Bacillus (shape)

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Bacteria display a large diversity of cell morphologies and arrangements.

A bacillus (plural bacilli) is a rod-shaped bacterium. Bacilli are found in many different taxonomic groups of bacteria. However, the name Bacillus, capitalized and italicized, refers to a specific genus of bacteria. The name Bacilli, capitalized but not italicized, can also refer to a less specific taxonomic group of bacteria that includes two orders, one of which contains the genus Bacillus. When the word is formatted with a lowercase and not italicized, 'bacillus', it will most likely be referring to shape and not to the genus at all. The shape bacillus can also be called rods. [1]

Bacilli usually divide in the same plane and are solitary, but can combine to form diplobacilli, streptobacilli, and palisades.[2]

  • Diplobacilli: Two bacilli arranged side by side with each other.
  • Streptobacilli: Bacilli arranged in chains.
  • Coccobacillus: Oval and similar to coccus (circular shaped bacterium).


There is no connection between the shape of a bacterium and its color in the Gram staining. MacConkey agar can be used to distinguish among Gram negative bacilli such as E. coli and salmonella.[4]

Examples of Gram positive bacilli[edit]

Examples of Gram negative bacilli[edit]



  1. ^ "The Size, Shape, And Arrangement Of Bacterial Cells". Midlands Technical College. Retrieved 8 August 2016. 
  2. ^ "Chapter 4: Functional Anatomy Of Prokaryotic And Eukaryotic Cells". Archived from the original on 23 September 2012. 
  3. ^ Kaiser GE. "Sizes, Shapes, and Arrangements of Bacteria". The Community College of Baltimore County. Retrieved 8 August 2016. 
  4. ^ "Gram Negative Bacilli". NYU School of Medicine. Archived from the original on 14 March 2009. 
  5. ^ "Gram negative bacilli". InfectionNet. Vancouver Island Health Authority. Retrieved 8 August 2016.