Morphology of Bacteria

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The unit of measurement used in bacteriology is the micron (micrometre)

  • 1 micron (μ) or micrometre (μm) – one-thousandth of a millimetre
  • 1 millimicron (mμ) or nanometer (nm) – one-thousandth of a micron or one-millionth of a millimetre
  • 1 Angstrom unit (Å) – one-tenth of a nanometer

The limit of resolution with the unaided eye is about 200 microns. Bacteria are
smaller which can be visualized only under magnification. Bacteria come in a great many sizes and several shapes. Most bacteria range from 0.2 to 2.0 μm in diameter and from 2 to 8 μm in length. (anoverall average size of 1 to 10 μm.)


Depending on their shape, bacteria are classified into several varieties

  1. Cocci (from kokkos meaning berry) are spherical or oval cells
  2. Bacilli (from baculus meaning rod) are rod-shaped cells
  3. Vibrios are comma-shaped curved rods and derive their name from their characteristics vibratory motility.
  4. Spirilla are rigid spiral forms.
  5. Spirochetes (from speira meaning coil and chaite meaning hair) are flexuous
    spiral forms
  6. Actinomycetes are branching filamentous bacteria, so-called because of the fancied resemblance to the radiating rays of the sun when seen in tissue
    lesions (from actis meaning ray and mykes meaning fungus)
  7. Mycoplasmas are bacteria that are cell wall deficient and hence do not
    possess a stable morphology. They occur as round or oval bodies and as
    interlacing filaments.


Cocci are usually round but can be oval or elongated. When they divide to reproduce, they remain attached to each other.

Classification According to Plane of Division


Those Cocci that remain in pairs after dividing are called diplococci.


Those cocci that divide and remain attached in chainlike patterns are called streptococci.


Those cocci that divide into two planes and remain in groups of four are known as tetrads.


Those cocci that divide into three planes and remain attached in cube-like groups of eight are called sarcina.


Those cocci that divide into multiple planes and form grapelike clusters or broadsheets are called staphylococci.


Bacillus is the rod-like bacteria e.g. Bacillus anthracis

Single bacillus

Most bacilli appear as single rods, called single bacilli.


Those bacilli which appear in pairs after division called Diplobacilli


Those bacilli found in chains in a single plane are called Streptobacilli.


Bacteria that maintain single shape called monomorphic or the bacteria having the same shape.


Some bacteria can have many shapes known as pleomorphic. e.g: Corynebacterium, Pyogenesis.

  • Some bacteria occur in the shape of star e.g.Stella.
  • Some bacteria occur in the shape of rectangle e.g. Haloarcula.
  • Some bacteria also appear triangular in shape.

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