Introduction to Microbiology:
Definitions:
Microbiology: is the study of microscopic organisms, which
are defined as any living organism that is either a single cell
(unicellular), a cell cluster, or has no cells at all (acellular).
Microbe: is a member of a large, extremely divers, group of
organisms that are lumped together on the basis of one property.
The fact that, normally, they are so small that they cannot be
seen without the use of a microscope.
Microbial nature:
Microbes are classified according to their nature into two groups:
1-Prokaryotic microbes.
2-Eukaryotic microbes.
Prokaryotes
Eukaryotes
Genetic material nature
DNA free in protoplasm
DNA is contained with a
membrane bound nucleus
Number of chromosomes
Only one chromosome.
Diploid copies of each
chromosome.
Energy production
It depends on cell
membrane.
It depends on
Mitochondria.
Ribosome
70 s ribosome.
80 s ribosome.
Cell wall
Peptidoglycan
Polysaccharide
Normal Flora (Commensals):
Human body is inhabited by many different microorganisms,
which under normal conditions are harmless, and may be
beneficial.
Under abnormal circumstances, it may invade the tissue causing
infection. (opportunistic pathogens).
The most common sites of the body inhabited by normal flora
are: the skin, eye, mouth, upper respiratory tract,
gastrointestinal tract, and urogenital tract.
Distribution of normal flora in the body:
Skin:
Most common flora: Staphylococcus epidermidis and other
coagulase negative Staphylococci.
Eye:
The conjunctiva of the eye is colonized by :
1-Staphylococcus epidermidis.
2-Staphylococcus aureus.
3-Aerobic Corynebacterium diphtheroids.
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Mouth and nose:
The most common are :
Corynebacterium diphtheroids, Staphylococcus aureus, and
Staphylococcus epidermidis.
-The teeth and surrounding gingival tissue are colonized by
their own particular species, such as Streptococcus mutans.
Intestinal tract:
-The density of microbes in the stomach is low ( 103 to 105) per
gram of contents.
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-The density of organisms increases along the alimentary canal,
reaching 108 to 1010 bacteria per gram of contents in the ileum,
and 1011 per gram in large intestine.
The most common flora of large intestine is the anaerobic
bacteria such as Bacterioides species.
Urogenital tract:
- The primary components of adult vaginal normal flora are the
Lactobacillus species. Candida albicans is a minor member of
the normal flora of vagina.
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Beneficial functions of normal flora.
Harmful effects of normal flora.
: Read the Textbook pages: 9-10.
Classification of Microbes:
Classification of Bacteria:
Medically important Bacteria are divided into three groups:
1-Lacking Cell Walls: Example: Mycoplasma
2-Flexible (have Cell Walls):
Example: Spirochetes (Treponema).
3-Rigid Cell Walls:
A- Filamentous bacteria: (antibiotic producers).
Example: Streptomyces.
B- Simple Unicellular:
1-Obligate intracellular parasite:
Example: (Rickettsia , Chlamydia).
2-Free-living bacteria:
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Free-living bacteria:
A-Cocci:
Gram positive: Staphylococcus, Streptococcus.
Gram negative: Neisseria.
B-Bacilli:
Gram positive:
Gram negative:
Bacillus, Clostridium.
Enteric Gram’s negative rods : E.coli.
Nonentric rods : Brucella.
The Gram’s positive Cocci:
Staphylococci and Streptococci :
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The Gram’s positive Bacilli:
The Gram’s negative Bacilli and spirochetes:
Classification of Fungi:
Medical fungi can be divided into three groups:
1- Mould:
Branching filaments; hyphae, mycelium.
Usually 2 to 10 μm in width. Example: Trichophyton.
2- True Yeasts:
These are ovoid or spherical cells that reproduce asexually by
budding and sexually with formation of spores.
Example : Cryptococcus spp.
3- Dimorphic fungi:
Produce a vegetative mycelium in artificial media, but are
yeast like in infected lesions.
Example: Candida albicans.
Fungi Morphology:
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Classification of Protozoa:
Protozoa are unicellular, eukaryotic organisms with
protoplasm differentiated into nucleus and cytoplasm.
-The clinically important protozoa are divided into four
groups:
A-Amoeba: (cyst and trophozoite).
Example: Entamoeba species.
Move by pseudopodia.
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B- Mastigophora: Move by Flagella.
Gastrointestinal flagellates: Giardia lamblia
Urogenital flagellates: Trichomonas vaginalis
Tissue and blood flagellates: Trypanosoma.
Giardia trophozoite.
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C- Ciliophora: move by cilia.
Example: Balantidium coli.
D- Sporozoa: Obligate intracellular parasites.
Example: Plasmodium .
Balantidium coli: kidney shaped
-Macronucleus.
Plasmodium species inside RBCs
Classification of Viruses:
Viruses consist of DNA or RNA enclosed in a simple protein
shell known as a capsid.
General properties of viruses
They are very small in size, from 20-300 m.
They contain one kind of nucleic acid (RNA or DNA).
They are metabolically inert.
They are obligate intracellular parasites.
They are only seen by electron microscope.
Viral morphology:
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