Dr. Saqib Khan
Bacteria Classification


Bacteria are single-celled (unicellular) microorganisms devoid of a nucleus and other membrane-bound organelles. The term “prokaryotes” is occasionally used to refer to bacteria. A prokaryote is a cell that lacks a nucleus, thus the name, which refers to “before the nut” in Greek.


Bacteria (such as Leptospira, which causes severe sickness in livestock) have a bad reputation because of their association with illness in humans and other animals.

However, the actinomycetes are responsible for the production of antibiotics like nocardicin and streptomycin; other bacteria live symbiotically in the digestive system of animals (and humans) or other parts of their bodies or on the roots of some plants, turning nitrogen into a useable form.

Bacteria provide sourness to yogurt and sourdough bread, aid in decomposing organic waste, and form the foundation of the food chain in many ecosystems.

Bacteria are crucial due to their adaptability, speed of growth and reproduction, and history; the earliest fossils discovered are of species similar to bacteria and date back approximately 3.5 billion years.

Bacteria Classification

Bacteria Classification:

Bacteria Classification based on Shape:

In 1872, scientist Cohn categorized bacteria into four primary groups based on their morphologies.

A. Cocci:

These bacteria are either round or elliptical in shape and have just one cell. They might continue to exist as isolated cells or form larger structures by banding together. Here are some of them:

  • Monococcus: Micrococcus are characterized by a single, spherical cell, thus their other name. For example Micrococcus flavus.
  • Diplococcus: Diplococcus cells divide once in a certain plane, and the daughter cells stay linked to their parents once the process is complete. For example, diplococcus pneumoniae.
  • Streptococcus: In this case, cells continuously divide in a single plane to create a chain. For example, Streptococcus pyogenes.
  • Staphylococcus: Here, the cells are arranged in a haphazard fashion like grape clusters, with the divisions occurring on three different planes. For example, Staphylococcus aureus.
  • Tetracoccus: This is made up of two parallel planes intersecting at right angles to form four circular cells. For example, Gaffkya tetragena.
  • Sarcina: In this scenario, the cells divide into three different planes to produce cube-shaped clusters of 8 or 16 cells. For example, Sarcina lutea.
Cocci types

B. Bacilli:

These bacteria, which may take the form of rods or cylinders, often persist alone or in pairs. For example, Bacillus cereus.

C. Vibro:

Among bacteria, the vibro are the ones with comma-shaped curves, and they have just one genus. For example, Vibro cholerae.

D. Spirilla:

These microbes take the shape of a spiral or a spring, with many coils at their tips and a single flagellum. For example, Spirillum volutans.


Due to its presumed similarity to the sun’s rays in tissue lesions, the genus Actinomycetes (a kind of filamentous bacteria) has been given this name (actis means ray and mykes means fungus).

Mycoplasmas lack a cell wall, making them unstable in their physical form. They might have spherical or oval shapes, or they can be found in the form of tangled strands.

Bacilli & other bacterias

Bacteria Classification based on Mode of Nutrition:

A. Phototrophs:
  • These microbes can use light as a source of energy.
  • On the basis of their electron sources, phototrops are separated into two subgroups.
  • These bacteria are photoautotrophs, meaning they utilize light to generate energy and decrease inorganic substances like Hydrogen sulfide for use as an electron source. For instance, chromium okenii.
  • Photoorganotrophs: These microbes utilize organic chemicals like succinate to convert light into energy.
B. Chemotrophs:
  • Chemical substances provide energy for microorganisms.
  • They are incapable of engaging in photosynthesis.
  • On the basis of their electron source, chemotrops are further classified into two categories.
  • They are chemolithotrophs, meaning they get their nutrients by oxidizing them, and they use inorganic molecules like NH3 as an electron source to oxidize organic ones. Instance: Nitrosomonas.
  • They are chemoautotrophs, meaning they get their nutrients and energy from inorganic molecules like sugars and amino acids. case in point Pseudomonas pseudoflava.
C. Autotrophs:
  • These bacteria can synthesize their own nutrition from carbon dioxide.
  • There are two kinds of autotrophs, distinguished by the amount of energy put into the process of carbon dioxide assimilation.
  • These organisms can sustain themselves only by their metabolic processes (chemoautotrophs and photoautotrophs, respectively).
  • Photoautotrophs: They make use of light to absorb carbon dioxide. Based on where their electrons come from, we may further classify them. This would include both Photoorganotropic autotrophs and Photolithotropic autotrophs.
  • Chemoautotrophs: CO2 is assimilated using chemical energy in this process.
D. Heterotrophs:
  • The bacteria that rely on organic compounds for their carbon needs do not have the capacity to fix CO2.
  • The majority of human-harming bacteria are heterotrophic.
  • It’s true that certain heterotrophs are easy to care for since they have few dietary needs. Still, certain bacteria, classified as finicky heterotrophs, need unusual nutrients in order to thrive.

Bacteria Classification based on the Number of Flagella:

The bacteria may be divided into various groups according to their flagella:

  • Atrichos: There are no flagella in these bacteria. For example, Corynebacterium diphtheriae.
  • Monotrichous: The bacterial cell has a single flagellum linked to one end. For example, Vibro cholerae.
  • Lophotrichous: In order to move about, bacteria use a set of flagella linked to the cell’s terminus.
  • For example, Pseudomonas.
  • Amphitrichous: A swarm of flagella protruding from the bacterial cell’s two ends. For example, Rhodospirillum rubrum.
  • Peritrichous: Flagella are dispersed uniformly around the bacterial cell. For example, Bacillus.

Video Lecture:

Other Articles:


Do bacteria have the potential to kill?

Unexpected deaths in both children and adults are often caused by bacterial infections. 
Bacterial infections of the gastrointestinal, respiratory, and central nervous systems are the leading causes of unexpected mortality in children.

Which bacterial illness is the deadliest, in your opinion?

They won’t hurt you and may even be useful in certain situations. For instance, the bacteria that reside in your digestive tract (gut) play an important role in preparing the food you ingest for absorption by your body. But there are certain germs that may cause bacterial infections, which can lead to sepsis. To put it simply, sepsis is a potentially fatal reaction to an infection in the body.


By Dr. Saqib Khan

I am a medical professional and research scholar having vast experience in Computer-aided drug discovery and organic Synthetic Chemistry. I also have a passion for academic and medical writing.

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