Ultrastructure of Bacteria and Its Morphological Classification


Bacteria are unicellular, free living, microscopic microorganisms capable of performing all the essential functions of life. Bacteria possess both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Bacteria are prokaryotic microorganisms that do not contain chlorophyll. They found in water, soil, air, food and all natural environments. In this article we will see the ultrastructure of bacteria and its morphological classification.

Ultrastructure of Bacteria


Many prokaryotic microorganisms produce amorphous organic exopolymers that are deposited outside the cell wall as capsules, slime layers, glycocalyx, or sugar coats. The term capsule refers to the layer that is tightly attached to the cell wall, whereas the slime layer is a loose structure that frequently diffuses into the growth medium.

Microcapsules are thin layers of less than 0.2 µm, while macrocapsules are thick layers ranging from 0.2 µm to 10 µm. Capsulated bacteria produce smooth colonies on the surface of agar media. These bacteria are usually non- motile as flagella remains unfunctional in the presence of capsule.

Development of capsule is dependent on the existence of favourable environmental conditions such as sugar concentrations, blood serum or growth in a living host. Water is the main component of bacterial capsule comprises 98 percent. Capsules may be composed of a complex polysaccharide or polypeptide or hyaluronic acid. Capsule protects the cell form desiccation and antibiotic exposure and also keeps bacterial cell nourished.

Cell Wall

The cell wall is a distinguishing feature of bacterial cells, providing structural support. The composition of the cell wall varies by bacterial species. For example, Gram-positive bacteria have a thick peptidoglycan layer, whereas Gram-negative bacteria have a thinner peptidoglycan layer surrounded by an outer membrane.

The outer layer or cell envelope consists of two components: a cell wall and a cytoplasmic or plasma membrane. The plasma membrane contains protoplasm, which includes cytoplasm and cytoplasmic inclusions such as ribosomes, mesosomes, granules, vacuoles, and the nuclear body.

The cell wall may be encased in a viscid layer, which can be loose slime or organized as capsules. Many bacteria have filamentous appendages called fimbriae or pili. Many bacteria also possess flagella which are organs of locomotion.

Cell Membrane

The cytoplasm is surrounded by a phospholipid bilayer known as the cell membrane, or plasma membrane. It governs the movement of chemicals into and out of cells.

The cytoplasmic membrane is a thin (5 to 10 nm) layer lining the inner surface of the cell wall and separating it from the cytoplasm. Prokaryotic plasma membranes are less rigid that Eukaryotic membranes due to lack of sterols.


The cytoplasm comprises a variety of cellular structures, including the nucleoid (where genetic material is stored), ribosomes (which are involved in protein synthesis), and inclusions (storage granules).

The bacterial cytoplasm is a suspension of organic and inorganic solutes in a viscous water solution. Bacterial cytoplasm differs from higher eukaryotic microorganisms in that it lacks the endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes. It houses the nucleus, ribosomes, proteins, and other water-soluble components and reserve material. Most bacteria contain extrachromosomal DNA, also known as plasmid DNA.

Bacterial chromosomes

The term genome refers to all of an organism’s genetic material. The bacterial genome is made up of a single molecule of chromosomal deoxyribonucleic acid, or DNA, and is found in the nucleoid, a region of the bacterial cytoplasm that can be seen under an electron microscope. Unlike eukaryotic nuclei, bacterial nucleoids lack nuclear membranes and nucleoli.

General, it is believed that each strand of the replicating bacterial DNA binds to proteins at the location that will eventually become the cell division plane during DNA replication. Par proteins, for instance, are involved in the division of bacterial chromosomes into opposing poles of the cell during cell division. They physically push or pull the chromosomes apart, much like the eukaryotic cell’s mitotic apparatus, by binding to the DNA replication origin.


Ribosomes are the most prominent components found in the cytoplasm of bacteria. They take part in the synthesis of proteins. Their quantity fluctuates according to the 15000/cell rate of protein synthesis. The number of ribosomes increases with the rate of protein synthesis.

These particles are 200 Å diameter ribonucleoproteins that are distinguished by their sedimentation characteristics. The 70S ribosomes (S-Svedberg unit, the sedimentation unit) are the name given to the bacterial ribosomes. These ribosomes split into 50S and 30S particles when exposed to a low concentration of magnesium.


In some bacteria, particularly in Gram-positive bacteria depending upon the growth conditions the membrane appears to be infolded at more than one point such, such folding are called as mesosomes.

Two different kinds of mesomes exist: the central mesosome, also known as the septal mesosome, and the peripheral mesosome, also known as the lateral mesosome. Central mesosomes are found close to the centre of the cell and extend deep into the cytoplasm. This mesosome forms cross walls during cell division and is linked to the bacterial chromosome. It also plays a role in DNA segregation.

Peripheral mesosomes are not connected to nuclear material and are not confined to a central location. Mesosomes are also called chondroids and are visualized only under an electron microscope.


Extrachromosomal, circular, double-stranded, self-replicating elements are called plasmids. They have heavy metal, antibiotic resistance, and fertility-related genes. The enzyme aids in the production of toxins and bacteriocins that are not found in the chromosomes of bacteria.

The majority of plasmids range in size from 1 to 500 kb and make up between 0.5 and 5.0% of all bacterial DNA. Plasmids can be found in any number of cells. Plasmids can be categorized into different types based on the functions they perform. They are among others F (Fertility) factor, R (Resistance) plasmids Col (Colicin) plasmids, Ri (Root-inducing) plasmids, and Ti (Tumour-inducing) plasmids.

Intracytoplasmic inclusions

Numerous bacterial species generate cytoplasmic inclusion bodies, which resemble granules in a circular shape. They vary among species and are contingent upon the state of the culture.

Volutin or metachromatic or Babes-Ernst granules are highly refractive, basophilic bodies consisting of polymetaphosphate. They appear reddish when stained with polychrome methylene blue or toluidine blue. Lipid granules consist mainly of polymerised ß-hydroxybutyric acid. Polysaccharide granules can be stained with iodine.


Acid or ribonuclease hydrolysis can be used to illustrate the bacterial nucleus. After staining, they can be viewed with an electron microscope or a light microscope. They usually appear as one body per cell, oval or elongated in shape. The single, circular molecule that makes up the genome is double stranded DNA. It may open under certain conditions to form a long chain about 1000 µm in length.


Bacterial species produce spores inside the cell are called endospores and outside the cell are called as exospores. Endospores are produced one endospore per cell and are highly refractile, thick-walled structures. After germinating, each bacterial spore develops into a single vegetative cell. Thus, rather than being a means of reproduction, sporulation in bacteria is a means of preservation.

Spores exhibit remarkable resistance against desiccation, staining, chemicals used for disinfection, radiation, and heat. They aid bacteria in long-term survival in adverse conditions and are still viable after centuries. All medically significant bacteria’s spores are eliminated by moist heat sterilization, which lasts 20 minutes at 121° C.

All endospores contain large amount of dipicolinic acid (DPA) with 10to 15 percent of the spore being dry weight. Spores can be detected by staining with a specially prepared stain i.e. Schaeffer-Fulton method along with heat.


Bacterial motility is facilitated by long, thin, hair-like cytoplasmic appendages called flagella. They are present on both Gram-positive and Gram-negative bacteria and have dimensions of 0.01 to 0.02 µm in diameter and 3 to 20 µm in length. These are the parts used for movement. Many bacteria, including some coccal forms, the majority of bacilli, and nearly all spirilla and vibrios, are motile due to the presence of flagella. Flagella are visible under a standard light microscope thanks to specific staining methods that thicken them via mordanting. They can be easily visualised under an electron microscope.

Pili or Fimbriae

Pili or Fimbriae range in size from 0.2 to 2 µm in length and have a diameter of about 5 to 7 nm. Gram-negative bacteria like Enterobacterium have hair-like appendages on the surface of their cell walls called pili or fimbriae. Fimbriae are composed of protein known as pilin. They are unrelated to motility and are found on motile as well as non-motile cells.

Morphological Classification of Bacteria

There are many types of bacteria, on the basis of their shape they are classified as-

Cocci: Cocci are small, spherical or oval cells. Example- Micrococcus.

Bacilli: The word bacilli derived from the Greek word ‘Bacillum’ which means ‘stick’. It is a rod-shaped cell. Example- Bacillus anthracis. When the length of the bacilli cell equal to the width of the cell they are called as coccobacilli. Example- Bracella

Vibrios: Vibrios are comma shaped or curved rods. Its name derived from its characteristic vibratory motility and resembles of comma shaped rods. Example- Vibrio comma

Spirilla: Spirilla are the long rigid rods with several curves or coils. Spirilla have helical shape and rapid bodies. Example- Spirillum ruprem.

Spirochetes: The spirochetes are a spiral form that is flexibly arranged. They are also slender in nature. Example- Treponema.

Actinomycetes: Bacteria with branching filaments are called actinomycetes. The existence of a hard cell wall is what gives it its distinctive shape. Example- Streptomyces species.

Mycoplasmas: Mycoplasmas are the cell wall deficient bacteria and due to that they do not possess a stable morphology. They occur as round or oval bodies with interlacing filaments. Example- Mycoplasma pneumoniae.


Bacteria are prokaryotic microorganisms that do not contain chlorophyll. They found in water, soil, air, food and all natural environments. Bacteria are prokaryotic microorganisms that do not contain chlorophyll. They found in water, soil, air, food and all natural environments. In the ultrastructure study of bacteria, we can know in detail about the capsule, cell wall, flagella, pili, cytoplasmic membrane, cytoplasm, ribosomes, mesosomes, nucleus, spores etc. We also able to understand the morphology of bacteria and its classification on the basis of shape like cocci, bacilli, vibrios, spirilla, spirochetes etc.

Frequently asked questions

What is the general size of the bacteria?

The general size of bacteria is ranges from 1 µm to 8 µm.

What is the function of capsule?

Capsule protects the cell form desiccation and antibiotic exposure and also keeps bacterial cell nourished.

What are plasmids?

Extrachromosomal, circular, double-stranded, self-replicating elements are called plasmids.

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