Classification of Microorganisms

Classification of Microorganisms


• System of classification

• Binomial system of classification

• Whittaker’s five kingdom classification

• Prokaryotes and eukaryotes

• Characteristics for bacterial classification

• Methods for bacterial classification

Learning objectives

At the end of this lecture, student will be able to

• Identify the importance of classification

• Outline the developments in taxonomy

• Differentiate between prokaryotic and eukaryotic cells

• Describe the relationship between different kingdoms

• Classify organisms based on their characteristics


• The science of classification – the classification of
living forms

• From the Greek word meaning ‘orderly arrangement’

of classification:

• To establish the relationships between one group of
organisms and another and to differentiate them

• Provides a common reference for identifying organisms

• To show degrees of similarities among organisms

History of

• Aristotle,
living organisms were categorized in just two ways, as either plants or

In 1735, the
Swedish botanist Carolus Linnaeus
introduced a Aristotle formal system of
classification dividing living organisms into two kingdoms – Plantae and

• 1857 – Carl von Nageli proposed that bacteria and fungi be
placed in the plant kingdom

• 1866 – Ernst Haeckel proposed the Kingdom Protista, to
include bacteria, protozoa, algae, and fungi

• Fungi were placed in their own kingdom in 1959


Advent of electron

German physicist
Ernst Ruska
and the electrical
engineer Max Knoll
constructed the prototype electron microscope in

• Ernst Ruska was awarded half of the Nobel Prize for
Physics in 1986 for his invention.

• The physical differences between cells became apparent

• Introduction of the term prokaryote in 1937 by Edouard
Chatton – to distinguish cells having no nucleus from the nucleated cells of
plants and animals

• In 1961, Roger Stanier defined prokaryotes: cells in which
the nuclear material (nucleoplasm) is not surrounded by a nuclear membrane

• In 1968, Robert G.E. Murray proposed the Kingdom

and Eukaryotic cell

• 1969, Robert H. Whittaker founded the five-kingdom system

• Prokaryotes were placed in the Kingdom Prokaryotae, or

• Eukaryotes comprised the other four kingdoms

The five
kingdom classification

Kingdom Monera

• All members are prokaryotes

• Includes organisms that are single-celled

• Considered as the most ancient living forms on earth

• Divided into two groups Archaebacteria and Eubacteria

Kingdom Protista

• Protists are eukaryotic organisms that cannot be
classified as a plant, animal, or fungus

• Mostly unicellular, but some, like algae, are

• Kelp, or ‘seaweed,’ is a large multicellular protist that
provides food, shelter, and oxygen for numerous underwater ecosystems

• Protists can be heterotrophic obtain energy by consuming
other organisms or they can be autotrophic, which means they obtain energy from
the environment through photosynthesis

• Protists primarily live in water, though some live in
moist soil

• Protists are grouped by how they move and how they obtain

• ‘”Animal-like” – also called protozoans (Latin for “First

• “Plant-like” – Example: algae

• “Fungus-like” – Example: Slime mold

Kingdom fungi

• The kingdom Fungi includes a diverse group of organisms
that are neither plant nor animal. 

• Unicellular or multicellular

• Absorb the nutrients

• Play an important role of ecological decomposers

Kingdom Plantae

• Includes every plant you could imagine from the moss
growing on the forest floor to the mighty, towering fir trees. 

• Plants are autotrophs, meaning they can make their own

• Multicellular

• Contain chlorophyll

Kingdom Animalia

• Animals are classified under the Kingdom Animalia

• Multicellular, Eukaryotes

• Heterotrophs, they depend on other organisms directly or
indirectly for food

• Most of the animals ingest food and digest in the internal

• Motile which means they can move independently and

The Three

Three distinctly different cell groups based on comparing
the sequences of nucleotides in ribosomal RNA 

• The eukaryotes 

• Two different types of prokaryotes- the bacteria and the

In 1978, Carl R. Woese proposed elevating the three cell
types to a level above kingdom, called Domain

• Organisms are classified based on the similarity in
ribosomal RNA in the three domain systems

Domain Eukarya

• Includes kingdoms animals, plants, fungi and protists

Domain Bacteria

• Includes prokaryotes 

Domain Archaea

• Includes prokaryotes that do not have peptidoglycan in
their cell walls

• Often live in extreme environments and carry out unusual
metabolic processes 

Archaea include three major groups

1. The methanogens, strict anaerobes that produce methane
from carbon dioxide and hydrogen 

2. Extreme halophiles, which require high concentrations of
salt for survival

3. Hyperthermophiles, which normally grow in extremely hot


• Every organism is assigned two names, or a binomial. 

• Genus name

• Specific epithet (species)

• Because this system gives two names to each organism, the
system is called binomial nomenclature.

The Taxonomic
Linnaeus developed the taxonomical hierarchy for his
classification of plants and animals

Species, Genus, Family, Order, Class, Phyllum, Kingdom, Domain

of Prokaryotes

• A prokaryotic species. Therefore, is defined simply as a
population of cells with similar characteristics

 Bacteria grown at a given time in media are called a

• A pure culture is often a clone. that is, a population of
cells derived from a single parent cell

• In some cultures, the same species may not be identical in
all ways. Such group is called a strain

• Strains are identified by numbers, letters, or names that
follow the specific epithet

of Eukaryotes


• Protists

• Eukaryotic organisms that didn’t fit into other kingdoms
were placed in the Protista


• Fungi

• Plants

• Animals

Kingdom Fungi

• Unicellular yeasts

• Multicellular molds

• Macroscopic species such as mushrooms.

• Fungus absorbs dissolved organic matter through its plasma

• Fungi develop from spores or from fragments of hyphae

Kingdom Plantae

• Includes some algae, Mosses, Ferns, Conifers and Flowering

• Multicellular

• To obtain energy, a plant uses photosynthesis

Kingdom Animalia

• Multicellular organisms

• Includes sponges, various worms, insects, and animals with

• Obtain nutrients and energy by ingesting organic matter


• Not classified as part of any of the three domains

• Viruses are not composed of cells

• They use the anabolic machinery within living host cells
to multiply

• A viral genome can direct biosynthesis inside a host cell

• Some viral genomes can become incorporated into the host

• Viruses are obligatory intracellular parasites

of microorganisms

In order to identify and classify microorganisms, we need to
learn their characteristics. Major characteristics of microorganisms:

• Morphological

• Chemical composition

• Cultural

• Metabolic: the way in which cells obtain and use their

• Antigenic: chemical components / antigens that are
characteristic of the cell

• Genetic: DNA base composition and Sequence of nucleotide
bases in the DNA

• Pathogenicity: Ability to cause disease

• Ecological: Habitat and distribution of the organism in
nature Metabolic

of Microorganisms – Morphological

Cell Shape

Cell Size

Cell arrangement

Special structures

Flagellar arrangement

Staining reactions

Motility: Motile
or Non-motile

of Microorganisms – Chemical

Chemical constituents
of the cell


• Presence of lipopolysaccharides in the cell wall of gram negative

• Presence of teichoic acid in gram positive cells

• Presence of DNA or RNA in virus

of microorganisms – Cultural


Organic compounds

Inorgainc compounds

Physical conditions
required for growth

Optimum temperature for growth – 40⁰C or 20 ⁰C 

Manner in which the
growth occurs


General methods of classifying bacteria

Three methods:

1. Intuitive method

2. Numerical taxonomy

3. Genetic relatedness

3. Genetic

      Based on the hereditary material,

% G + C values

a. DNA homology experiments

b. Ribosomal RNA homology
experiments and ribosomal RNA oligonucleotide cataloguing


• Classification give relationships between one group of
organisms and another and differentiates them

• Aristotle – classified living things as plants or animals

• Carolus Linnaeus- introduced the formal system of

• Robert H. Whittaker introduced the five-kingdom system

• Prokaryotes – cells having no nucleus

• Eukaryotes – nucleated cells

• Major characteristics of microorganisms used for

 Morphological

 Chemical composition

 Cultural

 Metabolic

 Antigenic

 Genetic

 Pathogenicity

 Ecological

• Methods used for bacterial classification

1. Intuitive method

2. Numerical taxonomy

3. Genetic relatedness

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