Bacteria Structure [ Simply explained]

Bacteria structure includes the following:

  1. Structures external to cell wall
  2. Cell wall
  3. Structures internal to cell wall


  1. Flagella
  2. Exo flagella
  3. Endo flagella
  4. Glycocalyx
  5. Capsule
  6. Slime layer
  7. Pilli
  8. Fimbriae


for further study in detail must read Flagella: An overview

It is the locomotory organ. It helps in motility. On the basis of locomotion, bacteria are classified as:

  • Motile bacteria
  • Non-motile bacteria
Flagella is presentFlagella is absent
The survival rate is better, Nutrition can be taken easily. they can move from adverse environmental conditions More flagella mean the survival rate increases For example peritrichous have a maximum survival rate  The survival rate is less
More chances to cause diseaseFewer chances to cause disease

On the basis of flagella, bacteria are classified as follows:


Bacteria that do not have flagella.


Bacteria that have single flagellum.


Bacteria thar have tuft of flagella at one end.


These bacteria have two or more than two flagella at both ends.


These bacteria have flagella all around the body.


Bacteria move in two ways:

  • RUN:

This is the straight-line movement of bacteria, which means bacteria don’t change their direction.


The jerking movement of bacteria due to the change in direction is called tumble (jump).

Run and tumble are due to the rotation of flagella either clockwise or anti-clockwise. Clockwise movements result in RUN and anti-clockwise movements result in TUMBLE.


The bacterial survival rate is maximum in liquids than in solids and then in gases. Bacteria are also affected by factors like light and chemicals that come from the outside. Bacteria contain enzymes for the recovery of their breakdown. Some of these enzymes need light and some work in the dark.

Bacteria also move in response to chemicals either organic or inorganic. Autotroph bacteria move due to the organic environment and heterotroph bacteria move due to the inorganic environment.

The movement of bacteria refers to as taxis. The movement due to nutrition is the positive taxis and movement due to adverse environment is the negative taxis.


Exo flagella have the following parts:

  • Filament
  • Hook
  • Basal body

It is present in the spirochete, and is also known as the axial filament. It is wrapped around the body of the bacteria and covered by a membrane or sheath. Rotation of the filament results in the movement of sheaths that move the spirochete in a spiral motion. Snake like movement is exhibited because of endo-flagella that is why it is known as a spirochete.


Treponema pallidum and Borrelia burgdorferi are spirochetes.


It is also known as the sugar coat. Many bacteria secrete a sticky substance on their surface that is known as the glycocalyx. It is a polymer of gelatin and composed of either polypeptide or polysaccharide or both. It is formed inside the cell and then secreted outside the cell wall. Hence it is present external to cell wall of the bacterium. It is described in two ways; capsule and slime layer.


If glycocalyx is strongly or firmly attached to the cell wall, it is known as a capsule.


If the sticky substance or glycocalyx is loosely attached to the cell wall, it is known as the slime layer.


          It plays a very important role in the virulence or pathogenicity of the bacterium. It protects the bacterium from the phagocytic effect of host immune system cells.


Streptococcus pneumonia can only cause pneumonia when the bacterium is covered by the polysaccharide layer of the capsule, otherwise, it can’t cause pneumonia.

          The sticky character of glycocalyx helps the bacteria to attach to the surfaces making biofilms. This type of glycocalyx that helps in making biofilms is known as an extracellular polymeric substance (EPS). This EPS helps the bacteria to form biofilms by surrounding them and helping them to survive in their natural environment by attaching to different surfaces. It also helps them to communicate.

          The attachment of bacteria helps them to grow on different surfaces like rocks, roots of plants, water pipes, tooth and even helps in attachment to other bacteria.

Gram negative bacteria have hair like appendages that are thinner and shorter than fimbriae that are used in the motility and genetic material transfer. These have proteins called pilin. This protein is attached helically to the core and is divided into two types, fimbriae, and pili.


These are either present on the poles of bacteria or can surround the whole cell. They vary in number in different prokaryotic cells. It helps in the attachment of bacteria hence it can facilitate the bacteria in forming biofilms on liquid, gases, or solids like rocks. They also facilitate bacteria to attach to the host body surfaces like epithelium or mucous membranes. This leads to the colonization of bacteria on that surface which results in the occurrence of the disease.


E. coli O157 adheres to the small intestine lining and causes severe diarrhea.


These are longer than fimbriae and can be one or two per bacterial cell. These are involved in the transfer of genetic material between bacteria and also help in motility. The pili bring the bacteria closer and help in the transfer of DNA from one bacterium to another, this process is known as conjugation and such pili are known as sex pili.

Cell wall

A lot of important things about bacteria have to do with their cell walls. In general, it helps determine the shape of a bacterium and gives it the strong structural support it needs to stay together even though its environment is constantly changing.

Most bacteria get their strength and stability from a unique macromolecule called peptidoglycan, which is found in their cell walls (PG). This substance is made up of a framework of long glycan chains that are linked together by short repeated peptide fragments.  Different types of bacteria have different amounts and different kinds of peptidoglycan. Gram positive cell wall and gram negative cell wall bacteria are differentiated by their cell wall structure

Because many bacteria live in water where there aren’t many solutes, they are always absorbing water through osmosis. If the peptidoglycan in the cell wall didn’t hold it together, it would break from the pressure inside.

Structures Internal to cell wall

The following structures are present internal to the cell membrane:


It is also known as the inner membrane, located inside the cell wall and encloses the cytoplasm or bacterial cell. The plasma membrane of bacteria is composed of phospholipids. Unlike eukaryotic cells, prokaryotic plasma membrane does not have sterols like cholesterol which is why prokaryotes have less rigid membranes than eukaryotes. Only a-typical bacteria i.e., Mycoplasma have sterols in their membrane.


Under an electron microscope, a plasma membrane seems like a double layer structure; two dark layers with a light space between them. The arrangement of phospholipid molecules is in two parallel rows which is why it is known as a lipid bilayer structure. The molecules of proteins are arranged in many ways and some proteins can easily be removed without disturbing the structure of the membrane. These proteins can be removed by mild treatments.  such proteins are known as peripheral proteins. There are some other proteins known as integral proteins that can only be removed by lipid bilayer disruption such as by using detergents.

On the outer surface of the membrane, many proteins and some lipids are attached to carbohydrates known as glycoproteins and glycolipids respectively. Glycoproteins and glycolipids help in cell protection and also lubricate the cell. They also help in interactions between cells.


  • It serves as the selective barrier, allowing only selective substances to enter or exit the cell. This happens through selective permeability like larger molecules cannot pass through the membrane because their size is larger than the pore size of integral proteins in the membrane, lipid soluble substances can pass easily, and ions can also pass but slowly.
  • The plasma membrane also plays an important role in the breakdown of larger molecules through enzymes and produces energy in the form of ATP.
  • Some bacteria have pigments and enzymes that are present in infoldings of membranes known as thylakoids, that are involved in photosynthesis.


The cytoplasm is known to be a substance inside the plasma membrane of the cell. It consists of 80% water and has enzymes (proteins), lipids, carbohydrates, low molecular weight substances, and high concentrations of inorganic ions.

The cytoplasm is aqueous, thick elastic, and semitransparent. It contains major structures like nucleoids (that contain DNA), ribosomes, and inclusion bodies (reserved deposits). Filaments of proteins are also present that are responsible for the helical or rod shaped structure of the bacterial cell.


It consists of a single circularly arranged thread of double stranded DNA that is long and continuous and known ad bacterial chromosomes. Nucleoid contains the genetic information of the cell that is necessary for the functions and structures of the bacterial cell. The bacterial chromosome does not have a nuclear membrane or nuclear envelope and also does not contain histones like eukaryotic chromosomes.

The shape of nucleoids can be elongated, spherical, or can have a shape like dumbbells. Plasma membrane proteins are responsible for DNA replication and new chromosome segregation during the division of cells. 20% of a cell’s volume consists of DNA in actively growing bacteria.

Bacteria also contain plasmid which is the extrachromosomal genetic molecule, circular and double stranded DNA molecule. Plasmids are not attached to the main chromosome of bacteria and can replicate independently. These are associated with the proteins of the plasma membrane. The genes on the plasmid are not essential for the survival of the bacterial cells. They can be transferred from one bacterium to other.  Plasmids can be lost or gained without causing any harm to cells.

They have many advantages:

  • Carry antimicrobial resistant genes
  • Plasmids are used in the manipulation of genes
  • Carry genes for tolerance of toxic metals
  • Have genes for toxin production
  • Contain genes for enzyme synthesis


Ribosomes are the sites for the synthesis of proteins. The actively growing cells which contain a high rate of synthesis of proteins have larger ribosomes’ number. Thousands of ribosomes are present in the cytoplasm of the prokaryotic cells and due to these ribosomes, the cytoplasm has a granular appearance.

Ribosomes consist of two subunits: one is a small subunit and the other one is a large subunit. Each subunit consists of protein and ribosomal RNA (rRNA). Prokaryotic ribosomes are smaller and less dense than eukaryotic ribosomes. They also contain a different number of proteins and rRNA molecules than eukaryotic cells.

The prokaryotic ribosomes are 70S ribosomes (S means Svedberg unit which is the relative sedimentation rate during ultra-high speed centrifugation. Rate of sedimentation is the function of weight, size, and shape of the molecules or particles). The smaller subunit of the 70S is 30S which has one rRNA and the larger subunit is 50S which has two rRNA molecules.


Cells accumulate excess nutrients and use them when the environment becomes deficient in nutrients. These reserve deposits are known as inclusions. There are many types of inclusions, as follows:

Metachromatic Granules:

Metachromatic granules are named from the fact they are stained red with some blue dyes like methylene blue. These granules are known to be volutin. Volutin refers to inorganic phosphate reserves that are used in ATP synthesis. These reserves are formed in an environment rich in phosphates. Such granules are present in fungi, protozoa, algae, and bacteria.


Sulfur granules are deposited in sulfur oxidizing bacterial cells and serve as reserves of energy. These bacteria formed energy by oxidizing sulfur or sulfur containing compounds known as sulfur bacteria.  


Many aquatic prokaryotes like cyanobacteria have hollow cavities known as gas vacuoles. Every vacuole has many rows of several gas vesicles that are hollow cylinders covered with protein. These vacuoles maintain the floating ability of cells in such a way that they may remain at depth in water which is sufficient for them to get an appropriate quantity of nutrients, light, and oxygen.


Acid fast bacteria require more lipids than others like mycobacterium. Excess lipids are reserved. The lipids are reserved in the form of butyric acid. Butyric acid presence can be checked by Sudan’s dye. Orange to red fluorescence will be observed.    


These are the reserves of starch and glycogen. The presence of these granules can be checked by an iodine test. Glycogen granules give red color and starch granules give blue color in the presence of iodine.


These inclusions have ribulose 1,5-bisphosphate carboxylase enzyme. This enzyme is used by photosynthetic bacteria like nitrifying bacteria and cyanobacteria for carbon dioxide fixation.


For further detail must read Endospore: Its definition, structure, and formation

When unfavorable environmental conditions come such as depletion in nutrition occur, Gram’s positive bacteria like Clostridium and Bacillus species, form resting cells known as endospores. These are dormant forms of bacteria. The endospores are formed inside the bacterial cell membranes. It is dehydrated cells with thick walls. Endospores can tolerate unfavorable conditions like high heat, dehydration, toxic chemicals, and radiation.

Endospore formation is the process of several hours and is known as sporulation and sporogenesis. Sporulation starts when any essential nutrient like carbon or nitrogen is depleted. Endospores can survive for thousands of years and when favorable conditions come can be converted into vegetative form.


  • Tortora, G. J., Funke, B. R., & Case, C. L. (2013). Microbiology: An introduction. Pearson.
  • Willey, J. M., Sandman, K. M., & Wood, D. H. (2020). Prescott’s microbiology (11th ed.). McGraw-Hill Education.
Ujala Shabbir
Ujala Shabbir

Ujala Shabbir is a microbiologist pursuing her MPhil in Microbiology at the University of Veterinary and Animal Sciences in Lahore. Her research is focused on the "preparation of calcium-conjugated FMDV vaccine and its comparative immunogenicity with other vaccine delivery systems".
She completed her Bachelor of Science degree in Applied Microbiology from the same university in 2021 with a CGPA of 3.72/4.
During her undergraduate studies, she took various microbiology-related courses and gained practical experience through internships.

Articles: 12

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