Introduction: The microbial
world, Classification,
Taxonomy, Nomenclature
Objectives
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To understand the broad classification of microbes
as bacteria, fungi, protozoa, helminths, viruses,
and prions
To know the differences between prokaryotes and
eukaryotes
To discuss the basis of bacterial taxonomy
To have an overview of the morphology,staining
and lifecycles of bacteria, fungi, protozoa, and
helminthes
To outline the immune response for protection
against microbial invasion
Three Domains

All living organisms can be classified into
one of three major groups called domains
Bacteria
 Archaea
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
Eukarya
}
= Prokaryote
= Eukaryote
Bacteria & Archaea
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Both have same shape, size & appearance
Major differences exist in their chemical
composition
Archaea
 lack peptidoglycan
 means "ancient" because use ancient
energy mechanisms
 Grow in harsh environments e.g high salt,
very high temp
Classes of infectious organisms
Eukaryotes
Arthropods (insects, ticks &
mites)
Helminthes (worms)
Fungi
Protozoa
Prokaryotes
Archaea
Bacteria
Viruses
Prions
Prokaryotes
Vs.
Eukaryotes
Features of
cells
Prokaryotic
Eukaryotic
Size
0.3-2µm
2-20µm
Chromosome
Single, circular
Multiple
Nucleus
No nuclear envelope;
no nucleoli
Membrane bound;
nucleoli present
Membrane
bound
organnelles
Not present
Present
Ribosomes
70S ribosomes (50S
& 30S subunits)
80S ribosomes (60S
& 40S subunits
Cell wall
Unique chemical
components,
peptidoglycan
Not present (except
in plant cells), no
peptidoglycan
Plasma
membrane
No carohydrates;
most lack sterols
Carbohydrates &
sterols present
Mitochodria
Cytoskeleton
& Chloroplasts
No
Yes
Chloroplast only in
plant & algae
Domain
Kingdom
Phyla
Class
Order
Family
Genus
Species
Microbial taxonomy is commonly called prokaryotic taxonomy. The widely accepted
prokaryotic taxonomy is Bergey’s Manual of Systematic Bacteriology, first published in
1923 by the American Society for Microbiology.
Bacterial Naming
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
Adopts the species and generic names
Species:
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Genus (plural: Genera)
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basic unit of classification
collection of strains with common characteristic
group of bacterial organisms having in common several structural,
biochemical & physiological traits
Group of species make up the genus
Every organism is identified by its genus and species
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The generic name is the first name and starts with a capital
letter
The species name is the second name and starts with a
small letter
Example:
genus
Staphylococcus
Staphylococcus
species
aureus
epidermidis
Bacterial classification - mixture of
characteristics

Macroscopic
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Microscopic
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Aerobic, Anaerobic, Microaerophilic
Reproduction:
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Shape: cocci, rods, curved, or spiral
Staining properties
Respiration
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Colonies: color, size, shape, smell
Haemolytic/ non-haemolytic
Special growth requirements
Sporing, non-Sporing
Biochemical characteristics
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Fermentation of sugars
Production of enzymes

Antigenicity
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Molecular (Genotype)
Microscopic

Size, shape and configuration of the
organisms
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cocci, rods, curved, or spiral
Ability to retain the Gram stain

gram-positive or gram-negative
Gram-negative bacilli
Gram-positive cocci
Classification of Bacteria
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True bacteria
Spirochaetes
Mycoplasmas
Riickettsiae & Chlamydiae
Filamentous bacteria
PROKARYOTES
Diagrammatic structure of a bacterium
Comparison of Eukaryotic microbes
Fungi
Protozoa
Helminths
Single or
Single celled Multicellular
Cell
organization multicellular
Source of
energy
Organic
compounds
Organic
compounds
Organic
compounds
derived from
host
Size
Microscopic
or
macroscopic
Microscopic
Macroscopic
Fungi

Diverse group of saphrophytic
organisms
 getting nutrients from dead organic
matter

Two basic forms:
 Filamentous, mold-like:
 Consist of threads(mycelia) that
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Yeast cells e.g. Candida
elongate and branch
Example: Mold & mushrooms
Yeast-like, single cells, round
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Divide by budding, larger than bacteria
Example: Candida
Fungal mould
Parasites Classification
Parasites
Protozoa
pseudopods, sporozoites
flagellates, ciliates
Helminths
(Worms)
Platyhelminths Nemathelminths
(Flat worms) (round worms)
Cestoda
(Tape worms)
Trematoda
(Flukes)
Protozoa

Single-celled eukaryotes, larger than bacteria
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Some are motile with flagella
Can have 2 forms during the lifecycle:
Trophozoite- larger form, metabolically active,
motile, dividing
 Cyst- smaller form, metabolically inactive, not
dividing, can survive harsh conditions

Some Protozoa (unicellular) of medical importance
Amoeboids
(Pseudopods)
Entamoeba
histolytica
Entamoeba coli)
Ciliates (cilia)
Zooflagellates
(flagella)
Balantidium coli Trichomonas
vaginalis
Giardia lamblia
Leishmania tropica
Sporozoa (no
locomotion)
Plasmodium
Toxoplasma
gondii
Cryptosporidium
E. coli (trophozoite)
E. histolytica (trophozoite)
Iodine stained wet
preparation
Showing: Cysts of
Entamoeba coli and
Entamoeba
histolytica
E. coli cyst
(more than 4 nuclei)
E. histolytica cyst
(4 nuclei)
Protozoa: Flagellates
Giardia lamblia :
Transmitted by faeco-oral route
Has a two life cycle stages:
Flagellated trophozoite & Cyst
Trichomonas vaginalis :
Sexually transmitted
One life cycle stage: Flagellated
trophozoite
PROTOZOA
Ring stage of P. falciparum
(malaria parasite)
Leishmania :
Amastigote form inside
macrophages
Helminths – worms
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Largest and multicellular
Most have 3 stages in lifecycle:
Egg
Larva
Adult
Helminths
Nematehelminths
(Nematodes or
round worms)
E.g. Ascaris
Platyhelminths
(flat worms
Cestodes
e.g. Tape worm
Trematodes
(Flukes)
e.g. Schistosoma
Viruses
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No cellular structure, not
living cells
Obligate intracellular
pathogens
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Viruses must enter host cells to
multiply & cause disease
Genome has DNA or RNA

surrounded by protein capsid
coat
Defense against microbes
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Innate immunity
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Inborn, non-specific
Adaptive immunity
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Acquired, specific, Immunological memory