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Microbiology
Is a science that deals with study of organisms which unable to be
seen by naked eye such as bacteria , viruses etc.
Medical microbiology
Is a branch of microbiology deals with micro-organism that causes
the diseases for human & other organisms and its divided several
branch e.g. :-
1 – bacteriology .
2 – virology .
3 – parasitology .
4 – mycology .
5 – immunology .
Microorganisms
belong to the Protista biologic kingdom include some eukaryotes and
prokaryotes, viruses, viroids, and prions , these are classified
according to their structure, chemical composition, and biosynthetic
and genetic organization.
B. Eukaryotic cells
contain organelles and a nucleus bounded by a nuclear membrane
these contain complex phospholipids, sphingolipids, histones, and
sterols , the lack a cell wall (plant cells and fungi have a cell wall) &
have multiple diploid chromosomes and nucleosomes. have relatively
long-lived mRNA formed from the processing of precursor mRNA,
which contains exons and introns. have 80S ribosomes and uncoupled
transcription and translation.
C. Prokaryotic cells
have no organelles, no membrane-enclosed nucleus, and no
histones; in rare cases, they contain complex phospholipids,
sphingolipids, and sterols.
have 70S ribosomes.
have a cell wall composed of peptidoglycan-containing muramic
acid.
are haploid with a single chromosome.
have short-lived, unprocessed mRNA.
have coupled transcription and translation.
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Typical bacteria
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have normal peptidoglycan.
may be normal flora or pathogenic in humans.
do not have a sexual growth cycle; however, some can
produce asexual spores.
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Mycoplasmas
are the smallest and simplest of the bacteria that are
self-replicating.
lack a cell wall.
are the only prokaryotes that contain sterols.
o
Rickettsia organisms
are obligate intracellular bacteria that are incapable of
self-replication.
depend on the host cell for adenosine triphosphate
(ATP) production.
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Chlamydiae
are bacteria-like obligate intracellular pathogens with
a complex growth cycle involving intracellular and
extracellular forms.
depend on the host cell for ATP production.
Bacteriology
Bacteria, along with blue-green algae, are prokaryotic cells. That is, in
contrast to eukaryotic cells, they have no nucleus; rather the genetic
material is restricted to an area of the cytoplasm called the nucleoid.
Prokaryotic cells also do not have cytoplasmic compartment such as
mitochondria and lysosomes that are found in eukaryotes. However, a
structure that is found in prokaryotes but not in eukaryotic animal cells is
the cell wall which allows bacteria to resist osmotic stress. These cell
walls differ in complexity among bacteria that are usually divided into
two major groups, the gram-positive and gram-negative organisms, which
reflect their cell wall structure. The possession of this cell wall, which is
not a constituent of animal cells, gives rise to the different antibiotic
sensitivities of prokaryotic and eukaryotic cells.
Limited number of bacterial species are responsible for the majority of
infectious diseases in healthy individuals. Due to the success of
vaccination, antibiotics, and effective public health measures, until
recently, epidemics were felt to be a thing of the past. Due to the
development of antibiotic resistant organisms, this situation is changing
rapidly.
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All humans are infected with bacteria (the normal flora) living on their
external surfaces (including the skin, gut and lungs). We are constantly
also exposed to bacteria (including those that live in air, water, soil and
food). Normally due to our host defenses most of these bacteria are
harmless. In compromised patients, whose defenses are weakened, these
bacteria often cause opportunistic infectious diseases when entering the
bloodstream (after surgery, catheterization or other treatment modalities).
When initiated in the hospital, these infectious diseases are referred to as
nosocomial. Some common bacteria found in the normal flora include
Staphylococcus aureus, S. epidermidis and Propionibacterium acnes
(found on the skin) and Bacteroides and Enterobacteriaceae found in the
intestine (the latter in much smaller numbers).
Koch's postulates (modified)
1. The organism must always be found in humans with the infectious
disease but not found in healthy ones.
2. The organism must be isolated from humans with the infectious disease
and grown in pure culture.
3. The organism isolated in pure culture must initiate disease when re-
inoculated into susceptible animals.
4. The organism should be re-isolated from the experimentally infected
animals.
In the recent years additional postulates was added
5. infection should be lead to development of an immune response against
pathogen.
Postulates 3. and 4. are extremely important in definite proof of the role
of agent in human disease. However, this depends on the ability to
develop animal models that resemble the human disease. In many cases
such models do not exist.
II. Bacteria
A. Bacterial shape
can usually be determined with appropriate staining and a light
microscope.
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is usually round (coccus), rod-like (bacillus), or spiral with most
species; cocci and bacilli often grow in doublets (diplococci) or
chains (streptococci). Cocci that grow in clusters are called
staphylococci.
may be pleomorphic with some species, such as Bacteroides.
is used, along with other properties, to identify bacteria.
is determined by the mechanism of cell wall assembly.
may be altered by antibiotics that affect cell wall biosynthesis (e.g.,
penicillin).
Bacterial structure
B. Bacterial nucleus
is not surrounded by a nuclear membrane, nor does it contain a
mitotic apparatus.
is generally called a nucleoid or nuclear body.
consists of polyamine and magnesium ions bound to negatively
charged, circular, supercoiled, double-stranded DNA; small
amounts of RNA; RNA polymerase; and other proteins.
C. Bacterial cytoplasm
contains ribosomes and various types of nutritional storage
granules.
contains no organelles.
D. Bacterial ribosomes
have a sedimentation coefficient of 70S and are composed of 30S
and 50S subunits containing 16S, and 23S and 5S RNA,
respectively.
are the sites of action of many antibiotics that inhibit protein
biosynthesis.
have proteins and RNAs that differ from those of their eukaryotic
counterparts.
form the basis for the selective toxicity of antibacterial protein
synthesis inhibiting agents, which affect 70S ribosomes (e.g.,
erythromycin) but not 80S ribosomes.
are membrane-bound if engaged in protein biosynthesis.
E. Cell (cytoplasmic) membrane
is a typical phospholipid bilayer.
ﻣﺘﻌﺪد اﻟﺸﻜﻞ
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contains the cytochromes and enzymes involved in electron
transport and oxidative phosphorylation.
contains carrier lipids and enzymes involved in cell wall
biosynthesis.
contains enzymes involved in phospholipid synthesis and DNA
replication.
contains chemoreceptors.
is responsible for selective permeability and active transport, which
are facilitated by membrane-bound permeases, binding proteins,
and various transport systems.
is the site of action of certain antibiotics, such as polymyxin.
G. Plasmids
are small, circular, nonchromosomal, double-stranded DNA
molecules.
are capable of self-replication.
are most frequently extrachromosomal, but may become integrated
into bacterial DNA.
contain genes that confer protective properties, such as antibiotic
resistance, virulence factors, or their own transmissibility to other
bacteria.
H. Transposons
are small pieces of DNA that move between the DNA of
bacteriophages.
are bacteria or plasmids not capable of self-replication.
code for antibiotic resistance enzymes, metabolic enzymes, or
toxins.
may alter expression of neighboring genes or cause mutations to
genes into which they are inserted.
I. Cell envelope
is composed of the macromolecular layers that surround the
bacterium.
always includes a cell membrane and a peptidoglycan layer.
includes an outer membrane layer in gram-negative bacteria.
may include a capsule, a glycocalyx layer, or both.
contains antigens that frequently induce a specific antibody
response.
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Cell wall
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refers to that portion of the cell envelope that is external to
the cytoplasmic membrane and internal to the capsule or
glycocalyx.
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confers osmotic protection and gram-staining characteristics.
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is composed of peptidoglycan, teichoic and teichuronic
acids, and polysaccharides in gram-positive bacteria.
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is composed of peptidoglycan, lipoprotein, and an outer
phospholipid membrane that contains lipopolysaccharide in
gram-negative bacteria.
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contains penicillin-binding proteins.
Peptidoglycan
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is also called mucopeptide or murein and is unique to
prokaryotes.
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is found in all bacterial cell walls, except Mycoplasma.
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is a complex polymer that consists of a backbone, which is
composed of alternating N-acetylglucosamine and N-
acetylmuramic acid and a set of identical tetrapeptide side
chains, which are attached to the N-acetylmuramic acid and
that are frequently linked to adjacent tetrapeptide side chains
by identical peptide cross-bridges or by direct peptide bonds.
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contains the glycosidic bond between N-acetylmuramic acid
and N-acetylglucosamine, which is cleaved by the
bacteriolytic enzyme lysozyme (found in mucus, saliva, and
tears).
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may contain diaminopimelic acid, an amino acid unique to
prokaryotic cell walls.
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is the site of action of certain antibiotics, such as penicillin
and the cephalosporins.
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comprises up to 50% of the cell wall of gram-positive
bacteria but only 2%- 10% of the cell wall of gram-negative
bacteria.
Teichoic and teichuronic acids
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are water-soluble polymers, containing a ribitol or glycerol
residue linked by phosphodiester bonds.
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are found in gram-positive cell walls or membranes.
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are chemically bonded to peptidoglycan (wall teichoic acid)
or membrane glycolipid (lipoteichoic acid), particularly in
mesosomes.
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contain important bacterial surface antigenic determinants,
and lipoteichoic acid helps anchor the wall to the membrane.
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may account for 50% of the dry weight of a gram-positive
cell wall.
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Lipoprotein
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cross-links the peptidoglycan and outer membrane in gram-
negative bacteria.
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is linked to diaminopimelic acid residues of peptidoglycan
tetrapeptide side chains by a peptide bond; the lipid portion
is noncovalently inserted into the outer membrane.
Periplasmic space
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is found in gram-negative cells.
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refers to the area between the cell membrane and the outer
membrane.
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contains hydrated peptidoglycan, penicillin-binding proteins,
hydrolytic enzymes (including
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β -lactamases), specific
carrier molecules, and oligosaccharides.
Outer membrane
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is found in gram-negative cells.
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is a phospholipid bilayer in which the phospholipids of the
outer portion are replaced by lipopolysaccharides.
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protects cells from many things, including harmful enzymes
and some antibiotics, and prevents leakage of periplasmic
proteins.
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contains embedded proteins, including matrix porins
(nonspecific pores), some non-pore proteins (phospholipases
and proteases), and transport proteins for small molecules.
Lipopolysaccharide
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is found in the outer leaflet of the outer membrane of gram-
negative cells.
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consists of lipid A, several long-chain fatty acids attached to
phosphorylated glucosamine disaccharide units, and a
polysaccharide composed of a core and terminal repeating
units.
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is negatively charged and non-covalently cross-bridged by
divalent cations.
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is also called endotoxin; the toxicity is associated with the
lipid A.
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contains major surface antigenic determinants, including O
antigen found in the polysaccharide component.
J. External layers
Capsule
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is a well-defined structure of polysaccharide surrounding a
bacterial cell and is external to the cell wall. The one
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exception to the polysaccharide structure is the poly-D-
glutamic acid capsule of Bacillus anthracis.
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protects the bacteria from phagocytosis and plays a role in
bacterial adherence.
Glycocalyx
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refers to a loose network of polysaccharide fibrils that
surrounds some bacterial cell walls.
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is sometimes called a slime layer.
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is associated with adhesive properties of the bacterial cell.
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is synthesized by surface enzymes.
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contains prominent antigenic sites.
K. Appendages
Flagella
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are protein appendages for locomotion.
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consist of a basal body, hook, and a long filament composed
of a polymerized protein called flagellin.
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may be located in only one area of a cell (polar) or over the
entire bacterial cell surface (peritrichous).
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contain prominent antigenic determinants.
Pili (fimbriae)
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are rigid surface appendages composed mainly of a protein
called pilin.
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exist in two classes: ordinary pili (adhesins), involved in
bacterial adherence, and sex pili, involved in attachment of
donor and recipient bacteria in conjugation.
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are, in the case of ordinary pili, the colonization antigens or
virulence factors associated with some bacterial species,
such as Streptococcus pyogenes and Neisseria gonorrhoeae.
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may confer antiphagocytic properties, like the M protein of S
pyogenes.
L. Endospores
are formed as a survival response to certain adverse nutritional
conditions, such as depletion of a certain resource.
are metabolically inactive bacterial cells that are highly resistant to
desiccation, heat, and various chemicals.
possess a core that contains many cell components, a spore wall, a
cortex, a coat, and an exosporium.
contain calcium dipicolinate, which aids in heat resistance in the
core.
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germinate under favorable nutritional conditions after an activation
process that involves damage to the spore coat.
are helpful in identifying some species of bacteria (e.g., Bacillus
and Clostridium).
are not reproductive structures.