Morphology of Bacteria: Size, Shape and Arrangement

Morphology of Bacteria: What is pleomorphism? What are the types of bacteria? Do all bacteria have similar shapes and sizes? Bacteria are microorganisms that belong to the phylum Monera. Morphology of Bacteria deals with the bacterial outer structure, shape and sizes. To know more about the Morphology of Bacteria, scroll down the article.

What are Bacteria?

Bacteria are prokaryotic microorganisms and are unicellular in nature. Since they lack a membrane-bound nucleus, they are less complex than other living creatures. Although only some bacteria can be seen by our naked eyes while the rest are microscopic in nature, they display a wide range of shapes, sizes, and structures.

Fig: Bacteria

What are the Characteristics of Bacteria?

Some of the basic characteristics of bacteria are as follows:

I. Bacteria are microorganisms with only one cell and without a nucleus or membrane-bound organelles.
II. A plasmid is an additional circular of genetic material found in some bacteria.
III. They do not possess internal organisation and their DNA is often sequestered into a region of the bacterial cell known as the nucleoid.
IV. Bacteria have a cell wall that is often made of peptidoglycan, which is a complex molecule consisting of sugars and amino acids.
V. Bacteria may be found in all types of environments on Earth, including soil, rock, seas, and even polar snow.
VI. The binary fission method is used by bacteria to reproduce. The bacteria, which is a single cell, splits into two identical daughter cells throughout this procedure.
VII. These bacteria can be pathogenic or non-pathogenic in nature.

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What is the Morphology of Bacteria?

i. A bacteria’s greatest distinguishing characteristic is its morphology or shape. It’s a certain species’ distinguishing feature.
ii. The shape of a bacterium cell is determined by its morphology, which also determines the pathogenicity of the bacteria.
iii. The adaptation and evolution of bacteria are influenced by their morphological characteristics.
iv. The peptidoglycan (murein) in the bacterial cell wall and is a polymer of sugars that alternates \({\text{N}}\)-acetylglucosamine (NAG) connected to \({\text{N}}\)-acetylmuramic acid (NAM) and amino acids in a peptide chain.
v. The varying morphology and form of bacteria are due to changes in the polymer’s structure and thickness.

Different Sizes of Bacteria

i. Bacteria are about a tenth of the size of eukaryotic cells. Bacteria range in size from \(0.5\) to \(5\mu {\text{m}}\) on average.
ii. They can, however, be as little as \(0.3\,{\text{m}}\) and as big as \(0.7\,{\text{mm}}{\text{.}}\)
iii. The unaided eye’s resolution limit is around \(200\) microns, and because many bacteria are smaller than this, they are invisible to the human eye.
iv. Thiomargarita namibiensis, which may grow to be half a millimetre long, and Epulopiscium fishelsoni, which can grow to be \(0.75\) millimetres long, are among the biggest bacteria.
v. Members of the genus Mycoplasma are the smallest bacteria, measuring about \(0.3\,\mu {\text{m}}\) and being as small as the smallest viruses.
vi. The diameter of common bacteria such as Escherichia coli ranges from \(1.1\) to \(1.5\,\mu {\text{m}}{\text{.}}\)

Different Shapes of Bacteria

i. The majority of bacteria have a rigid cell wall that gives them a defined form while also protecting their internal components.
ii. Even while the majority of bacteria have this characteristic, they vary in shape, allowing them to be categorised into distinct categories depending on their morphologies.
iii. The bacterial cell wall and cytoskeleton are responsible for a broad range of forms.
iv. The common categories of bacteria based on their shapes are as follows:

A. Cocci

a. The bacteria that are oval or spherical in shape are known as cocci bacteria.
b. These can be solitary or connected to one another in a group. When grouped together, they seem flattened.
c. Coccoid forms are thought to have evolved from rod-shaped creatures throughout evolutionary time.

Arrangements of Cocci

a. Cocci bacteria can be organised individually, in pairs, in four-cell groups, in chains, in clusters, or in eight-cell cubes.
b. During cell division, these cells stay together. The bacteria’s form is altered by the plane of cell division.
c. Cocci have a gram-positive cell wall with a thick peptidoglycan layer or a gram-negative cell wall with a thin peptidoglycan layer.
d. According to the arrangement of cells, the cocci are again divided into the following subtypes:

1. Monococcus – It is a bacterial species that consists of a single cell.

2. Diplococcus
i. When two bacterial cells form a pair, this configuration occurs (joined together).
ii. Some cells in this arrangement may be spherical, while others may be flattened, elongated, or bean-shaped.
iii. Examples: Streptococcus pneumoniae.

3. Streptococcus
i. In this type, the bacteria are organised in long chains here.
ii. These bacteria belong to the Streptococcaceae family, which is characterised by Gram-positive bacteria and a lack of motility.
iii. Streptococcus pneumonia, Streptococcus pyogenes, and Streptococcus mutans are some examples.

4. Tetrads
i. Tetrad bacteria are organised in a group of four cells that remain connected during cell division and development in the attachment.
ii. When the cells split into two planes, this pattern occurs.
iii. Aerococcus, Pediococcus, and Tetragenococcus are some examples.

5. Staphylococcus
i. Bacteria organised in grape-like clusters make up this kind of arrangement.
ii. This is caused by cell division in both planes and is characterised by immotile and Gram-positive organisms.
iii. Staphylococcus haemolyticus, Staphylococcus capitis, Staphylococcus aureus, and other bacteria are examples.

6. Sarcinae
i. The bacterial cells form an eight-cell cluster in this configuration.
ii. This happens when the cells divide in a perpendicular plane.
iii. The fact that these organisms are strictly anaerobic in nature.
iv. Sarcina lutea, Sarcina aurantiaca, and Sarcina ventriculi are other examples.

Fig: Arrangements of Cocci

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B. Bacilli (Rod-shaped)

These are rod-shaped cells that, like cocci, can exist alone or in association with other cells. Bacilli bacteria were among the earliest to emerge, and their form is considered to be less favourable than that of other bacteria.

Arrangement of Bacilli

1. Bacillus
i. Bacilli are bacteria that are rod-shaped and exist as solitary cells.
ii. These bacteria are facultative anaerobes that may generate endospores
iii.Salmonella enterica subsp., Bacillus cereus and Salmonella choleraesuis are some examples.

2. Diplobacilli
i. Diplobacilli, like Diplococci, is found in pairs.
ii. The two cells do not divide and develop in an associated configuration after cell division.
iii. Coxiella burnetii, Klebsiella rhinoscleromatis, and Moraxella bovis are some examples.

3. Streptobacilli
i. Bacteria in this category are arranged in chains.
ii. This happens when a single chain of cells divides.
iii. Streptobacillus moniliformis, Streptobacillus felis, Streptobacillus Levaditi, and Streptobacillus hongkongensis are examples of streptobacilli.

4. Coccobacilli
i. As the name indicates, Coccobacilli is similar to both cocci and bacilli.
ii. Because they are smaller, they look stumpy.
iii. Chlamydia trachomatis, Gardnerella vaginalis, and Haemophilus influenzae are some of the examples.

5. Palisades
i. Palisades are bacilli bacteria that have a picket fence-like shape due to a bend at the site of division during cell division.
ii. They have the appearance of Chinese characters.
iii. Example – Corynebacterium diphtheria.

Fig: Arrangements of Bacilli

C. Spiral

This group of microorganisms includes bacteria that are either helical-shaped or curved or comma-shaped. The bacterium might have a corkscrew-like spiral or be slightly bent.

1. Vibrio
i. These are the comma-shaped bacteria that are slightly bent.
ii. Vibrio mytili, Vibrio anguillarum, Vibrio parahaemolyticus, and Vibrio cholera are some examples.

Fig: Vibrio

2. Spirochetes
i. Spirochetes are spiral bacteria that have a helical shape.
ii. These organisms are flexible and have an axial filament which helps in motility. These filaments are a key feature that distinguishes spirochetes from other bacteria.
iii. These filaments travel the length of the bacterium, aiding in the twisting of the bacteria’s motility.
iv. Examples include Leptospira interrogans, Treponema pallidum, etc.

Fig: Spirochetes

3. Spirilla (Helical-shaped/Corkscrew form)
i. Spirochetes have a similar structure to these bacteria, although they are more rigid.
ii. They, too, have a flagellum, but unlike spirochetes, they lack the endoflagella.
iii. Helicobacter pylori, Campylobacter jejuni, and Spirillum winogradskyi are some of the examples.

Fig: Spirillum

Other Shapes and Arrangements

Some of the other shapes and arrangements are as follows:
1. Filamentous Bacteria
i. These are filament-shaped bacteria that are thin, long, and are filamentous.
ii. They can split into mycelium-like branches that look like strands of hair or spaghetti.
iii. Actinomycetes are a good example.

2. Appendaged Bacteria
i. Appendaged bacteria are bacteria that generate a distinct structure, such as pillus or fimbriae.
ii. These bacteria are more virulent when compared to the other bacteria that do not form these appendages.
iii. Example – Neisseria gonorrhoea.

3. Club-shaped Rod Bacteria
i. One side of these bacteria is thinner than the other.
ii. Corynebacterium is a well-known representative of this group.

4. Box-shaped/ Rectangular Bacteria
i. Bacteria with a box-like form or that are rectangular in shape.
ii. Box-shaped bacteria have a rectangular form that resembles the shape of a box.
iii. Example – Haloarcula marismortui.

5. Triangular-shaped Bacteria
i. This group includes bacteria that are triangular in shape.
ii. Example: Haloarcula.

6. Pleomorphic Bacteria
i. The bacteria that belong to this group does not have or possess a specific shape. 
ii. They can alter the shape, although they appear to have a distinct morphology in pure cultivation.
iii. Examples: Mycoplasma.

7. Star-shaped Bacteria
i. The bacteria that look like stars or are star-shaped are included in this group of bacteria.
ii. Examples: Stella humosa.

8. Stalked Bacteria
i. These are the bacteria that possess a stalk on one end of the cell.
ii. Examples: Caulobacter crescentus.

Summary

Bacteria are unicellular organisms that are prokaryotic in nature. Through this article, we understood that bacteria comes in various sizes and shapes. There are many pathogenic bacteria that are responsible for different diseases in living organisms. There are some bacteria that do not have any shape and are considered pleomorphic like organisms.

The form of a bacterium cell is determined by its morphology, which also determines its virulence. The peptidoglycan (murein) in the bacterial cell wall is a polymer of sugars that alternates \({\text{N}}\)-acetylglucosamine (NAG) connected to \({\text{N}}\)-acetylmuramic acid (NAM) and amino acids in a peptide chain.

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FAQs

Q.1. What are the three different types of bacterial morphology?
Ans: The three different types of bacterial morphology are cocci, bacillus and spiral.

Q.2. What are the two main classifications of bacteria?
Ans: The two main classifications of bacteria are gram positive-bacteria and gram-negative bacteria.

Q.3. What do you mean by morphology?
Ans: Morphology is the study of the shape and structure of organisms.

Q.4. What are the main characteristics of bacteria?
Ans:  Some of the main characteristics of a bacteria are as follows:
i. Organelles that are membrane-bound are absent in bacteria.
ii. These are single-celled microorganisms that are typically small in size and are visible under a microscope.
iii. They have a cell wall, i.e., composed of peptidoglycan and may be distinguished using the gram staining technique.

Q.5. What is the most common type of bacteria?
Ans: The most common type of bacteria is Escherichia coli.

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