This document discusses the structure and composition of bacterial cells. It covers the key components of the bacterial cell envelope including the glycocalyx, cell wall, and cell membrane. The cell wall gives shape and structure, and its composition differs between gram-positive and gram-negative bacteria. The cytoplasm contains ribosomes for protein synthesis and may contain inclusion bodies like gas vacuoles or food reserves. The single, circular chromosome called the nucleoid contains the bacterial DNA. Some bacteria also contain extrachromosomal DNA in plasmids.

1. BACTERIA
Structure and Composition of bacterial
cell.
R.NAGALAKSHMI
ASST. PROF
THE OXFORD COLLEGE OF PHARMACY
BANGALORE

2. Definition and classification
Bacteria are prokaryotes, lacking well-defined nuclei and membrane-bound
organelles, and with chromosomes composed of a single closed DNA circle

3. 1. Cell Envelope - It is the outer covering of protoplasm of bacterial cell and consists of 3
components— glycocalyx, cell wall and cell membrane.
(i) Glycocalyx (Mucilage Sheath):
• It is the outermost mucilage layer of the cell envelope which consists of non-cellulosic
polysaccharides with or without proteins.
• Glycocalyx may occur in the form of loose sheath when it is called slime layer.
• If thick and tough, the mucilage covering is called capsule.
• Capsule - Some species of bacteria have a third protective covering, a capsule made up of
polysaccharides (complex carbohydrates). It protects bacterium from drying out and to
protect it from phagocytosis (engulfing) by larger microorganisms. The capsule is a major
virulence factor in the major disease-causing bacteria, such as Escherichia coli and
Streptococcus pneumoniae. Non-encapsulated mutants of these organisms are avirulent, i.e.
they don't cause disease.
• Glycocalyx gives sticky character to the cell. It is not absolutely essential for survival of
bacteria and other function include:
• (a) Prevention of desiccation,
• (b) Protection from phagocytes,
• (c) Protection from toxic chemicals and drugs,
• (d) Protection from viruses,
• (e) Attachment,
• (f) Immunogenicity and
• (g) Virulence.

4. (ii) Cell Wall: Composed of rigid cell wall made upof peptidoglycan, a protein-sugar
(polysaccharide) molecule.
• The wall gives the cell its shape and surrounds the cytoplasmic membrane,
protecting it from the environment.
• It also helps to anchor appendages like the pili and flagella, which originate in the
cytoplasm membrane and protrude through the wall to the outside.
• The strength of the wall is responsible for keeping the cell from bursting when
there are large differences in osmotic pressure between the cytoplasm and the
environment.
• Cell wall composition varies widely amongst bacteria which help to differentiate
between the two forms. When exposed to a gram stain, gram-positive bacteria
retain the purple color of the stain because the structure of their cell walls traps the
dye. In gram-negative bacteria, the cell wall is thin and releases the dye readily
when washed with an alcohol or acetone solution.
• Periplasmic space occurs between plasma membrane and cell wall.
• Wall is 20-80 nm thick in Gram positive bacteria. It is single layered and smooth.
• In Gram negative bacteria, wall is 8-12 nm thick, complex, wavy and two layered.
• The outer layer is also called outer membrane. It consists of lipopolysaccharides,
lipids and proteins.

5. (ii) Cell Wall: Conti…..
• The outer membrane has hydrophilic channels of 16-stranded (3-barrel proteins
called porins.
• The single layered cell wall of Gram positive bacteria and inner wall layer of Gram
negative is made up of pepidoglycan, proteins, non-cellulosic carbohydrates,
lipids, amino acids, etc.
• Peptidoglycan forms the structural network of the cell wall. It is also known as
murein or mucopeptide.
• Peptidoglycan consists of long glycan strands formed of repeating units of N-
acetyl glucosamine (NAG) and N-acetyl muranic acid (NAM). They are cross
linked by small peptide chains.
• Peptidoglycan constitutes 70-80% of wall in Gram positive bacteria. Lipid content
is little. 10-20% of wall in Gram negative bacteria is formed of peptidoglycan.
Lipid content is 20-30%.
• Amino acid present in the wall is diaminopimelic acid or lysine. In Gram positive
bacteria, the wall contains teichoic acids that form receptor sites and surface
antigens. In Mycobacterium and Nocardia, the wall contains long chain fatty acids
called mycolic acids.

6. • (iii) Plasma Membrane: It is selectively permeable covering of the cytoplasm
that forms the innermost component of cell envelope. Bacterial plasma membrane
or plasma lemma has a structure similar to that of a typical membrane. It is made
of a phospholipid bilayer with proteins of various types (extrinsic, integral, trans
membrane).
• It holds receptor molecules for detection and responding to different chemicals of
the surroundings. Bacterial membrane is metabolically active as it takes part in
respiration, synthesis of lipids and cell wall components.
2. Cytoplasm:
• It is crystallo-colloidal complex that forms the protoplasm excluding its nucleoid.
Cytoplasm is granular due to presence of a large number of ribosome's.
• Membrane bound cell organelles as found in eukaryotes are absent. However, all
biochemical pathways are found in prokaryotic cells.
• Various structures present in cytoplasm are as follows:
• (i) Mesosome
• ii) Ribosomes
• (iii) Chromatophores

7. • (i) Mesosome: It is a characteristic circular in growth from the plasma
membrane. It consists of vesicles, tubules and lamellae. Mesosme is of two types,
septal and lateral.
• Septal mesosome connects nucleoid with plasma membrane.
• It takes part in replication of nucleoid by providing points of attachment to the
replicated ones.
• Septal mesosome is also believed to help in septum formation.
• At the time of cell division, plasma membrane grows in the region where the
septal mesosme is present so that most probably it provides membranes for rapid
elongation.
• Lateral mesosme is not connected with nucleoid.
• It contains respiratory enzymes and is, therefore, often called chondrioid.
• It is believed to be equal to mitochondrion of eukaryotes. However, respiratory
enzymes are also present over the plasma membrane.

8. • ii) Ribosomes: They are small membrane less, submicroscopic ribonucleoprotein
entities having a size of 20 nm x 14-15 nm.
• Ribosomes are of two types, fixed and free.
• Fixed ribosomes are attached to the plasma membrane.
• Free ribosomes occur free in the cytoplasmic matrix. The ribosomes are 70S in
nature. Each ribosome has two subunits, larger 50S and smaller 30S.
• Ribosomes take part in protein synthesis.
• Free or matrix ribosomes synthesize proteins for intracellular use while fixed
ribosomes synthesize proteins for transport to outside.
• Ribosomes generally occur in helical groups called polyribosomes or polysomes.
In each polysome 4—8 ribosomes are attached to a single strand of messenger or
mRNA. It is a mechanism to synthesise several copies of the same protein.
(iii) Chromatophores: They are internal membrane systems of photosynthetic forms
which possess photosynthetic pigments.
• In purple bacteria the membranes are typical while in green bacteria they are non-
unit, non-lipid and proteinaceous.
• Chromatophores of green algae are called chromosomes.
• Photosynthetic pigments are bacteriochlorophyll, bacteriophaeophytin
(bacterioviridin) and carotenoids

9. 3. Nucleoid: It represents the genetic material of prokaryotes.
• Nucleoid consists of a single circular strand of DNA duplex which is supercoiled
with the help of RNA and polyamines to form a nearly oval or spherical
complex.
• The folding is 250-700 times. Polyamines or nucleoid proteins are different from
histone proteins.
• DNA of prokaryotes is considered naked because of its non-association with
histone proteins and absence of nuclear envelope around it.
• A cell can have 2 or more nucleoids but all are replicated copies of same
nucleoid.
• It is equivalent to a single chromosome of eukaryotes because nucleoid consists
of a single DNA double strand.
• Nucleoid may be directly attached to the plasma membrane or through the
mesosome.

10. 4. Plasmids: They are self-replicating, extra chromosomal segments of double
stranded, circular, naked DNA. Plasmids provide unique phenotypic characters
to bacteria.
• They are independent of main nucleoid.
• Some of them contain important genes like fertility factor, nif genes, resistance
factors and colicinogenic factors.
• Plasmids which can get associated temporarily with nucleoid are known as
episomes. Plasmids are used as vectors in genetic engineering.
5. Inclusion Bodies:
• They are non-living structures present in the cytoplasm.
• The inclusion bodies may occur freely inside the cytoplasm (e.g., cyanophycean
granules, volutin or phosphate granules, glycogen granules) or covered by 2-4
nm thick non-lipids, non-unit protein membrane (e.g., gas vacuoles,
carboxysomes, sulphur granules, PHB granules).
• On the basis of their nature, the inclusion bodies are of 3 types:
 gas vacuoles,
 inorganic inclusions and
 food reserve.

11. (i) Gas Vacuoles: They are gas storing vacuoles found in cyanobacteria, purple and
green bacteria and a few other planktonic forms. A gas vacuole is without any
covering of its own.
• The membrane is impermeable to water but is permeable to atmospheric gases.
• Gas vacuoles protect the bacteria from harmful radiations.
• They also constitute buoyancy regulation mechanism for their proper positioning
in water during daytime for photosynthesis.
(ii) Inorganic Inclusions: They include volutin granules, sulphur granules, iron
granules, magnetite granules, etc. Because of the ability to pick up different
colours with basic dyes, they are called metachromatic granules.
• Two common types of inorganic granules are volutin granules and sulphur
granules.
• Volutin granules are polymetaphosphates which function as storage reserve of
phosphate.
• Sulphur granules occur in bacteria living in sulphur rich medium like the one
which pick up hydrogen sulphide for obtaining reducing power in photosynthesis.
• Iron granules are similarly found in those bacteria which metabolise iron
compounds for obtaining energy. Aquaspirillum magnetotacticum contains
magnetosomes, which are vesicles having magnetite. The granules help the
bacteria to orientate themselves along geomagnetic lines.

12. (iii)Food Reserve:
• Blue green algae have cyanophycean starch or α-granules, β- granules or lipid
globules and cyanophycin or protein granules. In bacteria, starch is replaced by
glycogen. Neutral fats are absent. Instead poly-beta-hydroxy-butyrate or PBH
granules are present. A biodegradable plastic can be prepared from PBH. Protein
granules are present. Carboxysomes occur in photosynthetic forms.
6. Flagella
• Bacterial flagella are uni-stranded, equivalent to a single micro- tubular fibre. It is
about 20 nm (0.02 µm) in diameter and 1-7µm in length. Bacterial flagellum is
made up of 3 parts— basal body, hook and filament. Basal body is like a rod. It is
inserted in the cell envelope. The basal body bears ring-like swellings in the
region of plasma membrane and cell wall.
• There are two pairs of rings (L and P ring in cell wall and S and M rings
embedded in cell membrane) in Gram negative bacteria and only a single pair of
rings (S and M rings embedded in cell membrane) in Gram positive bacteria.
Hook is curved tubular structure which connects the filament with the basal body.
It is the thickest part of flagellum.

13. • Filament part is long tubular structure which causes turbulence in the liquid
medium. It is made up of protein called flagellin. Protein molecules are globular.
They are arranged in 3-8 spiral rows. It is believed that bacterial flagella perform
rotation type movement that brings about backward pushing of the water. It results
in the bacterium moving forward.

14. 7. Pili: The two terms have been used interchangeably for bacterial appendages which
are not involved in locomotion.
• Pili are small hair like projections emerging from the outside cell surface.
• Actually, pili (singular-pilus) are longer, fewer and thicker tubular outgrowths which
develop in response to F+ or fertility factor in Gram negative bacteria.
• They are made up of protein pilin.
• A donor bacterial cell having fertility factor develops 1-4 pili.
• Being long (18-20 (xm) they are helpful in attaching to recipient cell and forming
conjugation tube.
• Without pili, many disease-causing bacteria lose their ability to infect because
they're unable to attach to host tissue.
• Specialized pili are used for conjugation, during which two bacteria exchange
fragments of plasmid DNA.

15. 8. Fimbriae: Fimbriae are small bristle-like fibres sprouting from cell surface in
large number.
• There are 300-400 of them per cell. Diameter is 3-10 nm while length is 0.5-1.5
µm.
• They are distributed over the surface of the cell, and resemble fine hairs when
seen under the Electron microscope.
• Fimbriae are involved in attaching bacteria to solid surfaces (e.g., rock in water
body) or host tissues (e.g., urinary tract in Neisseria gonorrhoeae).
• Some fimbriae cause agglutination of RBC. They also help in mutual clinging of
bacteria.
• Fimbriae are essential for the virulence of some bacterial pathogens.[

16. 9. Ribosome: They are microscopic "factories" found in all cells, including bacteria.
• They translate the genetic code from the molecular language of nucleic acid to
that of amino acids—the building blocks of proteins.
• Proteins are the molecules that perform all the functions of cells and living
organisms.
• Bacterial ribosomes are similar to those of eukaryotes, but are smaller and have a
slightly different composition and molecular structure.
• Bacterial ribosomes are never bound to other organelles as they sometimes are
(bound to the endoplasmic reticulum) in eukaryotes, but are free-standing
structures distributed throughout the cytoplasm.
• There are sufficient differences between bacterial ribosomes and eukaryotic
ribosomes that some antibiotics will inhibit the functioning of bacterial ribosomes,
but not a eukaryote's, thus killing bacteria but not the eukaryotic organisms they
are infecting.

17. References:
• C:UsersuserDesktopMolecular Expressions Cell Biology_ Bacteria Cell
Structure.mhtml
• C:UsersuserDesktopComponents of Bacterial Cell_ 7 Components (With
Diagram).mhtml
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