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This text introduces microorganisms, differentiating prokaryotes (lacking a true nucleus, smaller, simpler) from eukaryotes (possessing a true nucleus, larger, more complex). Key differences include the presence of mitochondria and membrane-bound organelles, ribosome size (70S vs 80S), and sterol content in the cytoplasmic membrane. Viruses (smallest infectious agents, lacking cell structure), viroids (single-stranded RNA), prions (infectious proteins), and blue-green algae (which may produce toxins) are discussed. Bacteria are classified based on shape (cocci, bacilli, spirals), size, arrangement, and staining characteristics (Gram staining, Ziehl-Neelsen staining). Bacterial ultrastructures include the nucleoid (containing a circular DNA chromosome and plasmids), ribosomes (sites of protein synthesis), inclusion granules (energy reserves), mesosomes (involved in cell division), and the cytoplasmic membrane (responsible for transport, excretion, respiration, and cell wall biosynthesis). The bacterial cell wall, composed of peptidoglycan, maintains shape and protects against osmotic pressure. Gram-positive bacteria have a thicker peptidoglycan layer and teichoic acids, while Gram-negative bacteria have a thinner layer, an outer membrane with lipopolysaccharide (LPS), and a periplasmic space. Cell wall-deficient bacteria, such as Mycoplasma and L-forms, are also described.

Introduction to Microorganism
Prokaryote Eukaryote
Pro = before EU = True
Size Smaller Larger
Development Simple More developed
Nucleus No True nucleus
One chromosome is called
Nucleoid or Nuclear region
True nucleus
Nuclear
membrane
No nuclear membrane Surrounded by nuclear membrane
Mitochondria No mitochondria Contain mitochondria
Organelles No membrane bound
organelles
Contain membrane bound
organelles
Ribosomes 70S 80S
Cytoplasmic
membrane
No sterol (except mycoplasma) Contain sterol
Examples Bacteria Mycoplasma
Rickettsia Chlamydia
Blue green algae
Archaebacteria
Algae (except blue green) Protozoa
Slime molds Fungi Plants
Animals
Difference between Eukaryotes and Prokaryotes cells
Viruses, Viroids, Prions and Blue green algae
Viruses N Smallest infective agent N Have no cell structure
N Obligate intracellular parasites 
require host cells
Viroids Single stranded RNA (no protein)
Causes diseases in plants
Prions Infectious protein particles (no nucleic acids)
Blue green algae Do not cause infections
May produce potent toxins (drinking polluted waters)
MCQ
1- Which of the following microorganisms has a
nuclear membrane?
a- Viruses
b- Fungi
c- Prions
d- Bacteria
e- Viroids
2- Viruses have all the following characteristics EXCEPT:
a- They are one of the smallest infectious agents.
b- They have no cell structure.
c- They are obligate intracellular parasites.
d- They require the host biological machinery for their
replication.
e- They are prokaryotic.
• 3-
3- Prions:
a- Are single stranded circular RNA
b- Are devoid of proteins
c- Are infectious proteins devoid of nucleic acids
d- Are prokaryotic cells
e- Cause diseases in plants
Bacteria
their structure and organization
• Bacterial Morphology
• Bacteria are classified based on their morphological features such as:

1. Shape


2. Size


3. arrangement


4. staining characteristics.
   • Bacterial Size
   • Most bacteria range in size from 0.2-1.2 µm in width and 0.4-14 µm in
   length.
   Bacterial Shape and Arrangement
   Cocci Bacilli Spiral
   bacteria
   Definition Spherical organisms
   Singular = coccus
   Rod shaped (Stick)
   Singular = bacillus
   Types Diplococci: pairs of cells  Neisseria.
   Irregular grape-like clusters, 
   staphylococci.
   Chains of four or more  streptococci.
   May occur single, in
   pairs, or chains
   Coccobacilli: Short
   Vibrio: curved
   Spirilla: rigid
   Spirochetes:
   flexible

Staining Characteristics
Simple staining Differential staining
Using a single dye Requires more than one dye
Stained structures give the same
color
Distinguish between different types of
bacteria by giving different colors
Used for revealing the characteristics
of
size, shape, arrangement
Gram stain: The most important stain in microbiology
Divides bacteria into:

1. gram positive (violet)


2. gram negative (red)
   Ziehl-neelsen stain:
   Used to stain mycobacteria (acid fast bacilli)

Bacterial Ultra-Structures and their
Functions
• All bacteria have a nucleoid, ribosomes and a cytoplasmic
membrane.
• Most bacteria also have a cell wall and some are further
enveloped by a capsule or slime layer.
• Some types of bacteria have also various appendages as
flagella and pili.
Cytoplasm
• Few morphologically distinct components can be found within the
cytoplasm
• Nucleoid:
• Genetic information of a bacterial cell is contained in a single circular
molecule of double-stranded DNA, which constitutes the bacterial
chromosome.
• It is 1 mm long and is packed into a supercoiled state inside the cell.
• Plasmids:
• In many bacteria, additional genetic information is contained on plasmids which
are small circular extrachromosomal DNA molecules that can replicate
independently of the chromosome.
Ribosomes
 They are the site of protein synthesis in the cell.
 Ribosomes consist of protein and RNA.
 Prokaryotic ribosomes have a sedimentation constant of
• 70S, smaller than the 8OS ribosomes of
eukaryotes.
• This difference makes bacterial ribosomes a selective
target for antibiotic action
Inclusion granules
• These are granules of nutrient materials, usually phosphates,
sulphur, carbohydrates and lipids.
• Energy reserves are usually stored as glycogen, starch or poly-ꞵhydroxybutyrate.
• Phosphate is stored in metachromatic or volutin granules, which
are used for synthesis of ATP.
Mesosomes
 These are complex invagination of the cytoplasmic membrane.
 They are involved in cell division and sporulation.
 They also have a function analogous to the mitochondria in eukaryotes
providing a membranous support for respiratory
enzymes
Cytoplasmic Membrane
•Definition: It is a phospholipid protein bilayer similar to that of
eukaryotic cells except that, in bacteria, it lacks sterols.
•
• It has the following functions:
1.Selective transport:
• molecules move across the cytoplasmic membrane by simple
diffusion, facilitated diffusion and active transport.

1. Excretion of extracellular enzymes:
   a. Hydrolytic enzymes: which digest large food molecules into subunits
   small enough to penetrate the cytoplasmic membrane.
   b. Enzymes used to destroy harmful chemicals
   Example: penicillin-degrading enzymes.
   2.Respiration: The respiratory enzymes are located in the
   cytoplasmic membrane, which is thus a functional analogue of the
   mitochondria in eukaryotes.
   4- Cell wall biosynthesis:
   The cytoplasmic membrane is the
   site of:
   1.The enzymes of cell wall
   biosynthesis.
   2.The carrier lipids on which the
   subunits of the cell wall are assembled.


2. Reproduction:
    A specific protein in the membrane attaches to the DNA and
   separates the duplicated chromosomes from each other.
    A septum forms by the cytoplasmic membrane to separate the
   cytoplasm of the two daughter cells.


3. Chemotactic system:
    Attractants and repellants bind to specific receptors in the
   cytoplasmic membrane and send signals to the cell's interior.
    The cell then responds to the surface message.
   Cell Wall
    The bacterial cell wall is the structure that immediately surrounds
   the cytoplasmic membrane.
    It is 10-25 nm thick strong and relatively rigid, though having some
   elasticity
   • Structure of the cell wall
   • N The cell wall of bacteria is a complex structure.
   • N Its impressive strength is primarily due to peptidoglycan (murein or mucopeptide).
   • N Peptidoglycan: a complex polymer consisting of Nacetylglucosamine (NAG) and N- acetylmuramic acid (NAM) unique to
   bacteria.
   • A set of identical tetrapeptide side chains are attached to NAM

Gram-positive cell wall

1. Peptidoglycan:
   • N There are as many as 40 sheets of peptidoglycan.
   • N comprising up to 50% of the cell wall material.
   • N Despite the thickness of peptidoglycan, chemicals can readily
   pass through.


2. Teichoic acids:
   • N They are the polymer of ribitol or glycerol phosphate.
   • N They are found in the cell wall of most Gram-positive
   bacteria.
   • N Teichoic acids and cell wall associated proteins are the major
   surface antigens of the Gram-positive bacteria

Gram-negative cell wall

1. Peptidoglycan:
   • N It is much thinner.
   • N composed of only one or two sheets.
   • N comprising 5-10% of the cell wall material.


2. Outer membrane:
   • N It is phospholipid protein bilayer present external to the peptidoglycan layer.
   • N The outer surface of the lipid bilayer is composed of molecules of
   lipopolysaccharide (LPS) which consists of a complex lipid called lipid A chemically
   linked to polysaccharides.
   • N Lipid A of the LPS forms the endotoxin of the Gram-negative bacteria, while
   polysaccharides are the outermost molecules of the cell wall and are major
   surface antigens of the Gram-negative bacterial cell (somatic or O antigen).


3. Periplasmic space:
   • N It is the space between the cytoplasmic and outer membranes.
   • N It contains the peptidoglycan layer and a gel-like solution of
   proteins.
   •
   Functions of the cell wall

1. It maintains the characteristic shape of the bacterium.


2. It supports the weak cytoplasmic membrane against the high
   internal osmotic pressure of the protoplasm (5-25 atm.).


3. It plays an important role in cell division.


4. It is responsible for the staining affinity of the organism.
   • Cell Wall deficient Bacteria
   •
   • Mycoplasma:
    It is the only group of bacteria that exists naturally without cell wall.
    Mycoplasmas do not assume a defined recognizable shape, because they
   lack a rigid cell wall.
    These organisms are naturally resistant to cell wall inhibitors, such as
   penicillin and cephalosporins.
   • L. Forms:
   •
   They are wall defective or wall deficient bacteria.
   "L" stands for Lister Institute in London, where they were first discovered.
   L- Forms may develop from cells that normally possess cell wall, when
   they are exposed to hydrolysis by lysozyme or by blocking
   peptidoglycan biosynthesis with antibiotics, such as penicillin, provided
   that they are present in an isotonic medium.
   Some L. forms resynthesize their walls once the inducing stimulus is
   removed.
   • Others  permanently lose the capacity to produce a cell wall.
   L. forms may survive antibiotic therapy.
   • Their reversion to the walled state can produce relapses of the overt
   infection