Bacteria| Introduction,Structure,Classification, Reproduction|Medical World

What is Bacteria?

Introduction

Bacteria commonly called as microbe, are one type of simple and microscopic unicellular living organism. These are prokaryotic organisms, which cannot pass through bacteria. Bacteria is a greek word, whose meaning is rod. Bacteria were discovered by Anton van Leewenhoek in the year 1676.

Bacteria are related to plants in having cell wall and autotrophic mode of nutrition. At least in some species. For this reason they are included under plant kingdom. However, the motility of certain bacteria due to the presence of flagella is often considered as animal character. Due to similarly with unicellular fungus yeast and unicellular protozoon, they are now a days considered as a living organism under separate kingdom Protista.

Definition

One type of aerobic or anaerobic , smallest and simplest unicellular microscopic amazing characteristic organism, found occur in airline water, in soil, in living organisms etc. and familiarly known as microbe, is called bacteria.

Bacteria are separate group of microbes, possessing both plant and animal characters. They do not posses true nucleus and is prokaryotic in nature. They multiply mainly by means of binary fission.

Features of Bacteria

Bacteria have the following characteristics features:

1. They are smallest known as cellular organism under living world. Their diameter is less than 3micro.
2. Generally they are unicellular, but sometimes certain bacteria may be united to form a filament.
3. Bacteria cell is surrounded by a cell wall.
4. They are mostly hetertcophic e.g. Neisseria but a few may be autotrophic.
5. The bacteria cell is without chlorophyll, hence they are heterotrophic . Certain bacteria like rhodospirillum etc.
6. They do not posses true nucleus; only one spiral DNA is not associated with protein.
7. They are devoid of mitochondria, Golgi apparatus , endoplasmic reticulum , chloroplast etc
8. Bacteria can take only liquid foos matters.
9. Mitotic posses of cell devision is absent within the bacterial cell.
10. Bacteria mainly reproduce by means of binary fission.

Why Bacteria are prokaryotic?

Bacteria are prokaryotic due to the following reasons:

1. The main chemical composition of the bacterial cell wall is mucopeptide
2. Nucleus is primitive without nuclear membrane and nucleus.
3. Cell membrane bounded cell organelles like mitochondria, endoplasmic reticulum, Golgi bodies and chloroplast are totally absent within the bacterial cell.
4. 70s nature of ribosomes are irregularly scattered within the cytoplasm
5. Cell devision process is amitotic

Why bacteria are regarded as plants?

Bacteria are regarded as plants due to the following reason:

1. Like plants, the cell wall of bacteria is also composed of lipid, protein and polysaccharide
2. Like plants, bacteria also reproduces by means of vegetative reproduction.
3. Like plants, bacteria can also synthesise vitamins.
4. The presence of bacteriochlorophyll in the body of some bacteria.

Structure

The Escherichia coli is a gram negative bacterium. They live in the intestine of human being. Certain special strains of E.Coli causes dysentery, giving trouble to human being . The body of E.coli is divided into two main parts,

1. Outer covering
2. Protoplast

Outer covering

They are consists of the following terms-

Outer membrane

It is consists of the following terms-

Cell wall

The protoplast of bacteria cell is surrounded externally by a rigid, thick, elastic cell wall. Basically the cell wall is composed of 10-20% complex nitrogenous material called mucopeptide, lipoprotein, lipopolysaccharide, and phospholipid. Cell wall does not stain with gram stain.

Function

Gives the shape and integrity of the cell. It protects the cell protoplast from external injury.

Slime layer or Capsule

The cell wall is encircled by a slime layer. It is a thick sticky substance layer. This layer is formed by the secretary products of their cell. When the slime is conspicuously thick and composed mainly of polysaccharide and polypeptide then the rigid thick layer is membrane and called capsule.

Function- protects the bacterial cell from dryness.

Cell membrane

The outermost part of the cytoplasm is called cytoplasmic membrane or cell membrane. The semi-permeable membrane lies just below the cell wall. It is made up of protein and lipid.

Protoplast

Cytoplasm

the semi-liquid , semi-transparent, homogenous, colourless, jelly like granular substance lying between the cytoplasmic membrane and nuclear material is known as Cytoplasm. Cytoplasm contains carbohydrate, protein, lipid, mineral salts and other different materials. Cell organelles covered by cell membranes like plastid, mitochondria, Golgi apparatus, endoplasmic reticulum, etc are totally absent within the bacterial cell. But ribosome, mesosome, vesicle and different reserve substances are present within the cytoplasm.

70s Ribosome

The ribosome are tiny spherical ribo-nucleoprotein particles. They lie scattered within the cytoplasm and its nature is 70s .

Mesosome

The cytoplasm membrane extends within the cytoplasm of the bacterial cell forming tubular or loop like vesicular bodies called mesosome which contain various kind of respiratory enzymes.

Function

1. Takes part in the differential wall formation during cell devision.
2. Forms the replica DNA
3. It is supposed, that the other function of the mesosome is to establish connection between the cytoplasmic membrane and nuclear material.

Vacuole

Bacterial cell may posses one or more vacuole.

FunctionWithin the cytoplasm, where food and gases are stored.

Flagella

Body cell posses one or more whip like structure called flagella. These flagella originate from basal granules or blepharoplasts lying just outside the cell membrane .

Flagella helps in locomotion.

pili

On the outer side of the cell wall small thread like outgrowths present are called pili, which are different from the flagella.

Function- Act as an organ of adhesion for attachment and may form bridges during conjugation.

Storage product

Solid storage product are present in the cytoplasm.

Function-Starch, lipid, protein, glycogen, sulphur, vitamin etc are stored.

Classification of Bacteria

Bacterial classification on the basis of morphology

1. Coccus
2. Bacillus
3. Spirillum
4. Comma

Coccus

the bacteria included under this group are unicellular and spherical or oval shaped.

Example - Micrococcus flavus

Diplococcus

when two coccus lie together, then it is called diplococcus.

Example - diplococcus pneumoniae

Streptococcus

when many coccus unite together like a chain, then they are called streptococcus.

Example- streptococcus pyrogens.

Staphylococcus

when many coccus remain united together like bunch of grapes then they are called staphylococcus.

Example- staphylococcus aureus - a boil or wound causing bacteria.

Sarcina

when many coccus together form a cuboidal shaped structure then it is called sarcina.

Bacillus

his type of bacteria looks rod like their body possess one or many flagella.

Example - Bacillus typhi

Spirillum

they look like spiral . In many cases one or many flagella are present at their body ends.

Example- spirillum

Vibrio

this type of bacteria look like a comma shaped. Sometimes they are also called comma bacillus.

Example- vibrio cholerae

Bacterial classification on the basis of staining behaviour

They are divided into two terms-

1. Gram positive
2. Gram negative

Gram positive

Those bacteria which takes gram stain made up of crystal violet and iodine are called Gram positive bacteria

Gram negative bacteria

those bacteria which do not get coloured by gram stain are called gram negative Bacteria.

Reproduction

Asexual reproduction

Asexual reproduction takes place by means of endospores formation. Bacteria like clostridium bacillus etc possess the capacity of producing thick walled endospores within the body of parent cell possessing protoplast and genetic material. The endospores are resistant to heat, chemical and desiccation I.e. endospores can survive the most unfavourable environmental conditions. Many bacteria but not their endospores may be killed by means of diluted chemical like chlorine and iodine.

Sexual reproduction

They are follow as-

Conjugation

The two strains taking part is the male and female cells. Male cells are called genetic donors as they are capable of donating the F factor I.e. a circular piece of DNA representing sexuality. In female cells or recipients this factor is absent. So they are F-.

Transformation

This type of genetic recombination results in mixed character of both donor and recipient. This generally occurs when a strains of bacteria is grown in presence of dead cell culture filtrates or cell extracts of a related strains.

Transduction

In this process DNA of one bacterium is transported to another through bacterium is transported to another through bacteriophage serving as vector. In this process the transfer of genetic material from one bacterium to another bacterium occurs due to a virus agent.