Introduction to bacteria-I << Back
Introduction to bacteria-I
The purpose of this chapter is to describe the structural organisation of the prokaryotic cell i.e. Bacteria. The bacterial cell represents the simplest of all cellular organisms when seen under the microscope. Bacteria (plural word) is a prokaryotic structure. The singular for this word is “bacterium” . They do not have true nucleus. They have one chromosome of double-stranded DNA in a circular form. They reproduce by binary fission. They do not have any intracellular organelles. They belongs to kingdom Monera which is a very diverse group. There are some photosynthetic bacteria which can do photosynthesis--they don’t have chloroplasts, but their photosynthetic pigments are their cell membranes. These organisms are called blue green algae or cyanobacteria. They are the most common and omni present microorganisms on the earth.
Statement of cell theory
Structure of bacterial cell
A variety of structures are found in prokaryotic cells. Following image exhibits the typical structure of a cell.
All this structures are not found in all genus. Gram positive and Gram negative bacteria differs in their cell wall content. All bacterial cells were surrounded by a cell wall except mycoplasma. After cell wall there is a periplasmic space beneath which lies a plasma membrane. Bacteria does not contain the intracellular organelles so their internal structure is very simple. Genetic material consist of the circular chromosome lying in the nucleoid region. Ribosome and inclusion bodies are scattered in the cytoplasm. Many cells have flagella for locomotion. Pilli is also observed for sexual reproduction. Some bacteria also contain capsules surrounding their cell wall.
Morphology of bacteria
Generally four representative shapes of bacteria are observed.
Apart from the above sited shapes, some typical shapes of bacteria are also observed. But these shapes are restricted to few species only.
Pleomorphism: Some bacteria are variable in shape and lack a single, characteristic form and they are called as pleomorphic.
Pattern of cell arrangement
Bacterial cells may be categorised according to arrangement or cell grouping. Every bacterial species have their characteristic arrangement pattern. Variety of arrangement pattern is observed in cocci. Most common arrangement pattrns are
Bacteria range in size from about 0.2 to 5 micrometer. Smallest bacteria mycoplasma are about the same size as the largest viruses. (Pox virus). Cocci measures anywhere from 0.5 to 3.0 micrometer in diameter. Bacilli range from 0.2 to 2.0 micrometer in diameter and from 0.5 to 250 micrometer in length. Escherichia coli has the size of 1.1 to 1.5 micrometer wide by 2.0 to 6.0 micrometer long.
Habitats of bacteria
They are found in the bottom of the deepest oceans , at the peak of mount , inside the body of animals, and even in the frozen rocks and ice of Antarctica. One feature that has enabled them to spread so far, and last so long is their ability to go dormant for an extended period. They are found in the hot springs, frozen soils, acid drained mines, saline lakes.
Reproduction in Bacteria
Bacteria reproduce very rapidly. E.coli can reproduce in after every 20 minutes. Bacteria reproduce by two basic methods
Asexual reproduction involves only one parent. The offspring are exact copies of the parent.Bacteria splits into two cells using binary fission. Each daughter cell receives the same gentic material like parent cell.
It requre two parent cell. It involves the physical contact between the two cell. It occurs with the help of the conjugation tube. Here progeny cell have a mixture of the parent cells' traits. It occurs by the process of Conjugation.
Structures external to the cell wall
Majority of motile bacteria moves with the help flagella. It is a thread like appendages extending outward from the plasma membrane and cell wall. Size of the flagella is 15-20 micrometer long. The detailed structure of a flagellum can only be seen in the electron microscope.
Flagellation (Flagellar arrangement):There are different arrangement of flagella found on bacterial cell. Different pattern of flagellation are
Ultra structure of flagella:It consist of three parts.
Basal body is most complex structure of the flagella. In E.coli , basal body consist of four rings i.e. L,P,S,M rings. Gram positive bacteria have only two basal rings.
Chemical composition of Bacteria: It is made up of protein fibers. These protein fibers are made up of flagellin protein. Molecular weight of the flagellin protein is in the range of 20 kd to 60 kd. Flagellin protein of different bacterial species is not identical.
Mechanism of Flagellar movement:Bacteria moves when this flagellar helix rotates. Direction of flagellar rotation determines the nature of bacterial movement.Monotrichous polar flagella rotates counterclockwise during normal forward movement.Monotrichous bacteria stop and tumble randomly by reversing the direction of flagellar rotation. Peritrichous flagella forms bundle of the flagella and moves in the similar way. It is believed that flagella moves due to interaction of the rings present in the basal body.
Swimming motility: Spirochetes moves due to the axial filament present in between the periplasmic space. It is also called endoflagellum.
Gliding Motility: This movement occurs without flagella. Gliding bacteria are able to move on solid surface.
Question : 1 Mycoplasmas are bacterial cells that
Question : 2 The mode of reproduction which occurs in mycoplasma is
Question : 3 The microbiological term describing the spherical shape of the bacterium is
Question : 4 Which of the following is incorrect about capsules and slime layers?
Question : 5 Fimbriae are the bacterial appendages which
Question : 6 A bacillus bacterium with a single flagellum at each end is described as
Question : 7 The growth of flagellar filament in bacteria occurs by
Question : 8 Which of the following statement is correct regarding the ultra structure of flagella
Question : 9 Which of the following is incorrect regarding sex pili of bacteria
Question : 10 Which of the following is true regarding gliding motility
Introduction to structure of bacteria