What are Bacteria?
Bacteria are microscopic, unicellular, prokaryotic organisms. They do not have membrane-
bound cell organelles and lack a true nucleus, hence are grouped under the
domain “Prokaryota” together with Archae.
In a three-domain system, Bacteria is the largest domain. ( Living beings are classified
into Archae, Bacteria, and Eukaryota domain in the three-domain system.)
Bacteria, a singular bacterium, is derived from the Ancient Greek word “backērion” meaning
“cane”, as the first bacteria observed were bacilli.
The study of ‘Bacteria’ is called ‘Bacteriology’; a branch of ‘Microbiology’.
Watch Bacteria Create a Stunning Microscopic Flower Pattern
Evolution of Bacteria
Bacteria are considered as the first life-form to arise on the Earth about 4 billion years ago.
All other life-forms are evolved from the bacteria.
A hyperthermophile of about 2.5 – 3.2 billion years ago was the ancestor of bacteria and
archaea that are found in the present time.
Endosymbiotic association between different bacteria around 1.6 – 2.0 billion years ago give
rise to the first proto-eukaryotic cell, which gradually gives rise to eukaryotes.
Ecology (Habitat) of Bacteria
Bacteria are evolved to adapt and survive in any kind of ecological niches; from normal
to extreme environments. Hence, they are ubiquitous.
They are found in every possible habitat on the Earth; soil, air, and water. They are associated
with all the biotic and abiotic components of the Earth. They are essential components of
every ecosystem.
They are not only in normal ecological habitats but are also found in extreme environments.
Such bacteria are called extremophilic bacteria. They are found in extreme cold
(Psychrophiles), extremely hot (thermophiles), extreme pH (acidophiles and Alkaliphiles),
, extreme pressure (barophiles), anoxic environments (anaerobic), desertic area (xerophiles),
high radiation area, toxic wastes, barren sand and rocks, deep underground and mountain tip,
etc.
Ecology (Habitat) of Bacteria
Soil is the most heavily habituated place where they constitute about 0.5% W/W of the soil
mass. One gram of topsoil may contain as many as one billion bacterial cells.
It is estimated that there are approximately 2×1030 bacteria on the Earth, but only around 2%
of them are fully studied to date. Hence, there is a huge research gap on the diversity and
ecology of many unknown bacterial species.
Wide varieties of bacteria live in the body of all living beings, including higher plants,
animals, and even the human body. In an average human body (normal), there are about
1014 bacterial cells, while the body itself is made up of only 1013 human cells.
Structure of a Bacterial Cell
Bacteria are unicellular i.e. made up of a single cell. They are prokaryotes and their cells are
different from animal and plant cells. In general, the structure of bacteria can be studied as
external and internal structures;
Structure of a Bacterial Cell
External Structure of a Bacteria
It includes a cell wall and all the structures outside the cell wall.
1. Flagella (sing. Flagellum)
Flagella are long hair-like filamentous structures of about 4 – 5 μm long and 0.01 – 0.03 μm
in diameter. They confer motility to the bacteria. Flagella are divided into three parts;
filament, hook, and the basal body.
The filament is a threadlike part extending outside the cell wall. It is made up of flagellin
protein.
The hook is a short curved structure that joins filament with the basal body. It produces
repulsion like the propeller during the revolving of flagella.
The basal body is a set of rings embedded in the cell wall and plasma membrane. It consists
of 2 pairs of rings in Gram-Negative bacteria and 1 pair of rings in Gram-Positive bacteria. It
synthesizes polymers of the flagellum, produces energy for revolution, and regulates
movements of the flagellum.
Functions of Flagella
Responsible for motility
Aids in chemotaxis
Aids in bacterial pathogenicity and survival
2. Pili/Fimbriae
They are the short, hollow, non-helical filamentous structure of about 0.5 μm in length and
0.01 μm in diameter. They are exclusively found in Gram-Negative bacteria.
They are composed of protein ‘pilin’ arranged non-helically. They are short, numerous, and
straight than flagella.
Sex pili are a special kind of pili that take part in bacterial conjugation. They are larger than
usual pili; 10-20 μm in length. They are few in number, just 1-4 in number. They are further
classified into two types; F-pili and I-pili.
Functions of Pili/Fimbriae
Aids in adherence to host cells
Bacteria are microscopic, unicellular, prokaryotic organisms. They do not have membrane-
bound cell organelles and lack a true nucleus, hence are grouped under the
domain “Prokaryota” together with Archae.
In a three-domain system, Bacteria is the largest domain. ( Living beings are classified
into Archae, Bacteria, and Eukaryota domain in the three-domain system.)
Bacteria, a singular bacterium, is derived from the Ancient Greek word “backērion” meaning
“cane”, as the first bacteria observed were bacilli.
The study of ‘Bacteria’ is called ‘Bacteriology’; a branch of ‘Microbiology’.
Watch Bacteria Create a Stunning Microscopic Flower Pattern
Evolution of Bacteria
Bacteria are considered as the first life-form to arise on the Earth about 4 billion years ago.
All other life-forms are evolved from the bacteria.
A hyperthermophile of about 2.5 – 3.2 billion years ago was the ancestor of bacteria and
archaea that are found in the present time.
Endosymbiotic association between different bacteria around 1.6 – 2.0 billion years ago give
rise to the first proto-eukaryotic cell, which gradually gives rise to eukaryotes.
Ecology (Habitat) of Bacteria
Bacteria are evolved to adapt and survive in any kind of ecological niches; from normal
to extreme environments. Hence, they are ubiquitous.
They are found in every possible habitat on the Earth; soil, air, and water. They are associated
with all the biotic and abiotic components of the Earth. They are essential components of
every ecosystem.
They are not only in normal ecological habitats but are also found in extreme environments.
Such bacteria are called extremophilic bacteria. They are found in extreme cold
(Psychrophiles), extremely hot (thermophiles), extreme pH (acidophiles and Alkaliphiles),
, extreme pressure (barophiles), anoxic environments (anaerobic), desertic area (xerophiles),
high radiation area, toxic wastes, barren sand and rocks, deep underground and mountain tip,
etc.
Ecology (Habitat) of Bacteria
Soil is the most heavily habituated place where they constitute about 0.5% W/W of the soil
mass. One gram of topsoil may contain as many as one billion bacterial cells.
It is estimated that there are approximately 2×1030 bacteria on the Earth, but only around 2%
of them are fully studied to date. Hence, there is a huge research gap on the diversity and
ecology of many unknown bacterial species.
Wide varieties of bacteria live in the body of all living beings, including higher plants,
animals, and even the human body. In an average human body (normal), there are about
1014 bacterial cells, while the body itself is made up of only 1013 human cells.
Structure of a Bacterial Cell
Bacteria are unicellular i.e. made up of a single cell. They are prokaryotes and their cells are
different from animal and plant cells. In general, the structure of bacteria can be studied as
external and internal structures;
Structure of a Bacterial Cell
External Structure of a Bacteria
It includes a cell wall and all the structures outside the cell wall.
1. Flagella (sing. Flagellum)
Flagella are long hair-like filamentous structures of about 4 – 5 μm long and 0.01 – 0.03 μm
in diameter. They confer motility to the bacteria. Flagella are divided into three parts;
filament, hook, and the basal body.
The filament is a threadlike part extending outside the cell wall. It is made up of flagellin
protein.
The hook is a short curved structure that joins filament with the basal body. It produces
repulsion like the propeller during the revolving of flagella.
The basal body is a set of rings embedded in the cell wall and plasma membrane. It consists
of 2 pairs of rings in Gram-Negative bacteria and 1 pair of rings in Gram-Positive bacteria. It
synthesizes polymers of the flagellum, produces energy for revolution, and regulates
movements of the flagellum.
Functions of Flagella
Responsible for motility
Aids in chemotaxis
Aids in bacterial pathogenicity and survival
2. Pili/Fimbriae
They are the short, hollow, non-helical filamentous structure of about 0.5 μm in length and
0.01 μm in diameter. They are exclusively found in Gram-Negative bacteria.
They are composed of protein ‘pilin’ arranged non-helically. They are short, numerous, and
straight than flagella.
Sex pili are a special kind of pili that take part in bacterial conjugation. They are larger than
usual pili; 10-20 μm in length. They are few in number, just 1-4 in number. They are further
classified into two types; F-pili and I-pili.
Functions of Pili/Fimbriae
Aids in adherence to host cells
