Botany Notes “Eubacteria-Biological Classification” Chapter 2, for Class 12-CBSE


Bacteria are cosmopolitan and occur in every habitat wherever living or dead organic matter is present. Anton von Leeuwenhoek  discovered bacteria in rain water which had been allowed to stand for many days and tartar scrapped from teeth. In 1695, he published his work “The Secrets of Nature”.

A.V. Leeuwenhoek termed these microorganisms as dierkens which was later translated as animalcules by the Royal society. Term microbe for animalcules was coined by Se’ dillot, but the term microorganism was proposed by Pasteru.

  1. Ehrenberg first of all coined the word ‘bacteria’
  2. Louls Pasteur is considered father of modern microbiology. He introduced the term aerobic and anaerobic for the life in the presence or absence of oxygen respectively.
  3. Robert Koch, a German doctor , demonstrated that the anthrax disease of sheep was caused by bacteria. Koch had followed four experimental steps (Koch’s postulates) which help to establish a relationship between a microorganism and a disease.
  4. Smallest bacterium : Dialister pneumosintes
  5. Largest filamentous bacterium : Beggiatoa mirabilis

Shapes of Bacteria

Bacteria occur in four basic forms of shapes. These are – spherical (Cocci), rod shaped (Bacilli), Vibrio and Spiral. Though most bacterial species have cells that are of a fairly constant and characteristic shape, some species are pleomorphic (i.e., these can exhibit a variety of shapes), e.g., Rhizobium leguminosarum.

a.Coccus : Spherical or nearly spherical, aflagellate, sub-divided into six groups on the basis of cell arrangement :

  1. Monococcus – Only single cell represents the bacterium, e.g., Micrococcus luteus, M. roseus.
  2. Diplococcus – Cocci divide in one plane and remain attached in pairs, e.g., Meningococcus, Gonococcus, Diplococcus pneumonia.
  3. Streptococcus – Cocci remain attached to form chains of different lengths, e.g., Streptococcus lactis.
  4. Tetracoccus – Cocci divide in two planes at right angles to one another and form groups of four, e.g., Tetracoccus, Neisseria.
  5. Staphylococcus – Cocci divide in several planes resulting in formation of irregular bunches of cells, sometimes resembling a cluster of grapes, e.g., Staphylococcus aureus.
  6. Sarcinae – Cocci divide in 3 planes at right angles to one another and resemble cubical packets of 8 or more cells forming three dimensional figure, e.g., Sarcina lutea.

  1. Bacillus : Rod – like forms, either singly or may be arrange differently. They are generally flagellate. It is the most common of all the shapes . they are of following types :
  2. Monobacillus – The bacteria occur singly, e.g., Bacillus anthracis, Lactobacillus.
  3. Diplobacillus – Bacteria are arranged in pairs.
  4. Streptobacillus – Bacteria form a chain of rods, e.g., Streptobacillus.
  5. Palisadelike – If the cells are lined side by side like match sticks and at angles to one another.

  1. Spiral bacteria : Coiled forms of bacteria exhibiting twists with one or more turns are called spirilla, e.g., Spirillum volutans.
  2. Vibrio : Bacteria with less than one complete twist or turn are called vibrio. These resemble a comma (,) in appearance, e.g., Vibrio Cholerae.
  3. Stalked bacteria : The body of bacterium possesses a stalk, e.g., Caulobacter.
  4. f. Budding bacteria : The body is swollen at places, e.g., Rhodomicrobium.

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