Structure of Bacteria

Bacteria is a unicellular prokaryotic organism. The structure of the bacteria consists of three major parts: Outer layer (cell envelope), cell interior, and additional structures.

Outer layer (Cell envelope): It includes the cell wall of bacteria and the plasma membrane beneath it. The outer envelope acts as a structural and physiological barrier protecting the interior of the bacterial cell from the outer external environment. The function of the cell envelope is to protect the bacteria from the osmotic lysis and give the bacteria its shape.
Cell interior: The internal structure of the bacterial cell consists of the protoplasm, which consists of the cytoplasm, cytoplasmic inclusions (mesosome, ribosomes, inclusion granules), and single circular DNA.
Additional structures: It includes capsule, flagella, fimbriae, and spores.

Structure of bacterial cell — Structure of bacteria

Table of Contents

Cell wall

The bacteria’s cell wall is the outer rigid and chemically complex structure. It is in between the cell membrane and the capsule/slime layer. The cell wall of the bacteria maintains the shape of the cell and protects the bacteria from changes in osmotic pressure. The bacteria’s cell wall makes up 20-30% of the cell’s dry weight. The major component of the cell wall is the peptidoglycan layer. Peptidoglycan is the disaccharide and consists of the N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) as the sugar derivatives. Based on the structure of the cell wall, bacteria are classified into Gram-positive and Gram-negative bacteria.

Differences between Gram-positive cell wall and Gram-negative cell wall

Gram-positive cell wall	Gram-negative cell wall
Gram-positive cell wall is thick, i.e., 15-80 nm.	Gram-negative cell wall is thin, i.e., 2 nm.
Peptidoglycan layer is present in abundance.	Peptidoglycan layer is significantly less.
Lipid content is 2-5%.	Lipid content is 15-20%.
Teichoic acid is present.	Teichoic acid is absent.
Treatment with lysozyme makes protoplast.	Treatment with lysozyme makes spheroplast.

A. Gram-positive cell wall

Peptidoglycan is the main component of the Gram-positive cell wall that is thick and constitutes about 40-80 % of the dry weight of the cell wall. The Gram-positive cell wall consists of Teichoic acids and Teichuronic acid.

Read more: Teichoic acid

B. Gram-negative cell wall

Gram-negative cell wall contains four main components: lipopolysachharides, outer membrane, lipoprotein layer, and peptidoglycan.

Lipopolysaccharides

LPS is present in the outer membrane of the Gram-negative bacteria. It comprises lipid A, core oligosaccharide, and O polysaccharide.

Read more: Lipopolysaccharide

Outer membrane

The outer membrane in the Gram-negative bacteria protects the cell from various environmental stress and harmful components. It prevents the cell from the entry of the antibiotics and also protects the cell lysis from the lysozyme. The outer membrane of Gram-negative bacteria is the bilayered structure, and it consists of the following parts: porins, outer membrane proteins (OMPs), and other proteins.

Porins: Porin is present in large amounts in the outer membrane. Its main role is in maintaining outer membrane permeability and allowing the passive diffusion of low molecular weight hydrophilic substances like sugars, amino acids, and certain ions.

Outer membrane proteins (OMPs): Outer membrane proteins (OMPs) are embedded in the outer membrane. They are involved in nutrient uptake, membrane homeostasis, and virulence. The four major proteins involved in transmembrane diffusion of maltose and maltodextrins are Omp C, D, F, and PhoE & LamB. Omp A protein anchors the outer membrane to the peptidoglycan layer.

Lipoprotein layer

Lipoprotein stabilizes the outer membrane of the Gram-negative cell wall and is composed of the Braun lipoprotein.

Peptidoglycan

Peptidoglycan layer in the Gram-negative bacteria is less than that of the Gram-positive bacteria. It is 2-3 nm thin and is bound by the lipoprotein and the plasma membrane.

Read more: Peptidoglycan

Periplasmic space

The space between the cell membrane and the outer membrane in Gram-negative bacteria is called periplasmic space. It consists of different enzymes and proteins. E.g., hydrolytic enzymes and beta-lactamase binding proteins.

C. Acid-fast bacilli cell wall

The cell of acid-fast bacteria is resistant to most detergents and strong acids. It cannot be stained with the Gram stain, so we need to perform the Ziehl-Neelsen staining for its visualization. Acid-fast bacteria like Mycobacterium tuberculosis consist of mycolic acids linked to the arabinoglycan protein. Due to its structural composition, it resists the acid alcohol, called the acid-fast bacilli.

D. Cell wall deficient forms

There are many types of bacteria which is devoid of the cell wall. The different cell wall deficient forms of bacteria are protoplast, spheroplast, Mycoplasma, and L-forms.

Protoplast	When Gram-positive bacteria are treated with lysozyme, protoplast is formed. In the protoplast, the cell wall is absent, but its cytoplasmic membrane is intact.
Spheroplast	When Gram-negative bacteria are treated with lysozyme, spheroplast is formed.
*Mycoplasma*	Mycoplasma is a naturally occurring bacteria in which a cell wall is absent.
L-forms	L-form is named after the Lister Institute, London, where it was first demonstrated in the Streptobacillus moniliformis.

Read more: Cell wall deficient forms

Plasma membrane or cytoplasmic membrane

A plasma membrane or cytoplasmic membrane is a thin semipermeable membrane lying beneath the cell wall. Sterol-like cholesterol is absent in the bacterial cell membrane except in Mycoplasma, which makes it different from the eukaryotic cell membrane. The cytoplasmic membrane is a barrier and controls the metabolites’ inflow and outflow. It also helps in electron transport and oxidative phosphorylation.

Cytoplasm

The cytoplasm of the bacterial cell contains the ribosomes, mesosomes, and intracytoplasmic inclusion bodies.

Ribosomes

The ribosome of bacteria is the 70S which is made up of 30S and 50S subunits. Ribosomes serve as the site for protein synthesis.

Mesosomes

Mesosomes are analogs of the mitochondria present in eukaryotes. It is the main site of the respiratory enzymes in bacteria.

Intracytoplasmic inclusion bodies

Inclusion bodies are involved in storage, acting as a source of carbon, inorganic substances, energy, and reduction of the osmotic pressure. Examples of intracytoplasmic inclusion bodies: are metachromatic granules or volutin granules, starch inclusions, and lipid inclusions.

DNA

The bacterial cell contains circular double-stranded DNA. All the genetic information is encoded in the DNA.

Plasmid

Plasmids are the extrachromosomal DNA that can replicate on its own. Its size ranges from 1.5 kilobases (kb) pairs to 120 kb pairs.

Mitochondria and endoplasmic reticulum are absent in the bacterial cell.

Capsule

The capsule is the thin outer layer present in some bacteria. It protects the bacteria from phagocytosis. Non-capsulated bacteria are more prone to phagocytosis than capsulated bacteria. By performing the capsule staining, it can be visualized by the light microscope in which the capsule is seen as a hollow structure.

Examples of the capsulated bacteria:

Streptococcus pneumoniae
Haemophilus influenzae
Klebsiella pneumoniae
Neisseria meningitidis

Flagella

Bacterial Flagella help in locomotion. It protrudes out from the cell wall. Flagella are made up of flagellin protein. Some bacteria in which flagella is absent are called atrichous bacteria. The bacteria in which flagella are present are termed the flagellated bacteria. Based on the number and the position of flagella, bacteria are classified into four types.

Monotrichous: one flagellum present on one side of the body. i.e., polar flagella
Lophotrichous: a cluster of the flagella is present on one end of the body
Amphitrichous: either one or cluster of flagella is present on both ends of the body
Peritrichous: flagella present all over the body

Pili (Fimbriae)

These are the hair-like filaments extending from the cell surface. Pili are shorter and straighter than the flagella. It is made up of pilins protein. Pili helps in the adherence of the bacteria to the host cells. Sex pili help in the transfer of bacterial DNA during conjugation.

Spores

Spores are the resting or dormant form of bacteria. During starvation or under unfavorable conditions, bacteria like Bacillus and Clostridium spp. produces spores. The spores of the bacteria are resistant to boiling, disinfectants, and heating. Bacterial spores can be visualized by endospore staining. Examples of the spore former bacteria: Bacillus stearothermophilus, Bacillus anthracis, and Clostridium tetani.

References

Madigan, M. T., Martinko, J. M., Stahl, D. A., & Clark, D. P. (2011). BROCK Biology of Microorganisms (13th edition). Benjamin Cumming.
Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th edition). Tata McGraw-Hill.
Nelson, D., Lehninger, A., Cox, M., & Nelson, D. (2005). Lecture notebook for Lehninger principles of biochemistry (4th edition ). W.H. Freeman.