Anton van Leeuwenhoek discovered the bacteria in 1675 and actually called these organisms as animalcules.
Thereafter, no attention was given to the organisms for nearly two centuries. However, Louis Pasteur in 1864 and Robert Koch 1876 reported that these microorganisms could cause diseases. E.H. Haeckel (1886) placed bacteria in the kingdom of Protists along with fungi, algae and protozoa. Copeland (1956) suggested that the bacteria including Cyanobacteria were different and hence placed them in a new kingdom Monera.
However, bacteria have many features similar to the lower groups of plants. These characteristics are:
1. Occurrence of a definite and rigid cell wall which in few species contain cellulose.
2. The tendency of the organisms to grow in filaments.
3. The ability of the autotrophic bacteria to synthesize organic food from inorganic materials such as carbon dioxide and water.
4. The absorption of food in soluble form through cell wall.
5. Structure of bacterial cell and reproductive methods are roughly similar to the thallophytic plants.
Still bacteria have certain unique features which contributed to their segregation from the plant kingdom. They are:-
1. The cell wall composed mainly of peptidoglycans, i.e. acetyl glucosamine and acetylmuramic acid. In plants, cell wall component is mainly of cellulose.
(1) 2. There is no true nucleus since the nuclear membrane, nucleolus are absent. Nuclear histone proteins are also absent.
(2) 3. No cytoplasm organelle excepting ribosome present in bacterial cell. The ribosome is again of 70S type in contrast to 80S types of the eukaryotic organisms.
(3) 4. Sexual mode of reproduction which consists of meiosis, plasmogamy and karyogamy is absent in bacteria.
Status of Viruses
Russian botanist Dmitri Iwanowski (1892) discovered the viruses causing tobacco mosaic disease. Later, W.M. Stanley (1935) isolated these agents and identified them. He concluded that viruses are complex aggregation of only nucleic acids and proteins.
These are non cellular, ultramicroscopic structures. They grow and multiply only inside the living cells. Hence, their status in the living world is a long vexed question.
They were not even taken into consideration in any system of classification. However, some give the viruses the status of the living beings whereas others strongly object to it.
1. Growth and reproduction are chief identifying characters of living beings. Viruses possess both; they grow, reproduce and even undergo mutation.
2. Like living pathogenic organisms, they are transmissible. (Even the bacterial filter papers, that can hold back the smallest bacteria, cannot do so for viruses. They pass through these filters.)
3. In extremely dilute dosages, the viruses are effective in causing disease.
4. These are obligate parasites.
The viruses are hence, given the status of primitive microorganisms from which higher forms of life have evolved.
The adherents of nonliving hypothesis have observed that
(1) The viruses can be extracted in the form of crystals from the diseased tissues.
(2) They are quite inert, apparently lifeless outside the living cell. Hence, they can be treated like any chemical agents. They can neither metabolize nor multiply in this form.
(3) They are not infective themselves. However, once they come in contact of suitable host, they are capable of causing infection.
All these prompted the proponents of regressive theory to assume that the viruses are degenerate microorganisms being degraded from primitive, living organisms.
Therefore, viruses are referred to as cell components.
Adherents of the theory believe that the viruses are part of the cell. The nucleic acid is the genetic component which has the ability to leave the cell and be transferred to other cells.
It is independent enough to pass from cell to cell and impose its patterns on the infected host cells. Similar structures are shown by the cellular organelles like mitochondria and chloroplasts, but they have so strongly assimilated in the living cells that they have lost their ability of transmission from one cell to another.