AAT Bioquest

What are the shapes of bacteria?

Posted March 2, 2023


There are four basic bacterial shapes – spherical, rod-shaped, comma-shaped, and helical. Each bacterial species has a characteristic shape by which it is identified. 

Spherical shaped bacteria - Coccus 

Cocci are spherical-shaped bacteria. After dividing, the cells remain attached to one another in distinctive arrangements by which they are identified. Based on the cell arrangement, cocci bacteria can be classified into specific subtypes: 

  • Monococcus – Coccus bacteria exist as single spherical cells. 
  • Diplococcus – The cells in diplococcus bacteria are arranged in pairs after cell division. 
  • Streptococcus – After cell division, streptococcus cells join in a single plane in a chain-like arrangement. 
  •  Tetrads – In tetrads, cell division occurs in two planes, arranging the cells in groups of fours. 
  • Staphylococcus – In staphylococcus bacteria, cell division occurs in three different planes, arranging the cells in an irregular cluster that resembles a bunch of grapes. 
  • Sarcinae – Sarcinae bacteria occur as groups of eight cells. 

Rod-shaped bacteria – Bacillus

Bacilli are rod-shaped, gram-positive bacteria. Bacillus bacteria typically occur in chains and are found in soil and water. They can be divided into various sub-types, based on way the rod shaped cells are arranged: 

  • Bacillus – Bacillus bacteria occur as a single unattached cell that resembles a small rod. 
  • Diplobacilli – Diplobacilli occur as two rods attached to one another. They are found in pairs after cell division. 
  • Streptobacilli – After cell division, streptobacilli cells join in a single plane in a chain-like arrangement. 
  • Coccobacilli – With their oval shape and shorter size, coccobacilli resemble 
  • Palisades – After cell division, palisade bacilli bend, resulting in a structure that resembles a palisade fence.  

Comma-shaped bacteria – Vibrio

Vibrio are curved bacteria that appear like a comma with 1 – 3 whip-like structures at one end.  

Helical or spiral bacteria – Spirilla

Spirillas have a spiral or helical shape with whip-like flagella at one end. They are divided into two subtypes based on thickness, flexibility and motility of the cell. 

  • Spirillum – Spirillum are rigid bacteria that have external flagella. 
  • Spirochete – These are thin, flexible bacteria with internal periplasmic flagella. 

Most bacteria are monomorphic – they maintain a single shape. However, a few are pleomorphic – they are capable of assuming different shapes and lack a single characteristic shape.

Additional resources

The evolution of spherical cell shape; progress and perspective

Gram Staining

MycoLight™ Rapid Fluorescence Gram-Positive Bacteria Staining Kit