This document outlines the major characteristics used to classify microorganisms, including their morphological, chemical, cultural, metabolic, antigenic, genetic, pathogenic, and ecological characteristics. Morphological characteristics are determined microscopically and include cell shape, size, and structure. Chemical characteristics involve the organic and inorganic compounds that make up microbial cells. Cultural characteristics refer to an organism's nutritional and environmental growth requirements. Metabolic characteristics describe how microbes obtain and use energy. Antigenic characteristics involve surface compounds that trigger immune responses. Genetic characteristics have been elucidated through DNA and RNA analysis. Pathogenic characteristics determine if microbes can cause disease. Ecological characteristics describe an organism's natural habitat.

1. MAJOR CHARACTERISTICS OF
MICROORGANISMS

2. 1 Morphological characteristics. Cell shape, size, and
structure; cell arrangement; occurrence of special structures and
developmental forms; staining reactions; and motility and
flagellar arrangement
2 Chemical composition. The various chemical constituents
of the cells
3 Cultural characteristics. Nutritional requirements and
physical conditions required for growth, and the manner in which
growth occurs
4 Metabolic characteristics. The way in which cells obtain
and use their energy, carry out chemical reactions, and regulate
these reactions
5 Antigenic characteristics. Special large chemical
components (antigens) of the cell, distinctive for certain kinds of
microorganisms

3. 6 Genetic characteristics. Characteristics of the hereditary
material of the cell (deoxyribonucleic acid, or DNA); and occurrence
and function of other kinds of DNA that may be present, such as
plasmids
7 Pathogenicity. The ability to cause disease in various plants or
animals or even other microorganisms
8 Ecological characteristics. Habitat and the distribution of the
organism in nature and the interactions between and among
species in natural environments.

4. Morphological Characteristics
Unlike other kinds of microbial characteristics,
determination of morphological characteristics features
usually requires studying individual cells of a pure
culture. Microorganisms are very small and their size is
usually expressed in micrometers. The routine
examination of microbial cells requires the use of a
high-power microscope, usually at a magnification of
about 1,000 diameters.
The use of electron microscopy provides magnification
of thousands of diameters and makes it possible to see
fine details of cell structure. Numerous techniques are
available for the microscopic examination of -
microorganisms.

5. Chemical Characteristics
Microbial cells consist of a wide variety of organic
compounds. When cells are broken apart and their
components subjected to chemical analysis, each kind
of microorganism is found to have a characteristic
chemical composition. Both qualitative and quantitative
differences in composition occur among various
species. For example, the occurrence of
lipopolysaccharide in cell walls is characteristic of
Gram-negative bacteria but not Gram-positive bacteria;
on the other hand, many Gram-positive bacteria have
cell walls that contain teichoic acids, compounds not
found in Gram-negative bacteria. Fungal and algal cell
walls are very different in composition from those of
bacteria. A major distinction among viruses is made on
the basis of the kind of nucleic acid they possess,
namely ribonucleic acid (RNA) or deoxyribonucleic acid
(DNA).

6. Cultural Characteristics
Each kind of microorganism has specific growth
requirements. Many microorganisms can be grown
in or on a culture medium. Some microorganisms
can grow in a medium containing only inorganic
compounds, whereas others require a medium
containing organic compounds. Some require
complex natural substances (peptone, yeast, blood
cells, or blood serum), and some cannot as yet be
grown in an artificial laboratory medium and can be
propagated only in a living host or living cells.

7. Example
Rickettsias require a host in which to grow, such as
an animal, a fertilized chicken egg (chick embryo),
an arthropod, or a culture of mammalian tissue
cells. The host serves as a very complex 'medium'
for such nutritionally demanding microorganisms.
Physical conditions for growth.
 Temperature
 Gaseous Atmosphere
 Light

8. Metabolic Characteristics
The life processes of the microbial cell are a complex
integrated series of chemical reactions collectively
referred to as metabolism. The variety of these reactions
affords many opportunities to characterize and
differentiate various groups of microorganisms. For
instance, some organisms may obtain energy by
absorbing light, others by oxidizing various organic or
inorganic compounds, and others by redistributing the
atoms within certain molecules so that the molecules
become less stable. Organisms also differ in the ways in
which they synthesize their cell components during
growth.

9. Antigenic Characteristics
Certain chemical compounds of microbial cells are
called antigens. Antigenic characterization of a
microorganism has great practical importance. If
microbial cells enter the animal body, the animal
responds to their antigens by forming specific blood
serum proteins called antibodies, which bind to the
antigens. Antibodies are highly specific for the antigens
that induce their formation.
Because different kinds of microorganisms have
different types of antigens, antibodies are widely used
as tools for the rapid identification of particular
kinds of microorganisms.

10. Genetic characteristics
Since the last quarter of the 20th century, researchers
have accumulated a vast amount of information
elucidating in precise detail the chemical composition,
synthesis, and replication of the genetic material of
cells. Much of this research has been done by using
microorganisms, and techniques have been developed
that permit experimentation at the molecular level. For
instance, experiments determining the degree of
similarity between different organisms’ DNA and RNA
have provided new insights for the classification of
microorganisms. Test kits are available for the
identification of microorganisms, particularly bacteria,
by DNA probes.

11.  Since the invention of recombinant DNA
technology in 1973, techniques have been
developed whereby genes from one cell can be
transferred to an entirely different cell, as when
a gene is transferred from an animal cell to a
bacterium or from a bacterium to a plant cell.
Recombinant DNA technology opened the door
to many new medical and industrial applications
of microbiology, and it plays a central role in
genetic engineering.

12. Pathogenesis
Some microorganisms cause diseases of
humans, other animals, and plants. Such
microbes are called pathogens. Pathogens are
identified by the hosts they infect and the
symptoms they cause; it is also important to
identify the specific properties of the pathogen
that contribute to its infectious capacity—a
characteristic known as virulence. The more
virulent a pathogen, the fewer the number needed
to establish an infection.

13. Ecological Characteristics
o The habitat of a microorganism is important in
characterizing that organism. For example,
microorganisms normally found in marine
environment generally differ from those in
freshwater environments. The microbial
population of the oral cavity differs from that of
the intestinal tract. Some kinds of
microorganisms are widely distributed in
nature, but others may be restricted to a
particular environment. The relation of an
organism to its environment is often complex
and may involve special characteristics of the
organism that are not yet known.

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