This presentation is based on detailed ultrastructure of bacteria.

2. ◦ Bacterial cell ultrastructure is a fascinating topic that delves into the intricate details of these
tiny microorganisms. From their unique cell walls to their specialized internal structures, bacteria
have evolved to survive and thrive in a wide range of environments.
◦ In this presentation, we will explore the various components of bacterial cell ultrastructure and
how they contribute to bacterial physiology and pathogenesis. By understanding these features,
we can gain a deeper appreciation for the complexity and adaptability of these remarkable
organisms.

3. Cell Wall
◦ The bacterial cell wall is a complex and dynamic structure that provides structural support and protection to the cell. It is
composed of peptidoglycan, a unique polymer made up of alternating units of N-acetylglucosamine (NAG) and N-
acetylmuramic acid (NAM), cross-linked by short peptide chains.
◦ In addition to its structural role, the cell wall also plays a critical role in maintaining cellular homeostasis. It acts as a
semi-permeable barrier, regulating the movement of molecules in and out of the cell. Additionally, it contains enzymes
involved in important cellular processes such as cell division and nutrient uptake.

5. Cell Wall Biosynthesis
◦ Biosynthesis pathway of PG
in Escherichia coli:
The biosynthesis pathway
starts with the formation of
Park’s nucleotide in cytoplasm,
followed by binding to a lipid
component to produce lipid II.
◦ Finally, lipid II is translocated
across the cytoplasmic
membrane and then inserted
into the existing PG through
transglycosylation and
transpeptidation reactions.
◦ Abbreviations:
◦ PBPs, penicillin-binding
proteins; PG, peptidoglycan;
SEDS, shape, elongation,
division, and sporulation
enzyme family.
Radkovet al., 2018

6. Gram Positive Cell Wall
◦ Gram positive cell wall comprises of a thick cell wall which
is composed of peptidoglycan and large amounts of other
polymers such as teichoic acids.
◦ Teichoic acids are polymers of glycerol or ribitol joined by
phosphate groups. Some teichoic acids are covalently
linked to peptidoglycan and are referred to as wall teichoic
acids. Others are covalently connected to the plasma
membrane; they are called lipoteichoic acids. Wall of
teichoic acids extend beyond the surface of the
peptidoglycan. They are negatively charged and help give
the cell wall its negative charge. Teichoic acids are not
present in other bacteria.
◦ There is a presence of periplasmic space which lies
between the plasma membrane and the cell wall and is so
narrow that it is often not visible by electron microscopy.
The periplasm has relatively few proteins; this is probably
because the peptidoglycan sacculus is so porous that
many proteins translocated across the plasma membrane
pass through the sacculus.

7. Gram Negative Cell Wall
◦ Gram-negative cell walls are more complex than typical Gram-
positive walls. One of the most striking differences is the paucity
of peptidoglycan. The peptidoglycan layer is very thin (2 to 7 nm,
depending on the bacterium) and sits within the periplasmic
space.
◦ The periplasmic space is much larger than that of a typical Gram-
positive cell, ranging from about 30 to 70 nm wide. Some studies
indicate that it may constitute about 20 to 40% of the total cell
volume.
◦ The outer membrane lies outside the thin peptidoglycan layer. It
is linked to the cell by Braun’s lipoprotein, the most abundant
protein in the outer membrane. This small This external layer of
the Gram-negative cell is comprised of lipopolysaccharides
(LPSs). These large, complex molecules contain both lipid and
carbohydrate, and consist of three parts: (1) lipid A, (2) the core
polysaccharide, and (3) the O side chain.
◦ Lipid A contains two glucosamine sugar derivatives, each with
fatty acids and phosphate attached. The fatty acids of lipid A are
embedded in the outer membrane, while the remainder of the
LPS molecule projects from the surface.
◦ The core polysaccharide is Joined to lipid A and is constructed of
10 sugars, many of them unusual in structure.
◦ The O side chain or O antigen is a polysaccharide chain
extending outward from the core. It has several peculiar sugars
and varies in composition between bacterial strains.

8. Plasma Membrane
◦ The plasma membrane is a crucial component of bacterial cell ultrastructure. It acts as a selectively permeable barrier, controlling the
movement of molecules in and out of the cell. This is essential for maintaining cellular homeostasis and ensuring that metabolic processes can
occur efficiently.
◦ In addition to its role as a physical barrier, the plasma membrane also contains various proteins that are involved in transport, signaling, and
other important functions. For example, some transport proteins allow nutrients to enter the cell, while others pump out waste products or
toxins. Signaling proteins can detect changes in the environment and trigger appropriate responses within the cell.

9. Bacterial Membrane Transport
◦ Bacteria can transport nutrients into the cell even when the concentration
of a nutrient inside the cell is higher than the concentration outside. Thus,
they can move nutrients up a concentration gradient. This is important
because bacteria often live in nutrient-poor habitats. In view of the
enormous variety of nutrients and the complexity of the task, it is not
surprising that bacteria use several different transport mechanisms: passive
diffusion, facilitated diffusion, primary and secondary active transport, and
group translocation.
https://themedicalbiochemistrypage.org/wp-content/uploads/2020/04/membrane-transporters.jpg

10. Passive Diffusion
◦ Passive diffusion, often called diffusion or simple diffusion,
is the process by which molecules move from a region of
higher concentration to one of lower concentration; that
is, the molecules move down the concentration gradient.
◦ The rate of passive diffusion depends on the size of the
concentration gradient between a cell’s exterior and its
interior.
◦ A large concentration gradient is required for adequate
nutrient uptake by passive diffusion (i.e., the external
nutrient concentration must be high while the internal
concentration is low).
◦ Most substances cannot freely diffuse into a cell. However,
water and some gases, including O2 and CO2, easily cross
the plasma membrane by passive diffusion. H2O also
moves across membranes by passive diffusion.
https://i2.wp.com/cms.jackwestin.com/wp-content/uploads/2020/02/Passive-transport-
types.jpg?resize=1025%2C516&ssl=1

11. Facilitated Diffusion
◦ During facilitated diffusion, substances move
across the plasma membrane with the assistance
of transport proteins that are either channels or
carriers.
◦ Channels, are proteins that form pores in
membranes through which substances can pass;
they are often involved in facilitated diffusion.
Channels show some specificity for the
substances that pass through them, but this is
considerably less than that shown by carriers,
which are far more substrate specific.
◦ The rate of facilitated diffusion increases with the
concentration gradient much more rapidly and at
lower concentrations of the diffusing molecule
than that of passive diffusion.

12. Active Transport
◦ Active transport is the transport of solute molecules
to higher concentrations (i.e., against a
concentration gradient)with the input of metabolic
energy.
◦ Three types of active transport are observed in
bacteria: primary active transport, secondary active
transport, and group translocation.
◦ They differ in terms of the energy used to drive
transport and whether the transported molecule is
modified as it enters. Active transport resembles
facilitated diffusion in that it involves carrier
proteins.
https://www.sciencefacts.net/wp-content/uploads/2020/03/Active-Transport.jpg

13. Primary Active Transport
◦ Primary active transport is mediated by carriers
called primary active transporters. They use
energy provided by ATP hydrolysis to move
substances against a concentration gradient
without modifying them. Primary active
transporters are uniporters; that is, they move a
single molecule across the membrane ATP-
binding cassette transporters (ABC transporters)
are important primary active transporters.
◦ Most ABC transporters consist of two
hydrophobic membrane-spanning regions
(domains) with two ATP-binding domains facing
the cytoplasm. The membrane spanning
domains form a pore in the membrane, and the
ATP binding domains bind and hydrolyze ATP to
drive uptake. Most ABC transporters employ
solute-binding proteins to deliver the molecule
to be transported to the transporter.

14. Secondary Active Transport
◦ Secondary active transport couples the potential
energy of ion gradients to transport of substances
without modifying them. Secondary active
transporters are cotransporters. They move two
substances simultaneously: the ion whose gradient
powers transport and the substance being moved
across the membrane. When the ion and other
substance both move in the same direction, it is called
symport. When they move in opposite directions, it is
called antiport. On the other hand, when a single
molecules transport to one direction, it is called
uniport.

15. Group Translocation
◦ The distinguishing characteristic of group
translocation is that a molecule is chemically
modified as it is brought into the cell.
◦ The best-known group translocation system is the
phosphoenolpyruvate: sugar phosphotransferase
system (PTS), which is observed in many bacteria.
The PTS transports a variety of sugars while
phosphorylating them, using
phosphoenolpyruvate (PEP) as the phosphate
donor.
◦ PEP is a high-energy molecule that can be used to
synthesize ATP, the cell’s energy currency. However,
when it is used in PTS reactions, the energy
present in PEP is used to energize sugar uptake
rather than ATP synthesis.

16. Cytoplasm
◦ The cytoplasm is the gel-like substance that fills the inside of a bacterial cell. It contains
various molecules such as proteins, nucleic acids, and enzymes that are essential for cellular
processes. The cytoplasm also serves as the site of many metabolic reactions, including
protein synthesis, energy production, and nutrient storage.
◦ One key component of the cytoplasm is the bacterial chromosome, which contains the
genetic information necessary for the cell to carry out its functions. Other important
structures found in the cytoplasm include ribosomes, which are responsible for synthesizing
proteins, and inclusion bodies, which store nutrients and other molecules needed by the cell.
Additionally, the cytoplasm contains a network of filaments called the cytoskeleton, which
helps to maintain the shape and structural integrity of the cell.

17. Cytoplasmic Inclusions
◦ Cytoplasmic inclusions are
specialized structures found
within bacterial cells that serve
a variety of functions.
◦ These structures can be
composed of various materials,
including lipids, proteins, and
carbohydrates, and their
composition often reflects the
nutritional status of the cell.
https://biologyeducare.com/wp-content/uploads/2019/11/Cytoplasmic-inclusion-bodie.jpg

18. Storage Inclusions
Poly-β-hydroxybutyrate (PHB)
◦ The structure of PHB inclusions has been well studied, and
PHB granules are surrounded by a single-layered shell
composed of proteins.
◦ Much of the interest in PHB and other PHA granules is
due to their industrial use in making biodegradable
plastics.
Polyphosphate granules & Sulphur globules
◦ Polyphosphate granules and sulphur globules are inorganic
inclusions observed in many organisms.
◦ Polyphosphate granules store the phosphate needed for synthesis
of important cell constituents such as nucleic acids. In some cells,
they act as an energy reserve, and polyphosphate also can serve as
an energy source in some reactions, when the bond linking the final
phosphate in the polyphosphate chain is hydrolyzed.
◦ Sulfur globules are formed by bacteria that use reduced sulfur-
containing compounds as a source of electrons during their energy-
conserving metabolic processes.
Sulphur globules

19. Carboxysome
◦ Carboxysomes are present in many cyanobacteria and
other CO2-fixing bacteria.
◦ Their polyhedral coat is composed of three different
proteins and is about 100 nm in diameter. Associated
with the shell is the enzyme carbonic anhydrase that
converts carbonic acid and bicarbonate into CO2.
Recall that biological membranes allow the free
diffusion of CO2.
◦ Carboxysome shell prevents CO2 from escaping so it
can accumulate. Enclosed within the polyhedron is
the enzyme ribulose-1, 5-bisphosphate
carboxylase/oxygenase (RubisCO). RubisCO is the
critical enzyme for CO2 fixation, the process of
converting CO2 into sugar. Thus, the carboxysome
serves as a site for CO2 fixation.
Microcompartments
◦ Microcompartments are relatively large polyhedron formed by
one or more different proteins.
◦ It is enclosed within the protein shell containing one or more
enzymes.
◦ Microcompartments includes te ethanolamine utilization (Eut)
microcompartment and propandiol utilization (Pdu)
microcompartments and carboxyosome.

20. Other Inclusions
Gas Vacuoles
◦ The gas vacuole provides buoyancy to some aquatic bacteria, many
of which are photosynthetic.
◦ Gas vacuoles are aggregates of enormous numbers of small, hollow,
cylindrical structures called gas vesicles.
◦ Gas vesicle walls are composed of many copies of a single small
protein. These protein subunits assemble to form a rigid cylinder that
is impermeable to water but freely permeable to atmospheric gases.
◦ Cells with gas vacuoles can regulate their buoyancy to float at the
depth necessary for proper light intensity, oxygen concentration, and
nutrient levels. They descend by simply collapsing vesicles and float
upward when new ones are constructed.
• Mostly aquatic bacteria have this type of inclusion bodies which allows them to
orient according to the Earth’s magnetic field. These are intracellular chains of
magnetite (Fe3O4) particles.
• Magnetotactic bacteria in the Southern Hemisphere generally orient southward
and downward, with the same result. Magnetosomes are intracellular chains of
magnetite (Fe3O4) or greigite (Fe3S4) particles.
• They are around 35 to 125 nm in diameter and enclosed within invaginations
of the plasma membrane. The invaginations contain distinctive proteins that
are not found elsewhere in the plasma membrane. For the cell to move
properly within a magnetic field, magnetosomes must be arranged in a chain.
A cytoskeletal protein called MamK is responsible for establishing a framework
upon which the chain can form.
Magnetosomes

21. Nuclear Material
◦ The nuclear material of bacterial cells is not
organized into a true nucleus, but instead
exists as a single circular chromosome
located in the cytoplasm.
◦ This chromosome contains all of the genetic
information necessary for the cell to survive
and reproduce.
◦ The chromosomes of most bacteria are a
circle of double stranded deoxyribonucleic
acid (DNA), but some bacteria have a linear
chromosome.
◦ Most bacteria have a single chromosome,
but some bacteria, such as Vibrio cholerae
and Borrelia burgdorferi have more than one
chromosome.
◦ Many bacteria carry a single copy of their
chromosome (monoploid), but others are
known to be polyploid.

22. Extranuclear Genetic Elements
◦ Extranuclear genetic elements are DNA molecules that exist outside
of the bacterial chromosome. One such example of extranuclear
genetic elements are Plasmids.
◦ Plasmids are circular pieces of DNA that can replicate
independently of the bacterial chromosome. They often carry genes
for antibiotic resistance or virulence factors, which can be
transferred between bacteria through horizontal gene transfer.
◦ Most known plasmids are circular. Plasmids have relatively few
genes, generally less than 30. Their genetic information is not
essential to the bacterium, and cells that lack them usually function
normally.
◦ Thus, regulation of plasmid and chromosomal replication are
independent. However, some plasmids can integrate into the
chromosome. Such plasmids are called episomes and when
integrated are replicated as part of the chromosome.

23. Types of Plasmids
◦ Fertility Plasmids (F plasmids): They carry the fertility genes (tra genes) for conjugation, the transfer of genetic
information between two cells.
◦ Resistant Plasmids: They contain genes that can helps in the development of resistance to antibiotic or poisons.
◦ Col Plasmids: They contain genes that encodes for the antimicrobial polypeptides called bacteriocins, a protein
that kills other strains of bacteria. The col proteins of E. coli are encoded by proteins such as ColE1.
◦ Virulence Plasmids: They contains vir genes which turn the bacterium into a pathogen.
Example : Ti Plasmid and Ri Plasmid.
◦ Degradative Plasmids: They are able to digest unusual substances like toluene and salicylic acid.
Example : TOL Plasmid of Pseudomonas putida.

24. Ribosomes
◦ Ribosomes are essential organelles found in all bacterial cells that
play a crucial role in protein synthesis.
◦ Unlike eukaryotic ribosomes, bacterial ribosomes are smaller in size
and have a different composition of proteins and RNA. This allows
them to be targeted by antibiotics that selectively inhibit bacterial
protein synthesis.
◦ During translation, ribosomes read the genetic code on messenger
RNA and use it to assemble amino acids into a polypeptide chain.
The rate at which this occurs can be regulated to control the
amount of protein produced by the cell.
◦ Bacterial ribosome are compound of ribosomal RNA (rRNA)
molecules. The small subunit contains 16S rRNA; 23S rRNA and 5S
rRNA molecules are present in the large subunit. Approximately 55
proteins make up the rest of the mass of ribosome: 21 in the small
subunit and 34 in the large subunit.

25. Endospores
◦ Endospores are highly resistant structures
that some bacterial cells can form in
response to harsh environmental conditions.
◦ They are formed through a process called
sporulation, which involves the
transformation of a vegetative cell into a
dormant endospore.
◦ Endospores can survive extreme
temperatures, radiation, desiccation, and
chemical disinfectants.
◦ Once conditions become more favorable,
endospores can germinate and give rise to
new vegetative cells.
◦ Examples of bacteria that can form
endospores include Bacillus sp. and
Clostridium sp.

26. Capsule
◦ The capsule is a layer of polysaccharides that surrounds the
bacterial cell wall. It is a key virulence factor in many
pathogenic bacteria, as it helps the bacteria evade the host
immune system and resist phagocytosis.
◦ In addition to its role in pathogenesis, the capsule can also
help bacteria survive in harsh environments by protecting
them from desiccation and other environmental stresses.
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTwvx090umvDIqIxW-JS0aCyaOY2LdAexacpA&usqp=CAU

27. Cysts
◦ Cysts are a type of dormant cell that
can form in some bacterial species.
Unlike endospores, which are formed
as a survival mechanism when
conditions become unfavorable, cysts
are typically formed as part of the
normal life cycle of certain bacteria.
◦ In general, cysts are thought to provide
a protective environment for bacterial
cells during periods of stress or
nutrient deprivation. They may also
play a role in facilitating the dispersal
of bacterial populations, as cysts can
be more resistant to environmental
stresses such as desiccation and UV
radiation than their vegetative
counterparts.
https://en.wikipedia.org/wiki/Microbial_cyst#/media/File:Entamoeba_histolytica_01.jpg

28. Glycocalyx
◦ A glycocalyx, is a network of polysaccharides that project from
cellular surfaces of bacteria, which classifies it as a universal
surface component of a bacterial cell, found just outside the
bacterial cell wall.
◦ A distinct, gelatinous glycocalyx is called a capsule, whereas an
irregular, diffuse layer is called a slime layer. This coat is extremely
hydrated and stains with ruthenium red.
◦ Bacteria growing in natural ecosystems, such as in soil, bovine
intestines, or the human urinary tract, are surrounded by
glycocalyx-enclosed microcolony.
◦ It serves to protect the bacterium from harmful phagocytes by
creating capsules or allowing the bacterium to attach itself to inert
surfaces, such as teeth or rocks, via biofilms.
◦ Streptococcus pneumoniae attaches itself to either lung
cells, prokaryotes, or other bacteria which can fuse their
glycocalyces to envelop the colony.
https://upload.wikimedia.org/wikipedia/commons/9/91/Bacillus_subtilis.jp
g

29. Conclusion
◦ In conclusion, we have explored the fascinating world of bacterial cell ultrastructure.
We learned about the unique features of gram-positive and gram-negative bacteria,
including their cell walls and plasma membranes. We also discussed the important
roles played by the cytoplasm, ribosomes, and endospores in bacterial physiology and
survival.
◦ Additionally, we examined the function of endospores and how they contribute to
bacterial survival. Finally, we touched upon the various extranuclear genetic elements
present in bacterial cells. Overall, through this presentation a comprehensive overview
of bacterial cell ultrastructure and the importance of its various components are
emphasized.

30. References
◦ Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New
York: Garland Science; 2002.
◦ Madigan MT, Martinko JM, Bender KS, Buckley DH, Stahl DA. Brock Biology of
Microorganisms. 15th edition. Boston: Pearson; 2018.
◦ Joanne M. Willey, Kathleen M. Sandman (Author), Dorothy H. Wood (Author), Lansing
M. Prescott. Microbiology. 11th edition. New York: McGraw-Hill Education; 2020.
◦ Radkov AD, Hsu YP, Booher G, Van Nieuwenhze MS. Imaging Bacterial Cell Wall
Biosynthesis. Annu Rev Biochem. 2018 Jun 20;87:991-1014. doi: 10.1146/annurev-
biochem-062917-012921. Epub 2018 Mar 29. PMID: 29596002; PMCID: PMC6287495.