A lecture of an immunologist

2. General bacteriology
By
Amal abd elsatar Ibrahim

3. Cell wall
Functions of
The cell wall
of cytoplasm, cytoplasmic inclusions (mesosomes, ribosomes, inclusion bodies, and
vacuoles) and single circular DNA.
The cell wall gives bacterium rigidity and shape
The cell wall provides structural and physiological
of the cell and the external environment.
The cell wall is responsible for the Gram staining reaction.
The cell wall prevent damage to the underlying protoplast
It
Both bacterial cell wall and plasma membrane are called bacterial cell
envelope .
wall:
protoplasm which consists
protects bacteria against osmotic lysis
bacterial cell
encloses the
barriers between the protoplasm
N.B.

5. Bacterial cell wall structure
The Gram-positive bacteria cell wall:
It is a uniformly thick layer external to the plasma membrane.
It is composed mainly of peptidoglycan (murein), within which are trapped
a variety of proteins, polysaccharides, and teichoic acids (polymers of
glycerol or ribitol).

8. The Gram-negative bacteria cell wall:
It appears thin and multilayered.
It consists of a relatively thin peptidoglycan sheet (single layer) between
the plasma membrane and a phospholipid-lipopolysaccharide outer
membrane.
The lipopolysaccharides (LPS) in the outer membrane are the key surface
antigens and endotoxins of Gram negative bacteria. They are composed
of a lipid A (principally responsible for the endotoxin activity) and confer
the serological identity (O antigen) of individual strains.
Other components of the outer membrane include: porin proteins (allow
entry to the periplasmic space from the outside), and non-porin proteins
(such as penicillin-binding proteins).
No teichoic acid present.

11. Acid-fast cell wall
Mycobacterium species, Nocardia species, and Corynebacterium species
have a modified Gram-positive cell wall.
They have a higher cell-wall lipid content which is due to mycolic acids,
which can also confer virulence characteristics.
Acid-fast organisms are so-called because once stained with red carbol
fuchsin dye, they are resistant to decolourization with acid-alcohol, a
property conferred by the cell wall lipids.

13. Peptidoglycan (murein)
Peptidoglycan is a constituent of the bacterial cell wall.
Peptidoglycan is found in all bacterial species except Mycoplasma and
Ureaplasma species.
Peptidoglycan structure:
It comprises a carbohydrate backbone (glycan chain) connected by short
peptides to forms a net-like macromolecule around the cytoplasmic membrane.
Variations in the peptide linkages are responsible for different bacterial cell wall
characters.
The carbohydrate component consists of two alternating amino sugars, namely N-
acetylglucosamine (NAGA) and N-acetylmuramic acid (NAMA). The alternating
sugars are connected by a β- (1,4)-glycosidic bond.

16. Each N-
acetylmuramic
acid is attached to
a peptide chain of
three to five
amino acids (L-
alanine, D-
glutamic acid, D-
alanine, meso-
diaminopimelic
acid, or L-lysine).
Cross-linking between amino acids in different linear amino sugar chains
occurs with the help of the enzyme DD-transpeptidase and results in a 3-
dimensional structure that is strong and rigid. The specific amino acid
sequence and molecular structure vary with the bacterial species.
N.B. L-alanine, D-glutamic acid, meso-diaminopimelic acid, and D-alanine
in the case of Escherichia coli (a Gram-negative bacterium) or L-alanine, D-
glutamine, L-lysine, and D-alanine with a 5-glycine interbridge between
tetrapeptides in the case of Staphylococcus aureus (a Gram-positive
bacterium).

17. Peptidoglycan
Peptidoglycan provides the strength of bacterial cell wall.
Peptidoglycan stabilize
with the cell membrane
interaction and growth.
Peptidoglycan maintains osmotic pressure of the cytoplasm,
meaning it is able to allow sugars, amino acids, and other ions into
the cell wall and maintain its association
as well as having roles in cellular
Function:

18. the cell as needed.
Peptidoglycan is also
reproduction.
involved in binary bacterial cell
fission during

19. Peptidoglycan Importance:
The peptidoglycan layer is substantially thicker in Gram-
positive bacteria (20 to 80 nanometers) than in Gram-
negative bacteria (7 to 8 nanometers). Thus, presence of
high levels of peptidoglycan is the primary determinant of
the characterization of bacteria as Gram-positive.
It is important in serotyping purposes in Gram-positive strains
because the specific amino acid sequence and molecular
structure vary with the bacterial species.

20. N.B. The action of Lysozyme on bacterial
cell wall:
Lysozyme is enzyme found in tears and constitutes part of the body's innate
immune system exerts its antibacterial effect by breaking the glycosidic bonds
between N-acetylglucosamine and N-acetylmuramic acid in the peptidoglycan.
Thus, Gram-positive bacteria has high susceptibility to lysozyme.
However, Gram-negative bacteria can be affected by the action of lysozyme
because the lipopolysaccharides in the cell wall requires calcium for their stability.

21. So, the deficiency in calcium (due to presence of chelating agents as EDTA) leads
to liberation of LPS and so peptidoglycan can be degraded by lysozyme.

22. Penicillin-binding proteins (DD-
transpeptidases)
Some antibacterial drugs such as penicillin interfere with the
production of peptidoglycan by binding to bacterial enzymes
known as penicillin-binding proteins or DD-transpeptidases.
Penicillin-binding proteins form the bonds between
oligopeptide crosslinks in peptidoglycan.
Mutations in genes coding for transpeptidases that lead to
reduced interactions with an antibiotic are a significant
source of emerging antibiotic resistance.

24. Point of differences Gram positive cell wall Gram negative cell wall
Cell wall composition The cell wall is thick; single layered.
Lipid
content of the cell wall is low,
whereas
Murein content is 70-80% higher).
The cell wall is thick; two layered.
Lipid
content is 20-30% (high), Murein content is
10-
20% (low).
Peptidoglycan layer Thick (multilayered) and contains
L-
lysine or m-diaminopimelic acid
Thin (single-layered) and contains no
lysine,
only m-diaminopimelic acid
Outer membrane Absent Present
Phospholipid Absent Present
Lipopolysaccharide Absent Present
Lipids and
lipoprotein
content
Low (acid-fast bacteria have
lipids
linked to peptidoglycan)
High (due to presence of outer membrane)
Teichoic acid Present Absent

25. Periplasmic space Absent Present, thick
Mesosome Mesosome is more prominent. Mesosome is less prominent.

26. Atypical forms of bacteria
Cell wall deficient forms such as Mycoplasma, spheroplast, protoplast or L-form.
Pleomorphic bacteria that show pleomorphism.
Involution forms:
1. Spheroplast is a cell from which the cell wall has been almost removed but not
the outer membrane component of the cell wall, as by the action of penicillin. It
refers to the spherical shape assumed by Gram-negative bacteria.
2. Protoplast is a cell from which the cell wall has been completely removed by
mechanical, chemical or enzymatic (lysozyme) means. It refers to the spherical
shape assumed by Gram-positive bacteria.
3. L-form bacteria refer to the spherical shape of cell wall-deficient bacteria either
Gram-negative or -positive bacteria, by the action of penicillin.

27. Periplasm (periplasmic space)
Definition: Periplasm is a gel-like consistency in the space between
the inner cytoplasmic membrane and outer membranes in (Gram-
negative Bacteria) or cytoplasmic membrane and cell wall (Gram-
positive Bacteria).
A much smaller periplasmic space is also present in Gram-positive
bacteria.
Function of periplasm:
Periplasmic proteins have various functions in cellular processes
including: transport and degradationIt contains a considerable
number of enzymes such as proteases, phosphatases, nucleases and
phospholipases.
It contains binding proteins which help in the transport of various
substrates.
It contains the chemoreceptors which responsible for sensing the
environment.

29. Bacterial surface structures
Capsules
Definition: The bacterial capsule is a thick viscous layer firmly attached to the cell wall of
some bacteria.
It is a well-organized layer and not easily washed off. It can be removed by vigorous
washing.
The capsule is formed inside the host body.
The capsule is antigenic (K- Antigen) and can be used to identify certain organisms.
The capsule is not essential for life, but the possession of capsule enhances the virulence
of some pathogenic bacteria and so enhances their ability to cause disease.
Composition:
Capsule consists of 98% water and 2% solids.
It usually consists of polysaccharide but can be composed of other materials such as
glycoprotein, polypeptide or both.
For examples: polysaccharide in Streptococcus mutans, polypeptide D-glutamic acid in
B. anthracis capsule, and peptidoglycan and muramic acid in E. coli bacterial capsule.

30. Types
of
capsul
e:
Macro-
capsul
e:
thickne
ss of

31. 0.2µm or more, visible
under light microscope.
Microcapsule: thickness less
than 0.2µm, visible under
Electron microscope
Capsule staining:
Because most capsules are
so tightly packed and are
non-ionic. They are
difficult to stain because
most standard stains
cannot adhere to the
capsule.
For examination under the
microscope, the bacteria
and their background are
stained darker than the
capsule, which doesn't
stain. When viewed,
bacterial
cells as well as the surface
they are stained dark, while
the capsule appear as a
halo around the cell. This is called negative staining technique.
The used stains are india ink, Anthony's stain method, nigrosinstian or Congo
red.

33. Function of capsule:
The capsule is not essential for life, but the possession of capsule makes some
pathogenic bacteria resistant to phagocytosis and so enhance their virulence and
their ability to cause disease.
The capsule prevents the cell from desiccation and drying. The capsular
polysaccharide bind significant amount of water making cell resistant to drying
Protection: The capsule protects bacterial cell from toxins, complement proteins, and
antibodies, mechanical injury, temperature, drying etc.
Attachment: The capsule helps in attachment on the surface.
Anti-phagocytic : Capsule resists phagocytosis by phagocytic cell.
Capsule prevents attachment of bacteriophage on cell surface.

34. Source of nutrition: capsule is source of nutrition when nutrient supply is low in cell.

35. Slime
Definition: A slime layer in bacteria is amorphous viscid secretion loosely attached to cell
wall and easily removed (by vigorous washing or sub culture). It is unorganized layer of
extracellular material that surrounds bacteria cells.
Composition: A slime layer consists mostly of exopolysaccharides, glycoproteins, and
glycolipids.
The function of the slime layer:
The slime layer is to protect the bacteria cells from environmental dangers such as
antibiotics and desiccation.
The slime layer also allows bacteria to adhere to smooth surfaces such as catheters.
It may permit bacterial colonies to survive chemical sterilization with chlorine, iodine, and
other chemicals. Therefore, the autoclaving are the best methods of decontamination.
N.B. A bacterial capsule is similar to slime layer in its composition (exopolysaccharides),
but is an organized structure and bound tightly to the cell surface that does not wash off
as easily as slime layers.
N.B. Capsule and slime layer are sometimes summarized under the term glycocalyx.

37. Appendages: Flagella, Fimbriae and pili
Flagella (flagellum)
Definition:
Bacterial flagella are long thin helical protein filaments (appendages) that are
anchored in the cytoplasmic membrane and extend through the cell wall into
the surrounding medium.
Flagellar antigen is called H antigen.
They are usually found on Gram-negative rods, but motile Gram-positive
organisms also exist. They are found on all of the spiral and curved bacteria for
their motility.
Flagellar number and arrangement vary from single (monotrichous) to multiple
over the whole surface (peritrichous) of bacterial cell.
The diameter of a flagellum is about 20 nanometers.

38. Structure of flagella:
This filament is composed of flagellin proteins
which have a low molecular weight Flagella have
three sections: flagellar filament,
flagellar hook near the cell surface, and
a basal body which attach the flagellum in
the cytoplasmic membrane and cell wall by ring-like
structures.
Function of flagella:
They are responsible for cellular motility (as primary function) (swimming and
They provideswarming motility on surfaces or in viscous fluids.
It also often functions as a sensory organelle, being sensitive to
the cell.
swarming movement).
and temperatures
chemicals outside

39. Signal transduction
Flagella are generally being important virulence factors.
accepted as

41. Importance:
Flagellar distribution is a genetically-distinct trait that is occasionally used to
characterize or distinguish bacteria.
For example, among Gram-negative rods, Pseudomonas has polar flagella to
distinguish them from enteric bacteria, which have peritrichous flagella.
Types of flagella distributed on bacteria:
Monotrichous: Single polar flagellum. Example: Vibrio cholerae
Amphitrichous: Single flagellum on both sides. Example: Alkaligens faecalis
Lophotrichous: Tufts of flagella at one or both sides
Example: Spirillum

42. Peritrichous: Numerous flagella all over the bacterial body. Example: Salmonella
Typhi
Atrichous: lacking flagella.

43. Recognition of flagella can be done by
Ordinary light microscope using specific stain (Leifson's stain)
Phase contrast microscope
Dark field illumination microscope
Electron microscope