Morphology Of Bacteria-Sizes, Shapes, Arrangements, Examples

Morphology Of Bacteria Overview

  • Bacteria are prokaryotic, unicellular forms of biological cells. These are less complex than other kinds of living things, since they don’t have a nucleus that is connected to a membrane.
  • While most of them are minute and only a few are visible to the naked eye, they exhibit a broad variety of forms, sizes, and structures.

Bacterial Size

  • The micron (micrometre), which is one thousandth of a millimetre, is the unit of measurement used in bacteriology.
  • In general, bacteria are only 10% the size of eukaryotic cells. Bacteria typically have a size of between 0.5 and 5 m.
  • They can, however, range in size from 0.3 m to 0.7 mm.
  • Since many bacteria are smaller than 200 microns, the unaided eye’s maximum resolution, they are not visible to the human eye.
  • Thiomargarita namibiensis, which can reach lengths of up to half a millimetre, and Epulopiscium fishelson, which may reach lengths of 0.75 mm, are two of the biggest bacteria.
  • Members of the genus Mycoplasma have a diameter of only 0.3 m, making them as tiny as the biggest viruses.
  • Common bacteria like Escherichia coli have a diameter that varies from 1.1 to 1.5 m.
  • Bacterial size has been found to play a crucial influence in the survival of organisms.
  • They may live and even thrive in a variety of unusual habitats, such as the vertical sediments in the marine environment, because of their microscopic size.
  • Because there are no other living things present in this environment, bacteria may make use of the resources.
  • In addition, bacteria’s tiny size encourages parasitism and the capacity for survival in low-nutrient environments.
  • The bacteria’s high surface area to volume ratio also enables them to absorb all the nutrients necessary for their continued development and reproduction.

Bacterial Shape

  • The majority of bacteria have a solid cell wall that gives them a defined form while safeguarding their interior parts.
  • Although most bacteria have this property, there are those that differ in shape, allowing them to be divided into several groups according to their shapes.
  • The bacterial cell wall and cytoskeleton control this vast range of forms.
  • Although bacteria come in a vast range of forms, each genus generally only displays a small subset of morphologies. This suggests that although bacteria can adopt a large variety of shapes, only the most adaptable ones are chosen by individual bacteria.
  • Different bacterial forms display various physical characteristics to the outside world, and these characteristics aid cells in coping with and adapting to environmental circumstances.
  • In the face of food acquisition, cell division, predators, adhesion to surfaces, passive dispersion, active motility, and internal or external differentiation, it has been discovered that bacterial form provides a measure of survival value.
  • Bacteria can be divided into the following common shape categories:


  • Cocci bacteria are defined as those that are spherical or oval in form.
  • These can either stay separate or be connected in groups. When grouped together, they take on a flattened appearance.
  • Coccoid forms are thought to have evolved from rod-shaped creatures during the course of evolution.

Bacilli (Rod-shaped)

  • These cells have a rod-like form and, like cocci, can be either solitary or attached to other cells.
  • Bacilli bacteria were among the first types of bacteria to appear, and it is believed that this form is less favourable than others. This is based on observations of the motion of filamentous E. coli cells, which, although motile and chemotactic, travel slowly and are unable to tumble to change direction.


  • This class of bacteria consists of those with curved or helical shapes (comma-shaped).
  • The bacteria might be spiral-shaped or just slightly bent.

Arrangements of Cocci

  • Cocci bacteria can be grouped in a variety of ways, including individually, in pairs, in groups of four, in chains, in clusters, or in eight-cell cubes.
  • During cell division, these cells stay together.


  • These bacteria are those that exist as a single cell.
  • This configuration happens when two bacterial cells form a pair (joined together).
  • In this configuration, some of the cells may continue to be spherical while others may take on a flattened, elongated, or bean-shaped appearance.
  • Examples include Neisseria gonorrhoea, Moraxella catarrhalis, Streptococcus pneumonia, and Enterococcus species.


  • Tetrad bacteria are grouped in a cluster of four cells that stay connected after cell division and continue to develop there.
  • When the cells divide into two planes, this configuration is what happens.
  • Examples include Tetragenococcus, Aerococcus, and Pediococcus.


  • The bacterial cells are arranged into a group of eight cells in this configuration.
  • This occurs when cells divide in a perpendicular plane.
  • Being a stringent anaerobe is a feature shared by several species.
  • Examples include Sarcina ventriculi, Sarcina lutea, and Sarcina aurantiaca.


  • The bacteria are grouped into long chains at this location.
  • These microorganisms belong to the family Streptococcaceae, which is distinguished by Gram-positive bacteria and a lack of motility.
  • Streptococcus mutans, Streptococcus pneumonia, and Streptococcus pyogenes are a few examples.


This kind comprises bacteria that are grouped into clusters that resemble grapes.

This is the outcome of cell division in both planes and is characterised by immobile, Gram-positive organisms.

Examples include Staphylococcus aureus, Staphylococcus audermidis, Staphylococcus haemolyticus, and Staphylococcus capitis.

Arrangement of Bacilli


  • Bacilli are rod-shaped solitary cells of bacteria that are present.
  • These microorganisms are facultative anaerobes and may create endospores.
  • Examples include Bacillus cereus, Salmonella choleraesuis, and Salmonella enterica subsp.


  • Diplobacilli also occur in pairs, like Diplococci.
  • The two cells do not split again after cell division and continue to grow together.
  • Examples include Moraxella bovis, Klebsiella rhinoscleromatis, and Coxiella burnetii.


  • The bacteria in this category are organized into chains.
  • Cell division in a single chain causes this to happen.
  • Streptobacilli moniliformis, Streptobacillus Levaditi, Streptobacillus felis, and Streptobacillus hongkongensis are a few examples.


  • Coccobacilli resemble both cocci and bacilli, as their names indicate.
  • These are smaller and hence seem stumpier.
  • Examples include Gardnerella vaginalis, Haemophilus influenza, and Chlamydia trachomatis.


  • Pallisades are a particular form of bacilli bacteria that have a bent point of division during cell division, giving them the appearance of a picket fence.
  • They resemble Chinese characters in appearance.
  • Consider the diphtheria-causing Corynebacterium diphtheria.

Arrangement of Spiral


  • These are microorganisms that have a comma-like form and are slightly bent.
  • Examples include Vibrio cholera, Vibrio parahaemolyticus, Vibrio mytili, and Vibrio anguillarum.


  • Spirochetes are helical-shaped, spiral bacteria.
  • These are axial filament-equipped and flexible, which aids in motility. These filaments are a key characteristic that sets spirochetes apart from other bacteria.
  • These filaments are present along the whole length of the bacterium and aid in twisting the bacteria’s movements.
  • Examples include Treponema pallidum, Borrelia recurrentis, and the Leptospira species (Leptospira interrogans).

Spirilla (Helical-shaped-Corkscrew form)

  • Although more rigid, these bacteria share a similar structure with spirochetes.
  • Although they lack the endoflagella found in spirochetes, they still contain a flagellum.
  • Examples include Spirillum winogradskyi, Helicobacter pylori, and Campylobacter jejuni.

Other Shapes and Arrangements

Appendaged Bacteria

  • Appendaged bacteria are those that develop distinctive structures like pilli or fimbriae.
  • Compared to other bacteria, those that produce these appendages are more virulent.
  • Consider the gonorrhea-causing agent Neisseria gonorrhoeae.

Box-or rectangle-shaped bacteria

  • Bacteria that resemble boxes have a rectangular form.
  • Haloarcula marismortui, as one example.

Club-shaped Rod Bacteria

  • One side of these bacteria is thinner than the other.
  • Corynebacterium is one of this group’s well-known representatives.

Filamentous Bacteria

  • These are long, slender, filament-shaped microorganisms.
  • They may split into mycelium branches that resemble spaghetti or hair strands.
  • Consider the actinomycetes.

Triangular-shaped Bacteria

  • Triangular-shaped bacteria are included in this category.
  • Consider the haloarcula.

Pleomorphic Bacteria

  • This group consists of bacteria that do not have a specific form.
  • Although they can modify their appearance, in pure culture they seem to have a distinct shape.
  • Examples include M. genitalium and Mycoplasma pneumoniae.

Stalked Bacteria

These are the bacteria whose cells have a stalk on one end.

Caulobacter crescentus, as an example.

Star-shaped Bacteria

  • This category includes bacteria with a star-like appearance or that are star-shaped.
  • For instance, Stella Humosa


  • Young K. D. (2007). Bacterial morphology: why have different shapes?. Current opinion in microbiology, 10(6), 596–600. https://doi.org/10.1016/j.mib.2007.09.009
  • Al-mohanna, Moshtaq & H., quine. (2016). MORPHOLOGY AND CLASSIFICATION OF BACTERIA.
  • Constantino MA et al.(2016). Helical and rod-shaped bacteria swim in helical trajectories with little additional propulsion from helical shape. Science Advances. DOI: 10.1126/sciadv.1601661
  • Image created using biorender.com.


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