- Bacteria are unicellular prokaryotes that reproduce through binary fission. Their size, shape, and structures vary between species. - External structures include capsules, flagella, pili, and endospores. Capsules protect pathogens and enable functions like adherence. Flagella and pili enable motility and attachment. - Endospores form in genera like Bacillus and Clostridium during nutrient depletion, allowing dormancy and dispersal. Endospores are highly resistant resting structures that germinate into vegetative cells under favorable conditions.

1. Protoplasts, Spheroplasts, and L Forms
If protoplasts from grampositive cells and spheroplasts (which retain outer membrane and
entrapped peptidoglycan) from gram-negative cells are able to grow and divide, they are called
L forms.
L forms are produced more readily with penicillin than with lysozyme, suggesting the need for
residual peptidoglycan.
Some bacterial species produce L forms spontaneously. The spontaneous or antibiotic-induced
formation of L forms in the host may produce chronic infections, the organisms persisting by
becoming sequestered in protective regions of the body. Because L-form infections are
relatively resistant to antibiotic (and phage) treatment, they present special problems in
chemotherapy.

2. Mycoplasmas
The mycoplasms are cell wall-lacking bacteria containing no peptidoglycan
Mycoplasms lack a target for cell wall-inhibiting antimicrobial agents (eg,penicillins
and cephalosporins) and are therefore resistant to these drugs.
Some, such as Mycoplasma pneumoniae, an agent of pneumonia, contain sterols in
their membranes.
The difference between L forms and mycoplasmas is that when the murein is allowed
to reform, L forms revert to their original bacteria shape, but mycoplasmas never do.

3. Capsule
Many bacteria synthesize large amounts of extracellular polymer when growing in their natural
environments. With one known exception (the poly-D-glutamic acid capsules of Bacillus
anthracis and Bacillus licheniformis), the extracellular material is polysaccharide. The terms
capsule are frequently used to describe polysaccharide layers;

4. Capsule
The capsule contributes to the invasiveness of pathogenic bacteria—encapsulated cells are protected from
phagocytosis unless they are coated with anticapsular antibody.
The capsule plays a role in the adherence of bacteria to surfaces in their environment, including the cells of
plant and animal hosts.
S mutans, for example, owes its capacity to adhere tightly to tooth enamel to its glycocalyx. S mutans and
other species form the layer known as plaque on the tooth surface;acidic products excreted by these bacteria
cause dental caries. The essential role of the glycocalyx in this process—and its formation from sucrose—
explains the correlation of dental caries with sucrose consumption by the human population.
Because outer polysaccharide layers bind a significant amount of water, the glycocalyx layer may also play
a role in resistance to desiccation.

5. Flagella
flagella - 12–30 nm in diameter, organs of locomotion
Three types of arrangement are known:
monotrichous (single polar flagellum),
lophotrichous (multiple polar flagella), and
peritrichous (flagella distributed over the entire cell
A bacterial flagellum is made up of several thousand molecules of a protein subunit
called flagellin. The flagellum is formed by the aggregation of subunits to form a
helical structure.
The flagellins of different bacterial species presumably differ from one another in
primary structure. They are highly antigenic (H antigens), and some of the immune
responses to infection are directed against these proteins.
Taxis
Chemotaxis (movement toward the optimal concentration
of source of a chemical attractant),
Aerotaxis (movement toward the optimal oxygen
concentration),
Phototaxis (movement of photosynthetic bacteria toward
the light),
Electron acceptor taxis (movement of respiratory bacteria
toward alternative electron acceptors, such as nitrate and
fumarate).

6. Many gram-negative bacteria possess rigid surface appendages called pili (L “hairs”) or fimbriae.
They are shorter and finer than flagella; similar to flagella, they are composed of structural protein
subunits termed pilins.
Minor proteins termed adhesins are located at the tips of pili and are responsible for the attachment
properties. Two classes can be distinguished:
• ordinary pili, which play a role in the adherence of symbiotic and pathogenic bacteria to host
cells, and
• sex pili, which are responsible for the attachment of donor and recipient cells in bacterial
conjugation
The virulence of certain pathogenic bacteria depends on of “colonization antigens,” which are ordinary
pili. In one group of gram-positive cocci, the streptococci, fimbriae are the site of the main surface
antigen, the M protein.
Pili of different bacteria are antigenically distinct and elicit the formation of antibodies by the host.
Some bacteria such as N gonorrhoeae, are able to make pili of different antigenic types (antigenic
variation) and thus can still adhere to cells in the presence of antibodies to their original type of pili.
Similar to capsules, pili inhibit the phagocytic ability of leukocytes.
Pili (Fimbriae)

7. Endospores
Members of several bacterial genera are capable of forming Endospores. The most common endospore-forming
genera of bacteria are
Bacillus (the obligately aerobic G+ rods)
Clostridium (the obligately anaerobic G+ rods)
Thermoactinomyces,
Sporolactobacillus,
Sporosarcina,
Desulfotomaculum,
Heliobacter
The process, sporulation, is triggered by near depletion of any of several nutrients (carbon, nitrogen, or
phosphorous). Each cell forms a single internal spore that is liberated when the mother
cell undergoes autolysis. The spore is a resting cell, highly resistant to desiccation, heat, and chemical agents; when
returned to favorable nutritional conditions and activated,
the spore germinates to produce a single vegetative cell.
Endospores can thus be thought of as the dormant stage of a bacterial life cycle: vegetative cell endospore vegetative
cell. Endospores are also easily dispersed by wind, water, or through theanimal gut.

8. The sporulation process begins when nutritional conditions become unfavorable, near depletion
of the nitrogen or carbon source (or both) being the most significant factor. Sporulation
occurs massively in cultures that have terminated exponential growth as a result of this near
depletion.
Sporulation involves the production of many new structures, enzymes, and metabolites along
with the disappearance of many vegetative cell components.
Differentiation of a vegetative cell of B subtilis into an endospore takes about 7 hours under
laboratory conditions. Different morphologic and chemical events occur at sequential stages of
the process.
The heat resistance of spores is partly attributable to their dehydrated state and in part to the
presence in the core of large amounts (5–15% of the spore dry weight) of calcium dipicolinate.

9. SUMMARY
• Eukaryotic cells are characterized by a membrane-bound nucleus, an endoplasmic
reticulum, 80S ribosomes, and plastids (mitochondria and chloroplasts). The plasma
membrane is characterized by the presence of sterols (cholesterol). Prokaryotic cells
lack a true nucleus and are haploid. The cytoplasm contains 70S ribosomes, and
they do not have mitochondria and chloroplasts.
• The major functions of the cell membrane of prokaryotic
cells are
(1) selective permeability and transport of solutes;
(2) electron transport and oxidative phosphorylation, in aerobic species;
(3) excretion of hydrolytic enzymes and other proteins;
(4) bearing the enzymes and carrier molecules that function in the biosynthesis
of DNA, cell wall polymers, and membrane lipids; and
(5) bearing the receptors and proteins of the chemotactic
and other sensory transduction systems.

10. SUMMARY
• Most bacteria are classified as gram-positive or gram– negative according to their
response to the Gram-staining procedure. The differences between these two groups are
reflected by fundamental differences in their cell envelopes.
• Gram-positive cell wall consists of a plasma membrane and thick peptidoglycan layer;
the gram-negative cell wall consists of a plasma membrane, a thin peptidoglycan layer,
and an outer membrane containing lipopolysaccharide (endotoxin). The space between
the plasma membrane and outer membrane is referred to as the periplasmic space.
• Many bacteria synthesize large amounts of extracellular polymers. When this polymer
forms a condensed, well defined layer surrounding the cell that excludes particles, it is
referred to as a capsule. Capsules are an important virulence factor and protect the cell
from phagocytosis.
• Cell surface structures such as pili and flagella are important for attachment and
motility, respectively.
• The formation of endospores is a characteristic of the genera Bacillus and Clostridium
and is triggered by near depletion of nutrients in the environment. Endospores
(spores) are resting cells, highly resistant to desiccation, heat, and chemical agents;
when returned to favorable nutritional conditions and activated, the spore germinates
to produce a vegetative cell.

11. THE PROKARYOTIC CELL
• Bacteria are unicellular, and most of them multiply by binary fission .
• Bacterial species are differentiated by morphology, chemical composition, nutritional requiremrnts, biochemical activities, and source of
energy.
The Size, Shape, and Arrangement of Bacterial Cells
• Most bacteria are 0.2 to 2.0 µ in diameter and 2 to 8 µ in length.
• The three basic bacterial shapes are coccus (spherical), bacillus (rod -shaped), and spiral (twisted) .
• Pleomorphic bacteria can assume several shapes.
Structures External to the Cell Wall
Glycocalyx
• The glycocalyx (capsule, slime layer, or extracellular polysaccharide) is a gelatinous polysaccharide and/or polypeptide covering.
• Capsules may protect pathogens from phagocytosis.
• Capsules enable adherence to surfaces, prevent desiccation, and may provide nutrients.
Flagella
• Flagella are relatively long filamentous appendages consisting of a filamrnt, hook, and basal body.
• Prokaryotic flagella rotate to push the cell.
• Motile bacteria exhibit taxis; positive taxis is movement toward an attractant, and negative taxis is movement away from a repellent.
• Flagellar (H) protein is an antigen.
Axial Filaments
• Spiral cells that move by means of an axial filament (endoflagellum) are called spirochetes.
• Axial filaments are similar to flagella, except that they wrap around the cell.
Fimbriae and Pili
• Fimbriae help cells adhere to surfaces.
• Pili are involved in twitching motility and DNA transfer.

12. The Cell Wall
Composition and Characteristics (pp.85-87)
• The cell wall surrounds the plasma membrane and protects the cell from changes in water pressure.
• The bacterial cell wall consists of peptidoglycan, a polymer consisting of NAG and NAM and short chains of aminoacids.
• Penicillin interferes with peptidoglycan synthesis.
• Gram-positive cell walls consist of many layers of peptidoglycan and also contain teichoic acids.
• Gram-negative bacteria have a lipopolysaccharide-lipoprotein phospholipid outer membrane surrounding a thin peptidoglycan layer.
• The outer membrane protects the cell from phagocytosis and from penicillin, lysozyme, and other chemicals.
• Porins are proteins that permit small molecules to pass through the outer membrane; specific channel proteins allow other molecules to
move through the outer membrane.
• The lipopolysaccharide component of the outer membrane consists of sugars (0 polysaccharides), which function as antigens, and lipid A,
which is an endotoxin.
Cell Walls and the Gram Stain Mechanism
• The crystal violet-iodine complex combines with peptidoglycan.
• The decolorizer removes the lipid outer membrane of gramnegative bacteria and washes out the crystal violet.
Atypical Cell Walls
• Mycoplasma is a bacterial genus that nalUrally lacks cell walls.
• Archaea have pseudomurein; they lack peptidoglycan.
• Acid-fast cell walls have a layer of mycolic acid outside a thin peptidoglycan layer.
• In the presenee of lysozyme, gram-positive cell walls are destroyed, and the remaining cellular eontents are referred to as a protoplast.
• In the presence of lysozyme, gram-negative cell walls are not completely destroyed, and the remaining cellular contents are referred to as a
spheroplast.
• L forms arc gram-positive or gram-negative bacteria that do not make a cell wall.
• Antibioties such as penicillin interfere with cell wall synthesis.

13. The Plasma (Cytoplasmic) Membrane
• The plasma membrane encloses the cytoplasm and is a lipid bilayer with peripheral and integral proteins (the fluid mosaic model).
• The plasma membrane is selectively permeable.
• Plasma membranes contain enzymes for metabolic reactions, such as nutrient breakdown, energy production, and photosynthesis.
• Mesosomes, irregular infoldings of the plasma membrane, are artifacts, not true cell structures.
• Plasma membranes can be destroyed by alcohols and polymyxins.
• Movement across the membrane may be by passive processes, in which materials move from areas of higher to lower concentrationand no
energy is expended by the cell.
• In simple diffusion, molecules and ions move until equilibrium is reached.
• In facilitated diffusion, substances are transported by transporter proteins across membranes from areas of high to low concentration.
• Osmosis is the movement of water from areas of high to low eoncentration across a selectively permeable membrane until equilibrium is
reached.
• In active transport, materials move from areas of low to high concentration by transporter proteins, and the cell must expend energy.
• In group transloea tion, energy is expended to modify ehemicals and transport them across the membrane.
Cytoplasm
• Cytoplasm is the fluid component inside the plasma membrane.
• The cytoplasm is mostly water, with inorganic and organic molecules, DNA, ribosomes, and inclusions.
The Nucleoid
• The nucleoid contains the DNA of the bacterial chromosome.
• Bacteria can also contain plasmids, which are cireular, extrachromosomal DNA molecules.
Ribosomes
• The cytoplasm of a prokaryote contains numerous 70S ribosomes; ribosomes consist of rRNA and protein.
• Protein synthesis occurs at ribosomes; it can be inhibited by eertain antibiotics.
Inclusions
• Inclusions arc reserve deposits found in prokaryotic and eukaryotic eells.
• Among the inclusions found in bacteria are metachromatic granules (inorganic phosphate), polysaccharide granules (usually glycogen or
starch), lipid inclusions, sulfur granules, carboxysomes (ribulose 1,5-diphosphate earboxylase), magnetosomes (FeJ04), and gas vacuoles.
Endospores
• Endospores are resting structures formed by some bacteria; they allow survival during adverse environmental conditions.
• The process of endospore formation is called sporulation; the return of an endospore to its vegetative state is called germination.