Prokaryotic vs. Eukaryotic Cells
• Prokaryotic cells
– No Nucleus
– No Organelles
– Cell Wall of
peptidoglycan
– Binary Fission
– 1 circular chromosome
• Eukaryotic Cells
– Nucleus
– Organelles
– If cell wall, Cellulose or
chitin
– Mitosis
– Linear chromosomes
Prokaryotic and Eukaryotic Cells
Comparison between Prokaryotes and Eukaryotes
Term
Prokaryotes
Eukaryotes
Size
1-10 µm in diameter
10-100 µm in diameter
Cell wall
Existed
In plant cell (not animal cell)
nucleus
No nuclear envelope but
Nucleoid
True nucleus exists with
nuclear envelope
DNA
As fibre in the nucleoid
As Chromatin (DNA and
region (plasmids in some cases) protein)
Specialized
Most of them are absent
Organells
All are existed
Cell
2
division
Meiotic and/or Mitotic
By Binary Fission
Bacteria
• Most familiar of microbes that infect humans
• One-celled plants classified by shape and
arrangement
• Diseases such as strep throat and pneumonia
are caused by form of bacteria
• Can be categorized according to how
arranged; arrangement is way to identify exact
species
Bacterial Structure
Arrangement
• Cocci
–
–
–
–
–
diplococci
streptococci
tetrads
sarcinae
staphylococci
• bacilli
– diplobacilli
– streptobacilli
– coccobacilli
• spiral
– vibrio
– spirilla
– spirochete
Typical shapes of bacteria
7
Bacterial Cell Wall
 It is the outer most component of the bacterial
cell.
 It is located external to the cell membrane.
 Its structure differs between Gram positive and
Gram negative bacteria.
Cell wall
Situation: outmost
portion. 15-30nm
in thickness, 10%25% of dry
weight.
1884: Christian Gram: First publication for the Gram stain method)
Flagellum
Cell membrane
Nucleoid
Cell wall
Gram +
Pili
Gram Granule
Capsule
Cell (inner) membrane Outer membrane
Ribosomes
Cell wall
Comparison of the structures of gram-positive and gram-negative cell
envelopes. The region between the cytoplasmic membrane and the outer
membrane of the gram-negative envelope is called the periplasmic space.
gram –negative bacteria have a thin peptidoglycan covered
by an outer membran, whereas gram positive bacteria
have a thik peptidoglycan and no outer memmbran. these
differences explain why gram negative bacteria lose the
stain when exposed to a solvent in the gram stain process,
whereas gram positive bacteria retain the stain and remain
purple.
Cell wall
.
Gram stain is the most staning procedure Gram stain gram divedes bacteria
into Gram + and Gram –bacteria, an important step in classification
and identification–positive bacteria stain purple whereas gram –
negative bacteria stain pink. This is based on the ability of gram –
positive bacteria to retain the crystal violet- iodine complex in the
presence of a solvent usually acetone-alcohol. gram –negative
bacteria because have thin peptidoglycan lose the purple dye when
treated with acetone-alcohol and becom colorless and then stain pink
when exposed to a red dye such as safranin.
In Gram positive bacteria
 Peptidoglycan layer forms 50% of cell wall material.
 Peptidoglycan is responsible for cell wall rigidity ‫صالبة‬.
 Peptidoglycan is formed of N-acetyl glucosamine and Nacetyl muramic acid.
 Attached to each muramic acid molecule is a tetrapeptide.
Diaminopimelic acid is unique to the bacterial cell wall
Then the tetrapeptide chains are cross-linked together by
transpeptidase enzyme.
Penicillin inhibits transpeptidase enzyme. So it
inhibits cross linking of the bacterial cell wall
Cell wall :Common peptidoglycan layer
• A backbone of N-acetyl glucosamine and N-acetylmuramic acid: Both discovered
in Gram positive and Gram negative bacteria.
• A set of identical tetrapeptide side chain attached to N-acetyl-muramic acid:
different components in Gram positive and Gram negative bacteria.
• A set of identical peptide cross bridges: only in Gram positive bacteria
Special components of Gram positive
cell wall
Teichoic acid (anchored (‫ )الراسية‬to the peptidoglycan)
and Lipoteichoic acid ( anchored to cytoplasmic membran). They
facilitate the attachment to mucosal cells and induce septic shock
the same pathway as does endotoxin in gram negative bacteria
 In addition to peptidoglycan, there is teichoic acid
which:
 is highly immunogenic
 adheres the bacteria to mucosal surfaces.
 Lipoteichoic acid links peptidoglycan layer to the cell
membrane.
In Gram negative bacteria
 Inner peptidoglycan layer forms only 5-10% of the cell
wall material.
 Outer membrane layer formed of lipopolysaccharides:
 The lipid portion (lipid A) is called endotoxin. It is released
when the bacterial cells are lysed. It is highly toxic.
 The polysaccharide portion is called somatic or O antigen.
It is highly immunogenic (‫)مناعة قوية‬.
 The space between the inner and outer layers is called
periplasmic space which contains beta lactamase
enzyme that degrades beta lactam antibiotics.
Lipoprotein molecules cross-link (‫ )يربط‬the peptidoglycan
layer and the outer membrane.
Special components of Gram
negative cell wall
The outer membran of gram –negative bacteria contains
endotoxin (lipopolysaccharide, LPS).
Endotoxin consist of lipid A, which induces the fever and
hypotension seen in septic shock, and a polysaccharide (O
antigen) which is useful in laboratory identification.
Between the peptidoglycan and the outer membrane of
gram –negative bacteria lies the periplasmic space, which
is the location of beta lactamase (the enzymes that degrade
beta lactam antibiotics, such as penicillins and
cephalosporins).
In acid fast bacteria
 These bacteria contain high concentration of
mycolic acid and wax in their cell wall.
 These bacteria are not stained by Gram stain but
stained by Ziehl-Neelsen stain.
 Example: Mycobacterium tuberculosis.
 This bacteria resist de-colourization by sulfuric
acid. So, called acid fast bacteria.
Cell wall deficient bacteria
Mycoplasma
o
o
o
o
o
Lacks cell wall.
Pleomorphic (‫ )متعدد األشكال‬in shape.
Not affected by penicillin.
Not stained by Gram stain.
Only the bacteria that contains sterol in its cell membrane.
Protoplasts, spheroplasts, L-Forms
o
When bacterial cell wall is lost under the effect of certain
conditions (e.g. Antimicrobial agents) like penicillin:
 Gram positive bacteria form protoplasts (‫ البروتوبالستا محتوى الخلية البروتوبالزمي‬A
bacterial cell from which the rigid cell wall has been completely removed; the
bacterium loses its characteristic form.).

Gram negative bacteria form spheroplasts (A bacterial cell from which
the rigid cell wall has been incompletely removed. The bacterium loses its characteristic
shape and becomes round.).
o
o
If protoplasts and spheroplasts grow and divide, they are called
L-forms.
Unlike mycoplasma, L-forms can revert to the parental form on
removal of the cell wall inhibitor.
Wall-less forms of Bacteria.
• When bacteria are treated with 1) lytic enzymes for the cell
wall e.g. lysozyme or 2) antibiotics that interfere with
biosynthesis of peptidoglycan, wall-less bacteria are often
produced.
• Usually these treatments generate non-viable organisms.
• Wall-less bacteria that can not replicate are referred to as
spheroplasts (when an outer membrane is present) or
protoplasts (if an outer membrane is not present).
Bacterial cell wall
• Target for drugs that can attack and kill
bacteria without harming the host cell
• MANY ANTIBIOTICS are specifically directed at
Cell Wall Synthesis
– Penicillin
• works by damaging the pentaglycine crossbridges of
the peptidogylcan layer
• Works best against Gram (+) bacteria
lysozyme
• Digestive enzyme that damages bacterial cell
walls
• found in tears (‫)الدموع‬, saliva & mucus
• attacks the bond between NAM (Nacetylmuramic acid) & NAG (N-acetyl
glucosamine)
• Works best on Gram (+) bacteria
Functions of Cell Wall
• Maintains the shape of bacteria because of its rigidity.
• the rigid wall compensates (‫ )يعوض‬for the flexibility of
the phospholipid membrane .
• Providing attachment sites (target sites) for
bacteriophages
• Providing a rigid platform for surface appendagesflagella, fimbriae, and pili all emanate (‫ )ينبع‬from the
wall and extend beyond it (‫)ورائها‬
• Play an essential role in cell division
• Be the sites of major antigenic determinants ( ‫محددات‬
‫الـ‬antigen) of the cell surface.
• Resistance of Antibiotics
Functions of the cell wall
1)
2)
3)
4)
Protects bacteria from bursting in hypotonic solutions.
Contains endotoxin (lipid A) in Gram negative bacteria.
Contains somatic (O) antigen in Gram negative bacteria.
Determines cell reaction to Gram stain:
 Gram positive bacteria have less porous (‫ )مسام‬cell wall. So, they will not be
decolorized and will retain the violet dye and appear violet in color under the
microscope.
 But, Gram negative bacteria have more porous cell wall. So, they will be
decolorized and will not retain (‫ )تحفظ‬the violet dye. However, they will take the
last dye which is red in color and appear red under the microscope.
GOOD LUCK