Bacteria Definition and Shape Of Bacteria

Bacteria are small, single-celled organisms that can live in many different habitats. Some cause disease, while others are beneficial. Bacteria are classified as prokaryotes, which are organisms without a nucleus or other organelles.

Most bacteria are between 0.5 and 5.0 micrometers in size. That means that a single bacterium is about the width of a human hair! Bacteria are so small that they can only be seen with a microscope.

Shape Of Bacteria

Depending on their shape, bacteria are classified into several, A photo showing the types and shapes of bacteria is shown below.

Different morphological types of the bacteria.
Different morphological types of the bacteria


The cocci (kokkos, berry) are oval or spherical cells. These may be arranged in pairs (e.g., pneumococci, meningococci, and gonococci), tetrads (micrococci), chains (e.g., streptococci), and clusters (e.g., staphylococci).


The bacilli (bacillus, rod) are rod shaped. These bacilli may show either of the following arrangement:

  1. Coccobacilli: Length of the bacteria is approximately the same as its width, e.g., Brucella.
  2. Streptobacilli: These are arranged in chains, e.g., Streptobacillus.
  3. Comma shaped: They exhibit curved appearance, e.g., Vibrio.
  4. Spirilla: They exhibit rigid spiral forms, e.g., Spirillum.


Spirochetes (spira, coil; chaite, hair) are slender, flexuous spiral forms, e.g., Treponema.


Actinomycetes (actin, ray; mykes, fungus) are branching filamentous bacteria resembling fungi. They possess a rigid cell wall.

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