LECTURE 3:
Prokaryotic Cell Structure
and Function
Microbiology and Virology; 3 Credit hours
Atta-ur-Rahman School of Applied Biosciences (ASAB)
National University of Sciences and Technology (NUST)
COMPONENTS EXTERNAL TO THE CELL
WALL
• Capsules, Slime Layers, and S-Layers
• Pili and Fimbriae
• Flagella and Motility
– Chemotaxis
• Exospore
Capsules, Slime Layers
• Some prokaryotes have a layer of material
lying outside the cell wall.
• When the layer is well organized and not
easily washed off, it is called a capsule
• It is called a slime layer when it is a zone of
diffuse, unorganized material that is removed
easily.
• Glycocalyx; the glycoprotein, polysaccharide
Capsule, Slime Layers
Bacteria connected to each other and to the intestinal wall, by their
glycocalyxes, the extensive networks of fibers extending from the cells
Capsule
• Capsules are not required for growth and
reproduction in laboratory cultures, they do
confer several advantages when prokaryotes
grow in their normal habitats.
• Resist phagocytosis by host phagocytes
• Contain a great deal of water and can protect
against desiccation.
• Exclude viruses and most hydrophobic toxic
materials such as detergents.
• Aids in attachment to solid surfaces, facilitate
motility
S layer
• The surface structures on archaea
and bacteria are monomolecular
crystalline arrays of proteinaceous
subunits termed surface layers or
S-layers
• Since S-layers are monomolecular
assemblies of identical subunits,
they exhibit pores identical in size
and morphology.
• Has pattern like floor tiles
• S-layers are often lost during
prolonged cultivation under
laboratory conditions.
Surface Layer
• Because S-layer lattices possess pores identical in size and
morphology in the 2- to 8-nm range, they work as precise molecular
sieves, providing sharp cutoff levels for the bacterial cells
• A kind of periplasmic space is formed between the S-layer and the
plasma membrane where secreted macromolecules involved in
nutrient degradation, nutrient transport, and folding and export of
proteins could be stored
• Hyper-thermostable protease
• Adhesion sites for cell associated exoenzymes
• A quite interesting type of protective function was reported for the
S-layer lattice of Synechococcus GL-24, a cyanobacterium capable of
growing in lakes with exceptionally high calcium and sulfate ion
concentrations. The hexagonally ordered S-layer lattice functions as
a template for fine-grain mineralization and is continuously shed
from the cell surface to prevent clogging of further cell envelope
layers.
Surface Layer
The S-Layer. An electron micrograph of the S-layer of the
bacterium Deinococcus radiodurans
Pili and Fimbriae
Pili and Fimbriae
Fimbriae
• Procaryotes have short, fine,
hairlike appendages that are
thinner than flagella
• They are slender tubes composed
of helically arranged protein
subunits and are about 3 to 10
nm in diameter and up to several
micrometers long.
• A cell may be covered with up to
1,000 fimbriae
• Attach bacteria to solid surfaces
such as rocks in streams and host
tissues
• Locomotion
Pili
• Pili often are larger than
fimbriae (around 9 to 10 nm
in diameter).
• bacteria have about 1-10
sex pili (s., pilus) per cell
• They are genetically
determined by conjugative
plasmids and are required
for conjugation.
Pili and Fimbriae
The long flagella and the numerous shorter fimbriae are very evident in
this electron micrograph of the bacterium Proteus vulgaris
Flagella and Motility
• Bacterial flagella are slender, rigid structures,
about 20 nm diameter and up to 15 or 20 μm
long.
• Threadlike locomotor appendages extending
outward from the plasma membrane and cell
wall.
• Bacterial species often differ distinctively in
their patterns of flagella distribution and these
patterns are useful in identifying bacteria.
Arrangements of Flagella
THE BACTERIAL ENDOSPORE
• A number of gram-positive bacteria can form a
special resistant, dormant structure called an
endospore.
• These structures are extraordinarily resistant to
environmental stresses such as
– Heat, ultraviolet radiation,
– Gamma radiation, chemical disinfectants,
– Desiccation.
• In fact, some endospores have remained viable
for around 100,000 years
THE BACTERIAL ENDOSPORE
• Endospore position in the
mother cell (sporangium)
frequently differs among
species, making it of
considerable value in
identification.
• Endospores may be
centrally located, close to
one end (subterminal), or
definitely terminal
• Sometimes an endospore
is so large that it swells
the sporangium.
The Bacterial Endospore
Endospore Structure. Bacillus anthracis endospore
The Bacterial Endospore
• The spore often is
surrounded by a thin,
delicate covering called
the exosporium
• A spore coat lies beneath
the exosporium, is
composed of several
protein layers, and may
be fairly thick.
• The coat also is thought
to contain enzymes that
are involved in
germination
The Bacterial Endospore
• The cortex, which may occupy
as much as half the spore
volume, rests beneath the
spore coat.
• It is made of a peptidoglycan
that is less cross-linked than
that in vegetative cells.
• The spore cell wall (or core
wall) is inside the cortex and
surrounds the protoplast or
spore core.
• The core has normal cell
structures such as ribosomes
and a nucleoid, but is
metabolically inactive.
• As much as 15% of the spore’s dry weight consists of
dipicolinic acid complexed with calcium ions which is
located in the core.
• Aid in resistance to wet
• Heat, oxidizing agents, and sometimes dry heat
• Calcium dipicolinate stabilizes the spore’s nucleic
acids
• Specialized small, acid-soluble DNA binding proteins
(SASPs)
Endospore Germination
• The transformation of dormant spores into active
vegetative cells seems almost as complex a process as
sporogenesis.
• It occurs in three stages:
– activation,
– germination, and
– outgrowth
• Activation is a process that prepares spores for germination
and usually results from treatments like heating.
• It is followed by germination process is characterized by
spore swelling, rupture or absorption of the spore coat, loss
of resistance to heat and other stresses, release of spore
components, and increase in metabolic activity.
• Outgrowth; cell prepare for division