There are two types of classification system, viz. artificial classification system and natural classification system.
- Artificial Classification System: This system was proposed by Linneaus. It was based on vegetative characters and androecium structure.
- Natural Classification System: This system was used by George Bentham and Joseph Dalton Hooker. This system was based on natural affinities among organisms. Both external and internal features were considered; in this system
Two Kingdom Classification:
- Animal Kingdom
- Plant Kingdom
Five Kingdom Classification:
Robert H. Whittaker 1969
This is the most accepted system of classification. The five kingdoms are as follows:
- Plantae and
Whittaker used following criteria for classification of living beings:
- Cell Structure
- Thallus Organization
- Mode of Nutrition
- Reproduction and
- Phylogenetic Relationships
The bacteria are kept under the Kingdom Monera. They are prokaryotic and possess cell wall. The cell wall is composed of polysaccharides and amino acids. Bacteria can be autotrophic and heterotrophic. The autotrophic bacteria can be chemosynthetic or photosynthetic. The heterotrophic bacteria can be saprophytic or parasitic.
Based on their shape, bacteria are classified into four types:
- Spherical bacteria are called Coccus (pl.: cocci),
- Rod-shaped bacteria are called Bacillus (pl.: bacilli),
- Comma-shaped bacteria are called Vibrium (pl.: vibrio) and
- Spiral shaped bacteria are called Spirillum (pl.: spirilla)
Archaebacteria: These are believed to be the oldest living beings. The archaebacteria live in some of the harshest habitats; like sulphur springs, volcanic crater, etc. The different structure of their cell wall helps them in surviving in extreme conditions. Based on their habitats, the archaebacteria are classified as follows:
- Halophiles: They live in extremely salty areas.
- Thermoacidophiles: They live in hot spring.
- Methanogens: They live in marshy areas. They also live in the guts of the ruminant animals. They are responsible for production of methane from the dung of these animals.
Eubacteria: They are also called the ‘true bacteria’. They possess a rigid cell wall, and a flagellum (in motile bacteria). The cyanobacteria are also called ‘blue-green algae’ because they contain chlorophyll. The cyanobacteria can be unicellular or filamentous. They can live solitary or in colonies. The colony of cyanobacteria is usually surrounded by a gelatinous sheath. Some of the cyanobacteria are capable of nitrogen-fixation, e.g. Nostoc and Anabaena.
Heterotrophic: These are the most abundant organisms in nature. Most of them have economic significance for human beings. While many of them are beneficial for humans, many others are quite harmful.
Reproduction in Bacteria:
Bacteria usually reproduce by binary fission. Under unfavourable conditions, they reproduce by spore formation. They also reproduce by adopting a primitive type of DNA transfer from one bacterium to another. This is similar to sexual reproduction.