Bacteria are single-celled and are classified under the domain Prokaryota. As such, they lack membrane-bound organelles like those found in eukaryotes.
Although they are all microscopic organisms that can be found in various environments in nature, bacteria widely vary in size, shape, and arrangement.
* The majority of bacteria species are microscopic, however, studies have identified large bacteria that can be seen with the naked eye.
In general, bacteria are between 0.2 and 2.0 um - the average size of most bacteria. Research studies have shown their size to play an important role in survival over time.
Due to their small size, bacteria are able to exploit and thrive in various microenvironments. These include such environments as the vertical gradients in intertidal marine sediments in which various types of bacteria have been found. Because various microorganisms are absent in such environments, bacteria successfully exploit resources available and thrive in such environments.
The small size of bacteria is also beneficial for parasitism and oligotrophy. Bacteria can continue relying on a range of hosts (large and small) for their nutrition. In addition, they can also live and survive in environments that contain a low concentration of nutrients; for instance, a group of bacteria known as oligotrophic bacteria.
Bacteria have a high surface area to volume ratio that allows them to take up as many nutrients as possible for survival. In the process, they are able to continue growing and reproducing at a steady rate.
In bacteriology, the micron (micrometer) is the primary unit of measurement: I micrometer (um) is one-thousandth of a millimeter.
* Given that the limit of resolution for a human eye (naked eye) is between 100 and 200 um (about the diameter of a human hair) then the majority of bacteria cannot be seen with the naked eye.
The following are different sizes of bacteria:
According to many microbiology books, the average size of most bacteria is between 0.2 and 2.0 micrometer (diameter). However, there are some that hold this to range between 1 and 10 micrometers. This, however, only considers the diameter of the organisms and not the length.
For instance, whereas E. coli bacteria range between 1.1 and 1.5 um in diameters, B. anthracis range between 1.0 and 1.2um while B. subtilis range between 0.25 and 1.0um in diameter. They also vary in length when compared to each other.
There has been some debate about their classification but some of the smallest bacteria are microorganisms known as nanobacterium.
They are characterized by their slow growth under aerobic conditions as well as being Gram-negative in nature. Although they have been shown to range between 0.2 and 0.5um (200 to 500 nanometers) there have been studies where some were found to filter through 0.1um filters.
Some examples of these bacteria include Mycoplasmas (about 0.25um), Haemophilus influenzae which ranges between 0.2 and 0.3um and Mycoplasma gallicepticum which range between 200 and 300 nanometers in diameter.
* Although such bacteria as Mycoplasma lack a cell wall, they can exist independently. They are found in such hosts as human beings, plants and a variety of insects.
Some bacteria are large enough to see with the naked eye. For instance, whereas Schaudinnum bütschlii that measure between 4 and 5um in diameter are considered to be large bacteria, Thiomargarita namibiensis may grow to be as large as 0.75mm in diameter. This makes them some of the largest bacteria ever discovered. As such, they can be seen with the naked eye - when they reach such sizes.
* The Thiomargarita namibiensis is a Gram-negative bacterium that is coccoid in shape. It was discovered in ocean sediments in Namibia (Southwest of Africa).
Morphologically, bacteria are microscopic single-celled organisms that are small in size and lack membrane bound organelles. A majority of these organisms also have a cell wall and capsule that protects the inner contents of the cell where the nucleoid, ribosome, plasmid, and cytoplasm are found.
While a majority of bacteria share these characteristics, they vary in shape which allows different types of bacteria to be classified based on their general shape.
The following are the major categories of bacteria based on their shapes:
Cocci bacteria appear spherical or oval in shape. For the most part, the shape is determined by the cell wall of the organism and therefore varies from one type of cocci bacteria to another. Cocci bacteria may exist as single cells or remain attached to each other.
Attached Cocci bacteria include:
Diplococci bacteria - Diplococci bacteria are the type of cocci bacteria that occur as a pair (two joined cells).
Some examples of Diplococci bacteria include:
While some of these cells may be truly round shaped, others may appear elongated (ovoid) or bean-shaped/kidney shaped. For instance, some Neisseria cells may appear round while others are bean-shaped when viewed under the microscope.
Tetrad bacteria - Tetrad bacteria are arranged in groups of four cells. Following division, the cells remain attached and grow in this attachment.
Common examples of Tetrad bacteria include:
Sarcinae sarcina/Bacteria - Sarcina bacteria occur in groups of 8 cells. Unlike tetrads that divide into two planes, Sarcinae is produced through the perpendicular plane division.
Some of the characteristics associated with these bacteria include being strict anaerobes, Gram-positive bacteria and that measure between 1.5 and 3.0 um.
Examples of Sarcinae bacteria include:
Streptococci Bacteria- Streptococci bacteria are a type of bacteria that arrange in a chain form (resembling chains). A majority of these bacterial cells are also ovoid in shape and may form paired chains.
As members of the family Streptococcaceae, this group of bacteria is characterized by being non-motile, Gram-positive organisms.
Examples of Streptococcus bacteria include:
Staphylococci Bacteria- Staphylococci Bacteria are a type of bacteria that form grape-like clusters. This type of arrangement is the result of division that occurs in two planes. Two of the main characteristics of these organisms are that they are immobile, Gram-positive bacteria.
Examples of Staphylococci bacteria include:
Bacillus bacteria have the following traits:
Like cocci bacteria, bacillus bacteria are also arranged differently. While some exist as single, unattached cells (e.g. Salmonella enterica subsp, Bacillus cereus, and Salmonella choleraesuis), others are attached.
The following are the different types of bacillus arrangements:
Diplobacilli bacteria - Like Diplococci bacteria, Diplobacilli occur in pairs. Following cell division, the two cells do not separate and continue existing as a pair.
Examples of Diplobacilli bacteria include:
Streptobacilli - Streptobacilli bacteria occur as elongated chains. As such, they are the result of division on a single plane.
Common examples of Streptobacilli include:
Coccibacilli bacteria - Compared to other bacilli, Coccibacilli bacteria are shorter in length and thus appear stumpy.
Examples of Coccibacilli include:
* Palisades are another type of bacilli bacteria. Cell division of these bacteria results in a type of arrangement that resembles a picket fence. One of the best examples of palisades is the Corynebacterium diphtheria that is responsible for diphtheria.
Unlike cocci and bacilli bacteria, some types of bacteria appear curved when viewed under the microscope. However, they vary in shape making it possible to differentiate them from each other.
Vibrio bacteria - Generally, vibrio bacteria are comma-shaped and thus not fully twisted (curved rods).
Examples of Vibrio bacteria include:
Spirochete - Spirochetes are characterized by a helical shape. Spirochetes are also flexible and have been shown to produce mycelium. The movement involves the use of axial filaments, which is one of the distinguishing features between the bacteria and other types of bacteria.
* Axial filaments in spirochetes are located between the inner and outer membrane. They run along the length of the bacteria and are responsible for the twisting motion of the organisms.
Examples of Spirochetes include:
Spirilla bacteria - Like Spirochetes, Spirilla bacteria possess a helical shape. However, they are more rigid and have the typical flagella found in other types of bacteria.
Some examples of Spirilla bacteria include:
Some bacteria possess various odd shapes that distinguish them from other types of bacteria.
More on Cells:
Eukaryotes - Cell Structure and Differences
Prokaryotes - Cell Structure and Differences
Protists - Discovering the Kingdon Protista in Microscopy
Diatoms - Classification and Characteristics
Algae - Reproduction, Identification and Classification
Protozoa - Anatomy, Classification, Life Cycle and Microscopy
Archaea - Definition, Examples, Characteristics and Classification
Kevin D. Young. (2006). The Selective Value of Bacterial Shape. American Society for Microbiology.
Moshtaq Talip Al-mohanna. (2016). Morphology And Classification Of Bacteria. ResearchGate.
Siamak Yaghobee and Mojtaba Bayani. (2015). What are the nanobacteria?. ResearchGate.
James T. Staley. (1999). Size Limits of Very Small Microorganisms: Proceedings of a Workshop.