:

Cocci Bacteria

Cocci bacteria are spherical or oval and may exist as a single cell or remain attached to one another. The shape of bacteria is usually determined by the cell wall of the organism. Attached cocci bacteria include the following:

  • Diplococci bacteria – These are a type of bacteria that occur as a pair – meaning they are two joined cells. Types of diplococci bacteria include Streptococcus pneumoniae, Moraxella catarrhalis, Enterococcus spp, and Neisseria gonorrhea.
  • Most of these cells will be round in shape but some can appear elongated (ovoid) or even kidney-bean shaped. An example of this is the Neisseria cells which can be both round or kidney-bean shaped when viewed under a microscope.
  • Tetrad Bacteria – These are bacteria that are arranged in groups of four cells. The cells remain attached and grow in this attachment even after cell division. Examples of Tetrad bacteria include Pediococcus and Tetradenococcus.
  • Sarcinae sarcina/Bacteria – These bacteria occur in groups of 8 cells and are produced through the perpendicular plane division. The characteristics that are associated with this type of bacteria include being strict anaerobes, gram-positive, and measuring between 1.5 and 3.0 um. Some examples of this type of bacteria include: Sarcina aurantiaca, Sarcina lutea, and Sarcina ventriculi.
  • Streptococci Bacteria – This type of bacteria is arranged in the shape of a chain. They are usually ovoid in shape and sometimes can form paired chains. They are members of the Streptococcasceae family and are characterized by being non-motile and gram-positive. Some examples of this type of bacteria include Streptococcus pyogenes, streptococcus pneumoniae, and S. mutans.
  • Staphylococci Bacteria – These bacteria occur in grape-like clusters. They are a result of cell division that has occurred in two planes.

The two main characteristics of this type of bacteria are that they are immobile and gram-positive.

Examples of this type of bacteria include Staphylococcus epidermidis, Stahphylococcus haemolyticus, Staphylococcus aureus, and Staphylococcus capitis.

Bacillus Bacteria

Bacillus Bacteria are rod-shaped bacteria that form endospores and facultative anaerobes. Much like the cocci bacteria we discussed above, bacillus bacteria are all arranged differently as well.

Some are single and unattached such as the Salmonelle enterica subs bacteria, whereas others are attached. The following examples are different types of bacillus arrangements:

  • Diplobacilli Bacteria – Diplobacilli bacteria occur in pairs, just like diplococci bacteria do. The two cells do not separate after cell division and instead exist as a pair. Examples of this type of bacteria include Coxiella burnetii, Klebsiella rhinoscleromatis, and Moraxella Bovis.
  • Streptobacilli – These bacteria occur as elongated chains and are the result of division on a single plane. Examples of these bacteria include Streptobacillus moniliformis, streptobacillus levaditi, streptobacillus felis, and streptobacillus hongkongensis.
  • Coccibacilli Bacteria – These bacteria are short in length compared to other bacilli. Common examples of this type of bacteria include Chlamydia trachomatis and Haemophilus influenzae. 
  • Another type of bacilli bacteria is the palisade bacteria. These are an arrangement of bacteria that resemble a picket fence.
  • Some types of bacteria (that aren’t cocci or bacilli bacteria) appear different when viewed under the microscope. They instead appear curved and vary in shape. These bacteria can be differentiated from one another and include the following:
  • Vibrio Bacteria – These bacteria are usually comma-shaped. Examples include Vibrio mytili, Vibrio anguillarum, Vibrio paraphaemolyticus, and Vibrio cholerae.
  • Spirochete – These are characterized by being a helical shape. They are flexible and known to produce mycelium. They move using an axial filament and this is what distinguishes them from other types of bacteria.

An axial filament is responsible for the twisting motion of the organisms and is usually located between the inner and outer membrane.

Examples of the Spirochetes bacteria include leptospira, Spirochaeta, and treponema.

Spirilla Bacteria – These bacteria also possess a helical shape. They are more rigid and have the typical flagella found in other types of bacteria, different from Spirochetes.

Common examples of this type of bacteria include aquaspirillum, campylobacter jejuni, spirillum winogradskyi, and oceanospirillium.

More Types Of Bacteria (Based On Shape)

Sometimes bacteria may be more oddly-shaped. This distinguishes them from other types of bacteria that we have mentioned above. These types of bacteria include the following:

  • Rectangular Bacteria – Bacteria that appear rectangular for example the Haloarcula marismorui bacteria.
  • Star-shaped Bacteria – These bacteria look like stars, for example, the Stella humosa bacteria.
  • Haloarcula – These bacteria are triangular in shape.
  • Pleomorphic Bacteria- These bacteria have the ability to change in size depending on the different environments they are in.
  • Stalked Bacteria – These bacteria have a stalk on one end of the cell, for example, the C. crecentus bacteria. 

Final Thoughts

So, in conclusion, there are many numbers of bacteria that each possess different shapes ad arrangements and are what makes them unique.

Different bacteria can survive in different environments depending on their shape and how they are arranged.

Bacteria can be found all over the world and on most surfaces, even when they can’t be seen by the naked eye – which is most of the time.

Some bacteria, as we mentioned earlier, can be visible to the naked eye – however, they are usually so small that we’d only be able to see them when looking at them underneath a microscope!

Jennifer Dawkins

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