Word bacteria (singular bacterium) means small rod because the first bacteria were rod shaped. Bacteria are the smallest unicellular prokaryotic organisms (Organisms that lack true nucleus and membrane bounded organelles) that cannot be seen with naked eye but can be seen only with the help of microscope. Bacteria were discovered by a Dutch scientist Leeuwenhoek in 1676.
Origin of Life on Earth: Life on earth evolves billions of years ago. According to some scientists 4 billion of years ago life on earth evolves. The first living organisms evolved on earth were single celled prokaryotic organisms.
Occurrence: Bacteria found everywhere in the earth. It means they are found in water, on soil, in atmosphere, in the body of living organisms and also on the body of living organisms. Bacteria are present in specific environments such as hot springs, alkaline/acidic soil, highly saline environments, in highly polluted soils and waters. Kind and number of bacteria are different in different environments. Some bacteria are present in almost all environments and are termed as the normal flora.
Economic Importance of bacteria: Bacteria play important role in establishing balance in the universe. Some bacteria are beneficial for us, other living organisms and for the environment as well, while some bacteria are harmful as well for living organisms. Most important benefit of bacteria is decomposition process. Some bacteria produce useful products. Some bacteria living in the body of living organisms are important such as some bacteria are helpful in digestion process.
Size of Bacteria: Many scientists think that there exist some small living organisms that are not visible through naked eye. After the discovery of microscope Leuwenhoek for the first time observe these organisms and were named as animalocelues. He observed these small organisms in different substances like rain water, vinegar, etc.
Luis Pasteur further worked in this field. He proved that small organisms or microorganisms cause diseases and developed vaccines for different diseases. He also developed the process of pasteurization.
Robert Koch formulated the ‘germ theory of disease’. He isolated typical rod shaped bacteria with
squarish ends (baccilli) from the blood of sheep that had died of anthrax. He also isolated the bacteria for the first time that caused tuberculosis and cholera.
STRUCTURE OF BACTERIA:
All bacterial cells invariably have a cell membrane, cytoplasm, ribosome, and chromatin bodies. Lot of bacterial cells have a cell wall. The hard cell wall gives shape to the bacterial cell. Specific structures like capsule, flagella, pili, pimbriae and granules are found in some bacteria and are not found in all bacteria.
Bacteria range in size from about 0.1 to 600 nm over a single dimension. Bacteria have different sizes. Some bacteria are smallest in size (e.g., some members of the genus Mycoplasma) and are about 100 to 200 nm in diameter, approximately the size of the largest viruses (poxviruses). Some of the bacteria are largest in size, for example, Escherichia coli, a bacillus of about average size, is 1.1 to 1.5 nm wide by 2.0 to 6.0 nm long. Some spirochetes occasionally reach 500 nm and are largest size bacteria whereas Staphylococci and Streptococci are 0.75 – 1.25n in diameter and are of normal size bacteria. A huge bacterium Epulopiscium fishelsoni has been discovered in the intestine of Acanthurus nigrofuscus (Brown fish). It’s size is 600 micron meter by 80 micron meter.
Shape of Bacteria: On the basis of general shape, bacteria are classified into 4 main categories. These shapes are known as cocci (the round shaped bacteria), bacilli (the rod shaped bacteria) and spiral (The spring shaped bacteria) and vibrio (the comma shaped bacteria). Most of the bacterial cells have constant throughout entire life but some bacterial cells are pleomorphic and change their shapes.
- Cocci: They are round shaped bacteria and are further divided into following types.
a) Monococcus: They are single and are not arranged in groups.
b) Diplococci: They are found in groups of 2. It means that 2 bacteria are present in one group.
c) Tetrad: These round shaped bacteria are present in a group of 4.
d) Sarcina: These are groups of 8 round shaped bacteria.
c) Streptococci: These are in chain like manner.
d) Staphylococci: Such round shaped bacteria occurs in bundle.
- Bacilli: Bacilli are long rod shaped bacteria. They are further divided into following types.
a) Monobacillus: Mono means one. These are single rod shaped bacteria and not occurred in groups.
b) Diplobacilli: These rod shaped bacteria occur in groups of 2.
c) TetradBacilli (Tetra Baciili): Tetra means 4. These occur in group of 4 rod shaped bacteria.
d) Streptobacilli: They are rod shaped bacteria that occur in chain like manner.
e) Staphylobacilli: These rod shaped bacteria occur in bundle or bunch form.
- Spiral Shaped Bacteria: These are like the spring or look like a worm and are not occur in groups.
- Vibrio Shaped Bacteria: These bacteria are comma shaped and they also not occur in pairs.
Structure of Bacterial Cell: The structure of bacterial cell can be studied under the following headings.
- Flagella: Flagella are small hair or thread like thin structures that come out of the cell wall. These flagella are used for locomotion purpose. These flagella originate from the basal body. Flagella are composed of a protein called flagellin. Most of the bacilli and spirilla have flagella. Other bacteria very rarely have flagella.
Classification of Bacteria Based on presence or absence, attachment and number of flagella: On the basis of presence and number of flagella or attachment of flagella bacteria are classified into the following groups.
a) Atrichous: The bacteria that do not have any flagella or the bacteria that lack flagella are termed as atrichous. (The question arise here is how these bacteria move? These bacteria either do not show locomotion at all or move by a process called gliding).
b) Monotrichous: These bacteria have single polar flagella (1 flagellum).
c) Lopotrichous: These bacteria have a bunch or tuft of flagella at one pole only.
d) Amphitrichous: These bacteria have tuft or bunch of flagella at both the poles.
e) Peritrichous: In this the flagella surrounds the whole body of the bacterial cell.
Functions of flagella: The two main functions of flagella are as follows:
a) Motility: Primary function of flagella is to help bacteria in movement.
b) Chemotaxis: Second important function of bacterial flagella is to detect the chemical signals or stimulus. In this way the bacteria move according to the chemical stimulus, either away from the stimulus or towards the stimulus. This phenomenon is termed as chemotaxis.
Pilli (Fimbriae): Pilli are short rod like appendages on bacterial body. The Pilli help in reproduction (Conjugation). These are shorter than flagellum and are composed of protein called pillin. Pilli do not take part in locomotion or movement but can actively help in conjugation process.
Function of Pilli: Pilli are involved in mating process or reproduction. Pilli are also responsible for attachment of bacteria to various surfaces.
The Cell Envelope or the Outer Covering of Bacteria: In bacteria the covering that covers the protoplasm is termed as cell envelope. Cell envelopes include cell wall, slime, capsule, etc.
A) Capsule: Many bacteria produce a substance that make a protective layer outside the cell wall and is termed as capsule. Capsule is made up of polysaccharide units or proteins or of both. The capsule is sticky in nature and thicker.
B) Slime: It is loose as compared to capsule and is also called the loose capsule or slime capsule and is present in some bacteria. Slime provides pathogenicity and protects the bacteria against phagocytosis.
C) Cell Wall: Beneath the capsule and above the cytoplasmic membrane cell wall is present. Cell wall is rigid and gives a proper shape to bacteria. Cell wall protects bacteria from any damage (Osmotic lysis). It is absent in mycoplasma. Some bacteria lack cell wall like Mycoplasma.
- Gram Staining: Gram staining is a technique introduced by a Christian Mr Gram. On the basis of the cell wall characteristics the bacteria can be divided into 2 groups. These two types of bacteria are gram negative and gram positive bacteria.
>> Gram Positive Bacteria: These bacteria retain the primary stain that is purple.
>> Gram Negative Bacteria: These bacteria do not retain the primary stain and retain the secondary stain that is pink.
Difference between gram negative and gram positive bacteria: Difference in staining of gram negative and gram positive is due to the difference in structure of cell walls. The following table shows the difference between gram positive and gram negative bacteria.
Gram Positive Bacteria
Gram Negative Bacteria
ii. Techoic Acid
iii. Lipotechoic Acid
Chemical Composition of Cell Wall of Different Bacteria: The cell wall of most of the bacteria is composed of a molecule termed as peptidoglycan. Peptidoglycan is composed glycan chain linked with peptides. The cell wall also contains other chemicals like techoic acid, lipoproteins and lipopolysaccharides. The cell wall of archaebacteria does not contain peptidoglycan and their cell wall contains protein, glycoproteins and polysaccharides.