Bacteria – Discovery, Occurrence, Types and Structure

Bacteria are unicellular, microorganisms that are widely spread in occurrence. They inhabit almost every habitat.

They are of three shapes: cocci, bacilli, and spiral. Some bacteria do not have any specific shape because of the absence of cell walls are called pleomorphic.

They have flagella which are extremely thin, hair-like appendages that originate from the basal body. it is composed of flagellin.

Bacteria are also classified on the basis of flagella. Pilli is the hollow, non-helical filamentous appendages composed of pilin proteins. There is an outer complex layer called the cell envelope.

The capsule is a thick, gummy structure that gives sticky nature to the bacterial colonies. Slime is a soluble loose sheath of macromolecules that provides pathogenicity to bacteria.

Bacteria – A little Intro

Bacteria are microscopic, unicellular organisms that exist in their millions, which have cell walls but do not have organelles and an arranged nucleus, including some which can trigger the disease.

Microbiologists place bacteria in two major categories: eubacteria: Greek for “true bacteria” and a much smaller division, the archaeobacteria: Greek for “ancient bacteria”.

Discovery of Bacteria

It had actually long been believed that little animals exist which are too little to be seen with the naked eye. However, their discovery was linked to the creation of a microscopic lens. A Dutch Researcher “AntonieVan Leeuwenhoek” (1673) was the first to report the microorganisms such as bacteria and protozoa.

He used a simple microscope to describe bacteria and protozoa with accurate illustrations and descriptions and called these small animals as “animalcules”. He firstly observed small animals in the water, then verified these in saliva, vinegar, infusions, and other compounds.

The progress in understanding the nature and value of these tiny organisms has been sluggish. The existence of microorganisms was additionally confirmed by Louis Pasteur’s work. Pasteur led many discoveries in the field of microbiology and medicine.

His main achievements are the development of vaccines for disease anthrax, fowl cholera, and rabies. He also made significant contributions to the development of the pasteurization process and the advancement of fermentation industries. He proved that microbes might cause illness.

Germ Theory of Disease

Robert Koch created the ‘germ theory of disease’. He isolated common rod-shaped bacteria with squarish ends (bacilli) from the blood of sheep that had actually died of anthrax. Then he found germs that triggered tuberculosis and cholera.

He developed 4 postulates, which are the main pillars of the germ theory of disease’. These are utilized to find out whether the organism found in disease lesions is the causal agent of the disease or not.

  1. A specific organism can always be found in association with a provided disease.
  2. The organism can be isolated and grown in pure culture in the laboratory.
  3. The pure culture will produce the disease when inoculated into a vulnerable animal.
  4. It is possible to recuperate the organism in pure culture from the experimentally contaminated animal.

Koch and his associates created lots of strategies concerning shot, isolation, media preparation, maintenance of pure cultures, and preparation of specimens for microscopic examinations.

Occurrence of Bacteria

Bacteria are broad spread in their occurrence. They are present almost all over, in air, land, water, oil deposits, food, decaying organic matter, plants, humans, and animals. Their kind and number differ according to the area and ecological conditions. Some bacteria are constantly present and contribute towards the natural flora. Others exist in specific environments such as warm springs, alkaline/ acidic soil, highly saline environments, in extremely polluted soils and waters.

Mesophiles, or mesophilic bacteria, are the bacteria responsible for a lot of human infections. They grow in moderate temperatures, around 37 ° C. This is the temperature level of the human body.


Extremophiles, or extremophilic bacteria, can endure conditions considered too extreme for most life types.

Thermophiles can reside in high temperatures, up to 75 to 80 ° C, and hyperthermophiles can survive in temperature levels as much as 113 ° C.

Deep in the ocean, bacteria live in total darkness by thermal vents, where both temperature and pressure are high. They make their own food by oxidizing sulfur that originates from deep inside the earth.

Some other extremophiles are:

  • halophiles, present in a salty environment.
  • acidophiles, some of which live in environments as acidic as pH 0.
  • alkaliphiles, residing in alkaline environments approximately pH 10.5.
  • Extremophiles can make it through where no other organism can.
Types of Bacteria

The bacteria are divided into 3 categories.

  1. Cocci

The cocci are spherical or oval bacteria having one of a number of unique arrangements based upon their planes of division. If division remains in one plane it will produce either a diplococcus or streptococcus arrangement. When cocci occur in sets then the plan is diplococcus, whereas when cocci form a long chain of cells then the arrangement is called streptococci. When the division of cells is in two planes it will produce a tetrad arrangement. A tetrad is a square of 4 cocci.



Thirdly, when the division is in 3 planes, it will produce a sarcina arrangement. Sarcina is a cube of 8 cocci. When division occurs in random planes, it will produce a staphylococcus arrangement in which cocci are set up in irregular, often grape-like clusters.

Examples: Diplococcus pneumoniae and Staphylococcus aureus are some examples of cocci.

  1. Bacilli

Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or diplobacillus. Bacillus is a single cell of bacteria. Streptobacillus is a chain of bacilli. When rod-shaped bacteria occur in pairs then the arrangement of cells is called diplobacilli.

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Examples: Examples of rod-shaped bacteria are Escherichia coli, Bacillussubtilis, Pseudomonas.


  1. Spiral

The spiral-shaped bacteria are spirally coiled. Spirals can be found in one of 3 forms, a vibrio, a spirillum, or a spirochete. Vibrio is a curved or comma-shaped rod. Spirillum is a thick, stiff spiral. The spirochete is a thin, flexible spiral.

Examples: Examples of spiral-shaped bacteria are Vibrio, Hyphomicrobium.


Structure of Bacteria

All bacterial cells inevitably have a cell membrane, cytoplasm, ribosome, and chromatin bodies. The bulk have a cell wall, which offers shape to the bacterial cell. Particular structures like capsule, slime, flagella, pili, fimbriae, and granules are not present in all bacteria.


Bacteria vary in size from about 0.1 to 600 µm over a single dimension. Bacteria vary in size as much as fit. The smallest e.g., some members of the genus Mycoplasma are about 100 to 200 nm in diameter, around the size of the largest viruses (poxviruses). Escherichia coli, a bacillus of about average size, is 1.1 to 1.5 µm large by 2.0 to 6.0 µm long. Some spirochetes sometimes reach 500 µm in length whereas Staphylococci and Streptococci are 0.75 – 1.25 µm in diameter.

Flagella and their functions

These are very thin, hair-like appendages. They come out through cell wall and stem from the basal body, structure simply beneath the cell membrane in the cytoplasm. They are comprised of the protein flagellin. On the basis of the presence of flagella, pattern of the accessory of flagella, and the number of flagella present, bacteria are categorized into various taxonomic groups.

Atrichous means bacteria are without any flagella.

When a single polar flagellum is present then the condition is called monotrichous.

If tuft of flagella exists only at one pole of bacteria then these are lophotrichous flagella.

Amphitrichous is a condition when a tuft of flagella at each of two poles exists.

In peritrichous form, flagella surround the entire cell.


Most of the bacilli and spiral-shaped bacteria have flagella. Cocci really hardly ever have flagella.

The main function of flagella is to help in motility. With the assistance of flagella, flagellate bacteria can also detect and relocate the action to chemical signals which is a type of behavior called Chemotaxis.

Pili and their Functions

These are hollow, nonhelical, filamentous appendages. Pili are smaller than flagella and are not associated with motility. True pili are only present on gram-negative bacteria. They are made up of a unique protein called pilin. They are primarily associated with a mating process between cells called the conjugation process. Some pili work as a means of attachment of bacteria to various surface areas.


The cell wall: The external wrapping of bacteria

Bacterial surface and walls are extremely varied. Collectively complexes of layer external to the cell protoplasm are called as cell envelope and include capsule, slime, and cell wall.

Capsule: Bacteria produce pill, which is comprised of duplicating polysaccharide units, and of protein, or of both, the pill is firmly bound to the cell. It has a thicker, gummy nature that provides sticky characters to colonies of encapsulated bacteria. It provides pathogenicity to bacteria.

Slime: Some bacteria are covered with a loose, soluble shield of macromolecules which is called slime and slime supplies greater pathogenicity to germs and protects them against phagocytosis.

Cell Wall: Below the extracellular compounds and external to the cytoplasmic membrane cell wall exists. It is a rigid structure. It identifies the shape of the bacterium. The cell wall also secures the cells from osmotic lysis. The cell wall is only missing in mycoplasmas.

The cell walls of most bacteria have a special macromolecule called peptidoglycan. Its quantity differs in different types of bacteria. It is composed of a framework of long glycan chains cross-linked with peptide fragments. The intact cell wall also consists of chemical constituents such as sugar molecules, teichoic acid, lipoproteins, and lipopolysaccharides, which are connected to peptidoglycan.

Cell walls of archaeobacteria are different from eubacteria. They do not contain peptidoglycan. Their cell walls are composed of proteins, glycoproteins, and polysaccharides.

Cell Membrane

Simply beneath the cell wall is the cell membrane or plasma membrane. It is very thin, flexible, and totally surrounds the cytoplasm. The plasma membrane is extremely fragile in nature any damage to it leads to the death of the organism. Bacterial membranes differ from eukaryotic membranes in not having sterols such as cholesterol.

The cell membrane regulates the transportation of proteins, nutrients, sugar, and electrons or other metabolites. The plasma membranes of bacteria also contain enzymes for the respiratory metabolic process.

Cytoplasmic matrix

The cytoplasm of the prokaryotic cell does not have membrane-bound organelles and cytoskeleton (microtubules). The cytoplasmic matrix is the substance present in between the plasma membrane and the nucleoid. It has a gel-like consistency. Small molecules can move through it quickly. The plasma membrane and everything present within it are called protoplasts. Thus, the cytoplasmic matrix is a huge part of the protoplast. Other big discrete structures such as chromatin/ nuclear body, ribosomes, mesosomes, and granules and nucleoids are present in this matrix.



A bacterial cell unlike the cells of eukaryotic organisms lacks discrete chromosomes and a nuclear membrane. The nuclear material or DNA in bacterial cells occupies a position close to the center of the cell. This product is a single, circular, and double-stranded DNA molecule. It aggregates as an irregular shaped thick area called the nucleoid. This chromatin body is actually a very long molecule of DNA that is tightly folded so regarding it inside the cell element. Since bacteria have a single chromosome, they are haploid.


Many bacteria include plasmids in addition to chromosomes. These are the circular, double-stranded DNA particles. They are self-replicating and are not vital for bacterial growth and metabolism. They frequently consist of drug resistant, heavy metals, disease, and insect resistant genes on them.

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Ribosomes are composed of RNA and proteins. Some might be loosely attached to plasma membranes. They are protein factories. There are countless ribosomes in each healthy growing cell. They are smaller than eukaryotic ribosomes.


The cell membrane invaginates into the cytoplasm forming structure called as mesosome. Mesosomes are in the kind of vesicles, tubules, or lamellae. Mesosomes are involved in DNA replication and cell division whereas some mesosomes are also involved in the export of exocellular enzyme. Respiratory enzymes are also present on the mesosomes.

Granules and storage bodies

Since bacteria exist in an extremely competitive environment where nutrients are usually in short supply. They tend to store extra nutrients when possible. These may be glycogen, sulfur, fat, and phosphate. In addition, cells consist of waste materials that are subsequently excreted. For example, common waste products are alcohol, lactic acid, and acetic acid.

Multiple Choice Questions (MCQs) with Answers:

  1. What is the primary classification of bacteria based on shape?
    • A) Mesophiles
    • B) Cocci
    • C) Extremophiles
    • D) Archaeobacteria
    • Answer: B) Cocci
  2. Who was the first to report microorganisms such as bacteria and protozoa using a simple microscope?
    • A) Robert Koch
    • B) Louis Pasteur
    • C) Antonie Van Leeuwenhoek
    • D) Joseph Lister
    • Answer: C) Antonie Van Leeuwenhoek
  3. Which scientist developed the germ theory of disease and isolated bacteria causing anthrax and tuberculosis?
    • A) Louis Pasteur
    • B) Antonie Van Leeuwenhoek
    • C) Robert Koch
    • D) Joseph Lister
    • Answer: C) Robert Koch
  4. What is the primary function of flagella in bacteria?
    • A) Attachment
    • B) Protection
    • C) Motility
    • D) Conjugation
    • Answer: C) Motility
  5. Which bacterial structure provides a gummy nature to bacterial colonies and contributes to pathogenicity?
    • A) Pili
    • B) Flagella
    • C) Capsule
    • D) Slime
    • Answer: C) Capsule
  6. Which category of bacteria can survive in high temperatures, up to 75 to 80 °C?
    • A) Mesophiles
    • B) Thermophiles
    • C) Extremophiles
    • D) Psychrophiles
    • Answer: B) Thermophiles
  7. Which of the following bacterial shapes is characterized by a square of 4 cocci?
    • A) Diplococcus
    • B) Streptococcus
    • C) Tetrad
    • D) Sarcina
    • Answer: C) Tetrad
  8. What is the composition of bacterial cell walls in eubacteria?
    • A) Peptidoglycan
    • B) Proteins and glycoproteins
    • C) Cellulose
    • D) Chitin
    • Answer: A) Peptidoglycan
  9. Where does the DNA in bacterial cells aggregate, forming a region called the nucleoid?
    • A) Nucleus
    • B) Cytoplasm
    • C) Ribosomes
    • D) Mesosomes
    • Answer: B) Cytoplasm
  10. What is the primary function of pili in bacteria?
  • A) Motility
  • B) Attachment
  • C) DNA replication
  • D) Protein synthesis
  • Answer: B) Attachment
  1. Which bacterial structure is composed of pilin proteins and is primarily associated with the conjugation process?
    • A) Capsule
    • B) Flagella
    • C) Pili
    • D) Slime
    • Answer: C) Pili
  2. What is the function of mesosomes in bacterial cells?
    • A) DNA replication and cell division
    • B) Motility
    • C) Protein synthesis
    • D) Attachment
    • Answer: A) DNA replication and cell division
  3. In which category of bacteria are the cell walls composed of proteins, glycoproteins, and polysaccharides instead of peptidoglycan?
    • A) Eubacteria
    • B) Archaeobacteria
    • C) Cocci
    • D) Bacilli
    • Answer: B) Archaeobacteria
  4. What is the primary function of ribosomes in bacterial cells?
    • A) DNA replication
    • B) Motility
    • C) Protein synthesis
    • D) Attachment
    • Answer: C) Protein synthesis
  5. Which bacterial shape is characterized by a thin, flexible spiral?
    • A) Vibrio
    • B) Spirillum
    • C) Spirochete
    • D) Cocci
    • Answer: C) Spirochete
  6. What is the size range of bacteria?
    • A) 1 to 10 µm
    • B) 10 to 100 µm
    • C) 0.1 to 600 µm
    • D) 600 to 1000 nm
    • Answer: C) 0.1 to 600 µm
  7. Which scientist developed four postulates forming the basis of the germ theory of disease?
    • A) Antonie Van Leeuwenhoek
    • B) Louis Pasteur
    • C) Joseph Lister
    • D) Robert Koch
    • Answer: D) Robert Koch
  8. What are extremophiles?
    • A) Bacteria that cause extreme diseases
    • B) Bacteria living in extreme environments
    • C) Bacteria resistant to extreme temperatures
    • D) Bacteria with extreme motility
    • Answer: B) Bacteria living in extreme environments
  9. Which bacterial arrangement occurs when cocci form a long chain of cells?
    • A) Streptococcus
    • B) Tetrad
    • C) Sarcina
    • D) Staphylococcus
    • Answer: A) Streptococcus
  10. What is the primary function of the bacterial capsule?
    • A) Attachment
    • B) Motility
    • C) Protection
    • D) Pathogenicity
    • Answer: D) Pathogenicity
  11. In which environment are acidophiles bacteria commonly found?
    • A) Salty environment
    • B) Alkaline environment
    • C) Acidic environment
    • D) Highly polluted environment
    • Answer: C) Acidic environment
  12. Which bacterial shape is characterized by a comma-shaped rod?
    • A) Vibrio
    • B) Spirillum
    • C) Spirochete
    • D) Cocci
    • Answer: A) Vibrio
  13. What is the structure that provides a sticky nature to bacterial colonies?
    • A) Pili
    • B) Flagella
    • C) Capsule
    • D) Slime
    • Answer: C) Capsule
  14. What are plasmids in bacterial cells?
    • A) Circular, double-stranded DNA particles
    • B) Ribosomes
    • C) Mesosomes
    • D) Granules
    • Answer: A) Circular, double-stranded DNA particles
  15. Which bacterial structure is primarily associated with the mating process between cells?
    • A) Capsule
    • B) Pili
    • C) Flagella
    • D) Mesosomes
    • Answer: B) Pili


Frequently Asked Questions (FAQs) related to the Bacteria Tutorial:

1. Overview:

  • Q: What is the significance of bacteria in different environments?
    • A: Bacteria are widely spread microorganisms found in diverse habitats, contributing to natural flora, and playing vital roles in air, land, water, and various organic matter.

2. Discovery of Bacteria:

  • Q: Who was the first to observe microorganisms and describe bacteria?
    • A: Antonie Van Leeuwenhoek, a Dutch researcher in 1673, was the first to observe and describe microorganisms, including bacteria, using a simple microscope.

3. Germ Theory of Disease:

  • Q: What is the germ theory of disease, and who developed it?
    • A: The germ theory of disease, developed by Robert Koch, states that specific microorganisms are causative agents of diseases and can be isolated, cultured, and identified.

4. Occurrence of Bacteria:

  • Q: Where are bacteria commonly found, and how do their types vary?
    • A: Bacteria are present in air, water, soil, organic matter, plants, and animals. Their types and abundance vary based on environmental conditions, with extremophiles thriving in extreme environments.
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5. Types of Bacteria:

  • Q: What are the three main shapes of bacteria?
    • A: Bacteria can be cocci (spherical), bacilli (rod-shaped), or spiral (spirally coiled). Pleomorphic bacteria lack a specific shape due to the absence of cell walls.

6. Structure of Bacteria:

  • Q: What are the components of the bacterial cell envelope?
    • A: The cell envelope includes the capsule, slime, and cell wall. The capsule is a gummy structure, while slime provides pathogenicity to bacteria.

7. Size:

  • Q: What is the size range of bacteria?
    • A: Bacteria vary in size from about 0.1 to 600 µm, with different species exhibiting diverse sizes and shapes.

8. Flagella and their Functions:

  • Q: What is the role of flagella in bacteria?
    • A: Flagella are thin, hair-like appendages that aid in bacterial motility and chemotaxis, helping them move toward or away from chemical signals.

9. Pili and their Functions:

  • Q: How are pili different from flagella, and what is their primary function?
    • A: Pili are hollow, filamentous appendages primarily associated with the mating process between bacterial cells, known as conjugation, and some also serve as attachment structures.

10. The Cell Wall: The External Wrapping of Bacteria:

  • Q: What is the composition and function of the bacterial cell wall?
    • A: The cell wall, present external to the cytoplasmic membrane, provides shape, protects against osmotic lysis, and is primarily composed of peptidoglycan, although archaeobacteria have different cell wall compositions.

11. Cell Membrane, Cytoplasmic Matrix, Nucleoid, Plasmid, Ribosomes, Mesosomes:

  • Q: What are the key structures within the bacterial cytoplasm, and how do they contribute to bacterial function?
    • A: Structures such as the cell membrane, cytoplasmic matrix, nucleoid, plasmid, ribosomes, and mesosomes play crucial roles in bacterial DNA replication, protein synthesis, and cellular processes.

12. Granules and Storage Bodies:

  • Q: Why do bacteria store extra nutrients, and what are some examples of stored materials?
    • A: Bacteria store nutrients like glycogen, sulfur, fat, and phosphate due to the competitive environment. They also contain waste materials, excreted as byproducts like alcohol, lactic acid, and acetic acid.



In this comprehensive tutorial on bacteria, the exploration begins with an overview highlighting bacteria’s ubiquitous presence and their diverse shapes—cocci, bacilli, and spiral. The tutorial proceeds to introduce essential structures like flagella and pili, classifying bacteria based on these appendages. The outer complex layer known as the cell envelope, consisting of capsule and slime, is discussed, emphasizing their roles in pathogenicity.

The historical journey of bacterial discovery is traced back to Antonie Van Leeuwenhoek’s pioneering work with microscopes. The significance of Louis Pasteur’s contributions, including the development of vaccines and advancements in fermentation industries, is acknowledged. The Germ Theory of Disease, established by Robert Koch, is explained through four postulates that remain fundamental in identifying the causal agents of diseases.

The occurrence of bacteria is explored across various environments, highlighting the adaptability of mesophiles and extremophiles to different temperature and pH conditions. The tutorial categorizes bacteria into three main types—cocci, bacilli, and spiral—detailing their unique arrangements and providing examples.

A deep dive into bacterial structure covers components such as the cell wall, capsule, slime, flagella, pili, and granules. The size variability of bacteria, ranging from nanometers to micrometers, is discussed. The functions of flagella and pili, crucial for motility and attachment, respectively, are elucidated.

The external wrapping of bacteria, the cell wall, is dissected into its various components, emphasizing the presence of peptidoglycan in most bacteria and the differences in cell walls of archaeobacteria. The discussion extends to the cell membrane, cytoplasmic matrix, nucleoid, plasmid, ribosomes, mesosomes, and granules, elucidating their roles within the prokaryotic cell.

Overall, this tutorial provides a comprehensive understanding of bacteria, spanning their discovery, types, occurrence, and intricate structural details, making it a valuable resource for those seeking knowledge in microbiology.