Prescott’s Microbiology, 9th Edition
3
Bacterial Cell Structure
CHAPTER OVERVIEW
This chapter provides a description of the prokaryotic cell, including bacterial and archaeal cells. The
discussion begins with the general features of size, shape, and arrangement of prokaryotic cells. Then the
general features of biological membranes and the specific features of prokaryotic membranes are given. The
cell wall is discussed with special reference to the differences between the cell walls of gram-positive bacteria
and gram-negative bacteria, differential staining reactions, and the nature of S-layers. Important internal
structures of prokaryotes, such as the cytoplasmic matrix, ribosomes, inclusion bodies, and the nucleoid are
described. Structures external to the cell, such as pili, fimbriae, and flagella lead into a discussion of cell
motility and chemotaxis. The chapter concludes with a description of the structure and functions of bacterial
endospores.
LEARNING OUTCOMES
After reading this chapter you should be able to:
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list the characteristics originally used to describe prokaryotic cells
form an opinion on the “prokaryote” controversy using current evidence about bacterial cells
distinguish a typical bacterial cell from a typical plant or animal cell in terms of cell shapes and
arrangements, size and cell structure
discuss the factors that determine the size and shape of a bacterial cell
describe the fluid mosaic model of membrane structure and identify the types of lipids typically found in
bacterial membranes
distinguish macroelements (macronutrients) from micronutrients (trace elements) and provide examples
of each
provide examples of growth factors needed by some microorganisms
compare and contrast passive diffusion, facilitated diffusion, active transport, and group translocation,
and provide examples of each
discuss the difficulty of iron uptake and describe how bacteria overcome this difficulty
describe peptidoglycan structure
compare and contrast the cell walls of typical Gram-positive and Gram-negative bacteria
relate bacterial cell wall structure to the Gram-staining reaction
compile a list of the structures found in all the layers of bacterial cell envelopes, noting the functions and
the major component molecules of each
create a table or concept map that identifies the components of the bacterial cytoplasm and describes their
structure, molecular makeup and functions
distinguish pili (fimbriae) and flagella
illustrate the various patterns of flagella distribution
compare and contrast flagellar swimming motility, spirochete flagellar motility, and twitching and gliding
motility
state the source of energy that powers flagellar motility
explain why bacterial chemotaxis is referred to as a “biased random walk”
describe the structure of a bacterial endospore
explain why bacterial endospores are of particular concern to the food indusctry and why endosporeforming bacteria are important model organisms
describe in general the process of sporulation
describe those properties of endospores that are thought to contribute to its resistance to environmental
stresses
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Prescott’s Microbiology, 9th Edition
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describe the three stages that transform an endospore into an active vegetative cells (germination)
CHAPTER OUTLINE
The “Prokaryote” Controversy
A. Bacteria and Archaea are grouped together as prokaryotes because they lack a nucleus and
generally do not have membrane-bound organelles or an endomembrane system
B. Bacteria and Archaea are very distinct and the broad term, prokaryote, is misleading
II. A Typical Bacterial Cell
A. Shape, arrangement, and size
1. Most prokaryotes are spheres (cocci) or rods (bacilli)
a. During the reproductive process, some cocci remain attached to each other to form pairs
(diplococci), chains, clusters, square planar configurations (tetrads), or cubic
configurations (sarcinae)
b. Some rods are so short and wide that they appear to be ovals (coccobacilli); most rods
occur singly, but some form pairs or chains
2. Other shapes include: curved rods (vibrios); rigid helices (spirilla), and flexible helices
(spirochetes); a few are flat; filaments, which can produce a network, are called a mycelium;
bacteria that exhibit more than one form are called pleomorphic
3. Prokaryotic cells vary in size (generally 1 to 5 µm) although they are typically smaller than
most eukaryotic cells; there have been reports of nanobacteria (0.2 µm to less than 0.05 µm in
diameter) and of very large prokaryotes (up to 750 µm in diameter)
4. Microbial sizes provide advantages such as greater surface-to-volume ratios (small cells) or
resistance to predation (large cells)
B. Cell organization
1. Bacterial and archaeal cells often share a common organization
2. Bacterial and archaeal cells are surrounded by a cell envelope with complex cell walls; they
lack many internal features common in eukaryotic cells
III. Bacterial Plasma Membranes
A. The cell envelope includes the plasma membrane, cell wall, and other external layers of the cell
B. The plasma membrane serves several functions:
1. It retains the cytoplasm and separates the cell from its environment
2. It serves as a selectively permeable barrier
3. It contains transport systems used for nutrient uptake, waste excretion, and protein secretion
4. It is the location of a variety of crucial metabolic processes including respiration,
photosynthesis, lipid synthesis, and cell wall synthesis
5. It contains special receptor molecules that enable detection of and response to chemicals in the
surroundings
C. The fluid mosaic model of membrane structure
1. This model, proposed by Singer and Nicholson, states that membranes are lipid bilayers with
floating proteins
2. Cell membranes are very thin (5–10 nm thick); the lipids are amphipathic, having hydrophilic
(interact with water) head groups and long hydrophobic (insoluble in water) tails; the head
groups face out of the membrane while the tails are buried in the membrane to form bilayers
3. Two types of proteins are associated with the lipid bilayer of the membrane: peripheral
(loosely associated and easily removed) and integral (embedded within the membrane and not
easily removed)
D. Bacterial lipids
1. The plasma membrane of bacteria consists of a phospholipid bilayer with hydrophilic surfaces
and a hydrophobic interior; bacterial membranes lack sterols, but many contain sterol-like
molecules called hopanoids that help stabilize the membrane
I.
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Prescott’s Microbiology, 9th Edition
2.
Bacteria do not have membranous organelles, but can have internal membrane systems with
specialized functions such as photosynthesis or respiration
IV. Bacterial Cell Walls
A. The cell wall is a rigid structure that lies just outside the plasma membrane; it creates characteristic
shapes for the bacteria and protects from osmotic lysis and toxins, often while increasing
pathogenicity
B. Overview of bacterial cell wall structure
1. The cell walls of most bacteria contain peptidoglycan (murein)
2. The cell walls of gram-positive bacteria and gram-negative bacteria differ greatly, but both
have the periplasmic space between the cell wall material and the plasma membrane
C. Peptidoglycan structure
1. Peptidoglycan is a polysaccharide polymer composed of two sugar derivatives with peptide
linkers; the polysaccharide polymer is a linear chain of alternating N-acetylglucosamine
(NAG) and N-acetylmuramic acid(NAM) moieties
2. Polysaccharide chains of peptidoglycan are cross-linked via a peptide interbridge attached to
the sugar backbone via a short peptide chain; these peptides contain some amino acids not
found in proteins; cross-linking adds strength to the peptidoglycan mesh
3. Variations in peptidoglycan structure are seen in certain bacterial groups and can be diagnostic
D. Gram-positive cell walls
1. They consist of a thick wall composed of many layers of peptidoglycan and large amounts of
teichoic acids
2. Techoic acids are polymers with a glycerol and phosphate backbone that span the cell wall and
likely enhance its structural stability
3. The periplasmic space of gram-positive cells is usually thin and contains only a few secreted
proteins (exoenzymes)
4. Most gram-positive bacteria also have a layer of proteins (S-layer proteins) on the outer
surface of the peptidoglycan that have a role in wall synthesis and virulence
5. Acid-fast bacteria include mycolic acids in their cell walls
E. Gram-negative cell walls
1. The gram-negative cell wall is more complex than the gram-positive cell wall; and has a thin
layer of peptidoglycan surrounded by an outer membrane
2. The periplasmic space is often wide and contains many different proteins; some are involved
with energy conservation or nutrient acquisition
3. The outer membrane is composed of lipids, lipoproteins, and lipopolysaccharides (LPS);
Braun's lipoprotein attaches the outer membrane to the peptidoglycan
4. LPS are large complex molecules composed of lipid A, core polysaccharides, and O antigen
carbohydrate side chains; LPS stabilize the outer membrane, protect against some toxins, and
can cause strong host immunological responses, acting as an endotoxin
5. The outer membrane is more permeable than the plasma membrane because of porin proteins
that form channels through which molecules smaller than 600 daltons can pass
F. Mechanism of Gram staining
1. After staining with crystal violet, constriction of the thick peptidoglycan layer of gram-positive
cells during decolorization prevents the loss of the crystal violet stain
2. The thinner, less cross-linked peptidoglycan layer of gram-negative bacteria does not retain the
stain, and thus more readily decolorized when treated with alcohol
G. Cell walls and osmotic protection
1. The cell wall prevents swelling and lysis of bacteria in hypotonic solutions; in hypertonic
habitats, the plasma membrane shrinks away from the cell wall in a process known as
plasmolysis
2. Bacteria without cell walls (by removal with lysozyme or through peptidoglycan synthesis
inhibition by penicillin) called spheroplasts are osmotically sensitive
3. Mycoplasmas lack a cell wall and tend to be pleomorphic
V. Cell Envelope Layers Outside the Cell Wall
A. Capsules and slime layers
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Prescott’s Microbiology, 9th Edition
1.
Capsules and slime layers (also known as glycocalyx) are layers of polysaccharides lying
outside the cell wall; they protect the bacteria from phagocytosis, desiccation, viral infection,
and hydrophobic toxic materials such as detergents; they also aid bacterial attachment to
surfaces and gliding motility
2. Capsules are well organized, whereas slime layers are diffuse and unorganized
B. S-layers
1. S-layers are regularly structured layers of protein or glycoprotein outside of the cell wall
2. S-layers protect against ion and pH fluctuations, osmotic stress, and hydrolytic enzymes; can
help maintain cell shape and envelope rigidity, promote cell adhesion, and protect against host
defenses
3. murein
VI. Bacterial Cytoplasm
A. The cytoplasmic matrix is the substance bounded by the plasma membrane; it is often packed with
ribosomes and inclusion bodies
B. The prokaryotic cytoskeleton has homologs of the elements seen in eukaryotes, filaments of actin,
tubulin, and intermediate filament proteins
C. Intracytoplasmic membranes are often involved in energy metabolism in nitrifying and
photosynthetic bacteria; unique ladderene lipids are found in anammoxosomes
D. Inclusions
1. Many inclusions are granules of organic or inorganic material that are stockpiled by the cell
for future use; some are not bounded by a membrane, but others are enclosed by a singlelayered membrane
2. Storage inclusions include glycogen (carbon storage), poly--hydroxybutyrate (carbon
storage), polyphosphate granules (energy and phosphorus storage), sulfur globules (wastes),
and cyanophycin granules (nitrogen storage in cyanobacteria)
3. Carboxysomes are microcompartments that accumulate carbon dioxide and the enzyme
ribulose-1,5-bisphosophate carboxylase
4. Gas vacuoles are composed of hollow protein sacs that are filled with gases and used for
buoyancy control in aquatic environments; magnetosomes are magnetite granules that provide
orientation in the Earth's magnetic field
E. Microcompartments
1. These are not used just for storing substances for later use by the cell.
2. Relatively large polyhedrom formed by one or more different proteins.
3. Best studied are the carboxysomes
a. Found in many of the cyanobacteria and other CO2 fixing bacteria.
b. Converts carbonic acid into CO2
F. Other Inclusions
1. Gas vacuole
a. Lots of small, hollow structures known as gas vesicles.
b. Provide buoyancy to some of the aquatic bacteria.
2. Magnetosomes
a. Intracellular chains of magnetite.
b. Act as tiny magnets
G. Bacterial Ribosomes
1. Ribosomes are the site of protein synthesis (translation)
2. They are complex structures consisting of protein and rRNA (ribosomal RNA)
3. Bacterial and archaeal ribosomes are 70S with 50S and 30S subunits; although differences are
apparent in ribosomal protein and rRNA components, ribosomes are similar across cells from
all three Domains
VII. External Structures
A. Pili and fimbriae are short, thin, hairlike appendages that mediate bacterial attachment to surfaces
(fimbriae) or to other bacteria during sexual mating (sex pili); fimbriae tend to be narrower in
diameter and shorter than sex pili
B. Flagella
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Prescott’s Microbiology, 9th Edition
1.
Flagella are threadlike locomotor appendages extending outward from the plasma membrane
and cell wall; they may be arranged in various patterns:
a. Monotrichous—a single flagellum
b. Amphitrichous—a single flagellum at each pole
c. Lophotrichous—a cluster (tuft) of flagella at one or both ends
d. Peritrichous—a relatively even distribution of flagella over the entire surface of the
bacterium
2. The flagellum consists of a hollow filament composed of a single protein known as flagellin;
the hook is a short, curved segment that links the filament to the basal body, a series of rings
that drives flagellar rotation
3. Flagellar synthesis involves many genes for the hook and basal body, as well as the gene for
flagellin; new molecules of flagellin are transported through the needlelike structure (like a
type III secretion system) so that the growth of the flagellum is from the tip, not from the base
4. Archaeal flagella tend to be thinner with more than one type of flagellin subunit, are not
hollow, and have unique hook and basal body analogs
VIII. Bacterial Motility and Chemotaxis
A. Motility
1. Motility in prokaryotes is not aimless; responses are made to temperature, light, oxygen,
osmotic pressure, and gravity; chemotaxis is directed movement of bacteria either toward a
chemical attractant or away from a chemical repellent
2. Prokaryotic flagella rotate to create motion (like a propeller); the direction of flagellar rotation
determines the nature of bacterial movement: counterclockwise rotation causes forward motion
(called a run) and clockwise rotation disrupts forward motion (resulting in a tumble)
3. The basal body is the motor that drives the flagellum; it is powered by a proton motive force
4. Although archaeal flagella rotate in both directions, run and tumble movement is not observed
5. Prokaryotes can move by other mechanisms: in spirochetes, axial fibrils cause movement by
flexing and spinning; other prokaryotes exhibit twitching or gliding motility, a mechanism
involving pili by which they move along solid surfaces
B. Chemotaxis
1. The concentrations of attractants and repellents are detected by chemoreceptors in the
periplasmic space or the plasma membrane
2. Directional travel toward a chemoattractant (biased random walk toward attractant) is caused
by lowering the frequency of tumbles (twiddles), thereby lengthening the runs when traveling
up the gradient, but allowing tumbling to occur at normal frequency when traveling down the
gradient
3. Directional travel away from a chemorepellent (biased random walk away from repellent)
involves similar but opposite responses
IX. Bacterial Endospores
A. The bacterial endospore is a special, resistant, dormant structure formed by some bacteria; it enables
them to resist harsh environmental conditions
B. Endospore structure is complex, consisting of an outer covering called the exosporium, a spore coat
beneath the exosporium, the cortex beneath the spore coat, and the spore cell wall, which is inside
the cortex and surrounds the core
C. Endospore formation (sporulation) normally commences when growth ceases because of lack of
nutrients; it is a complex, multistage process
D. Transformation of dormant endospores into active vegetative cells is also a complex, multistage
process that includes activation (preparation) of the endospore, germination (breaking of the
endospore’s dormant state), and outgrowth (emergence of the new vegetative cell)
CRITICAL THINKING
1.
Discuss why the plasma membrane is considered the external boundary of the cell, even though other
structures outside of the plasma membrane are considered part of the cellular anatomy.
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
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Prescott’s Microbiology, 9th Edition
2.
Discuss the nature of flagellar-mediated movement of bacteria. In particular, discuss how the direction of
rotation affects the direction of movement, and speculate how this movement is altered in the presence of
chemical attractants and repellents for which the bacterium has the appropriate chemoreceptors. Give
plausible mechanisms of the response to chemical attractants and/or repellents.
3.
How does the lack of cytoskeletal scaffolding proteins and phagocytosis in bacteria influence how
nutrients must be brought into the bacterial cell?
4.
Explain why growing cells may be more sensitive to harsh environmental conditions and why the
dormancy of endospore state of certain bacteria accomplishes preservation of the bacterium.
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
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