What are bacteria?

Bacteria are single-celled, tiny creatures that may enter healthy tissues and grow rapidly. Bacteria are microscopic organisms that are tiny and unicellular. These are members of the prokaryote kingdom. They live in water, air, soil, and all-natural environments. They are used in industrial and therapeutic processes, and they support a wide range of plant and animal life. The first organism to appear on the planet. Bacteria-like creatures are the oldest known fossils. Bacteria can consume a wide range of organic and inorganic elements, and some may even survive in harsh conditions.

Types of bacteria

Bacteria are classified into four groups based on their basic shapes, they are:

Spherical shaped bacteria

Cocci are bacteria that are shaped like balls, while coccus is a single bacterium. Cell walls of cocci can be gram-positive with a thick peptidoglycan layer or gram-negative with a thin peptidoglycan coating. The cells do not split after cell division. The cell division plane changes the shape of the bacteria. Monococcus, diplococcus, streptococcus, and staphylococcus are the four subgroups of cocci depending on the arrangement of their cells.

Rod-shaped bacteria

The most common bacteria are rod-shaped bacteria, often known as Bacillus. After cell division, they might arise alone or in groups of numerous ells. Vibrio bacteria are curved rod-shaped bacteria. Bacillus bacteria are gram-positive bacteria that can be obligate aerobes or facultatively anaerobic. Bacillus anthracis or Bacillus anthrax are examples of the Bacillus bacterium.

Spiral bacteria

Spiral or helical bacteria are the most common. They are classified into two types based on cell thickness, flexibility, and motility: spirillum and spirochete. Bacteria of this form cause leptospirosis, Lyme disease, and syphilis.

Coma shaped bacteria

They are curved and resemble a comma. These types of bacteria are also called vibrio. These are mostly gram-negative bacteria. These bacteria also cause various food-borne diseases. Vibrio cholerae, Vibrio parahaemolyticus is an example of this bacteria.

Classification of bacteria

Grass-positive bacteria and gram-negative bacteria are the two types of bacteria that make up the majority of bacteria. Cell wall composition and gram stain teat reactivity are the basis for these classifications. Hans Christian Gram created the gram stain method, which uses the reaction of bacteria's cell walls to dyes and chemicals to identify them. The fundamental distinction between these bacteria is the makeup of their cell walls.

Gram-positive bacteria

Gram-positive bacteria have a cell wall that is made up of a thick layer of a material known as peptidoglycan. After gram staining, these bacteria became purple.

Gram-negative bacteria

On the surface of gram-negative bacteria is a single thin layer of peptidoglycan. Gram-negative bacteria have more complex cell walls than gram-positive bacteria. The crystal violet fades, and the pink safranin counterstain appears. The crystal violet fades and the safranin counterstain turns pink.

Structure of a bacterial cell

Bacterial cells have a simpler internal structure. All membrane-bound cell organelles, such as mitochondria, lysosomes, Golgi, endoplasmic reticulum, chloroplast, peroxisome, glyoxysome, and true vacuole, are absent. Bacteria do not have a membrane-bound nucleus or nucleolus. A nucleoid refers to the bacterial nucleus. The structure of the bacterial cells is categorized into two:

External components

The external components of the bacterial cell wall involve:

Cell wall

Each bacteria is surrounded by a hard cell wall comprised of peptidoglycan, a protein-sugar polymer. The cytoplasmic membrane is surrounded by the cell wall, which provides structure and protects the cell from the outside world. It also aids in the anchoring of appendages like pili and flagella, which protrude through the cell wall to the outside and emerge from the cytoplasm membrane.

Cytoplasmic membrane

The cytoplasmic membrane is a layer of phospholipids and proteins that encloses the bacterium's interior and regulates material flow in and out of the cell.


Some bacteria have a third line of defence in the form of a polysaccharide capsule. Capsules' primary purposes are to keep bacteria from drying out and to protect them from larger germs that phagocytose them. The capsule is an important virulence component of bacteria that cause diseases, such as E. coli and Streptococcus pneumonia. The capsule is 98 percent water with 2% polysaccharide, glycoprotein, or both polysaccharide and glycoprotein.


Flagella are hairlike structures that provide a method for germs to move around. They can be present on a bacterium's inside or outside, as well as on its entire surface. The flagella beat in a propeller-like motion to help the bacterium move toward nutrients, away from harmful chemicals, or toward light in photosynthetic cyanobacteria.


Pili, which are microscopic hairlike projections that arise from the cell surface, are found on many bacteria. Bacteria use these outgrowths to connect to other cells and surfaces such as teeth, intestines, and rocks. Many pathogenic bacteria lose their capacity to infect because they can't connect to host tissue without pili. Conjugation happens when two bacteria use specialized pili to exchange plasmid deoxyribonucleic acid fragments.


The sheath is a hollow tube-like structure that surrounds some bacteria that form chains or trichomes. Aquatic bacteria are mostly from the sheath.


A prosthesis is a semi-rigid expansion of the cell membrane and cell wall. Some prostheses are developed with a bud at the tip, which helps with reproduction. One bacterium may contain one or more prostheses.


The stalk is a non-living, tubular structure that looks like a ribbon. Bacterial excretory products are responsible for the formation of the stalk.

Internal components

The internal components of the bacterial cell involve:


The cytoplasm is a clear, viscous fluid found within the cell membrane. The cytoplasmic fluid contains all of the cell organelles and inclusions. It is made up of protein, lipids, water, and nucleic acid.


The nucleus is an essential component of the cell. It directs and controls all cell functions and stores the cell's hereditary information. A nucleoid is a bacterial nucleus that lacks a nuclear membrane, nucleoplasm, and nucleolus.


Ribosomes are globular granules that float freely in the cytoplasm. Ribosomes are regarded as the universal cell organelles because they can be found in both bacterial and eukaryotic cells. They translate the genetic information from the chemical language of nucleic acids to amino acids, the building blocks of proteins. Proteins are the chemicals that enable cells and living beings to carry out their functions. Eukaryotic ribosomes are similar to bacterial ribosomes in appearance. Bacterial ribosomes, on the other hand, are smaller and have a somewhat different molecular structure and composition. Because bacterial and eukaryotic ribosomes differ in certain ways, some antibiotics will disrupt bacterial ribosome function but not eukaryotic ribosome function, killing bacteria but not the eukaryotic organisms they infect.


A mesosome is a sac-like structure seen in gram-positive bacteria that is spherical or circular. Mesosoma's primary role is to serve as a location for bacterial cell respiration.

The figure shows the bacterical cell structure
CC BY-SA 4.0 | Image Credits: commons.wikimedia.org | Nejash Abdela

Use of bacteria

Bacteria are commonly thought to be harmful, although many of them are actually useful. We wouldn't exist if it weren't for them. Bacteria are most likely the source of the oxygen we breathe. Bacteria can be used for a variety of purposes, including:

Nitrogen fixation

When bacteria consume nitrogen, it is released for use by plants during the blooming season. Plants require nitrogen in the soil to survive, but they cannot produce it. Many plant seeds contain a small container of bacteria, which is used to ensure this when the plant grows.

Food technology

Lactic acid bacteria, such as Lactobacillus and Lactococcus, are utilised in the production of cheese, soy sauce, natto, vinegar, yoghurt, and pickles, as well as yeast and moulds, or fungi.

Human survival

Many microorganisms in the human body are essential for living. Bacteria in the digestive system break down nutrients such as complex carbohydrates into forms that the body can use. Some non-harmful bacteria can be employed to reduce disease transmission.

Context and Applications

This topic is significant for professional courses, especially for

  • Bachelor of Science in Microbiology
  • Master of Science in Microbiology
  • Doctor of Philosophy in Microbiology

Practice Problems

Question 1: Cells of bacteria are made up of___________.

  1. Cellulose
  2. Generic material
  3. Peptidoglycan
  4. Cell wall

Answer: Option 3 is correct.

Explanation: Peptidoglycan is a hard layer that surrounds the cytoplasmic membrane in most bacteria. It aids bacterial cells in their resistance to environmental stress and in maintaining cell shape throughout their life cycle.

Question 2: Bacillus is the term used for _____.

  1. Round bacteria
  2. Spherical bacteria
  3. Rod-shaped bacteria
  4. Cluster-shaped bacteria

Answer: Option 3 is correct.

Explanation: Rod-shaped bacteria are also called "bacilli. Vibrio bacteria are rod-shaped bacteria that are curved. Bacillus bacteria are gram-positive, obligate aerobes, or facultative anaerobic bacteria.

Question3: In bacteria, binary fission does not involve _____.

  1. Deoxyribonucleic acid duplication
  2. Cytokinesis
  3. Spindle formation
  4. Cell elongation

Answer: Option 3 is correct.

Explanation: Mitosis and binary fissions are contrasted. In many ways, binary fission in bacteria is comparable to mitosis in humans and other eukaryotes. Chromosomes are duplicated and separated in both cases, and the cell's cytoplasm is split into two cells. Amitotic spindle does not exist in bacteria.

Question 4: Cell division of blue-green algae is similar to ______.

  1. Virus
  2. Bacteria
  3. Brown algae
  4. Red algae

Answer: Option 2 is correct.

Explanation: Prokaryotic organisms include bacteria and cyanobacteria. They have similarities. They have lacked true cell organelles. They both possess nucleotides and they have similar cell division.

Question 5: Which of the following disease are caused by bacteria?

  1. Malaria
  2. Mumps
  3. Diphtheria
  4. Tuberculosis

Answer: Option 3 is correct.

Explanation: Diphtheria is an acute infection caused by strains of bacteria called Corynebacterium. Weakness, sore throat, and fever are the symptoms of this.

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