Bacteria can have different shapes based on how they divide and grow. The three basic shapes of bacteria are: coccus, bacillus, and spiral¹²³.

• Coccus: These are spherical or oval bacteria that can be single or arranged in groups. Some common arrangements of cocci are:
  • Diplococcus: Two cocci in a pair (e.g. Streptococcus pneumoniae) ¹.
  • Streptococcus: A chain of cocci (e.g. Streptococcus pyogenes) ¹.
  • Tetrad: A square of four cocci (e.g. Micrococcus luteus) ¹.
  • Sarcina: A cube of eight cocci (e.g. Sarcina ventriculi) ¹.
  • Staphylococcus: A cluster of cocci, often grape-like (e.g. Staphylococcus aureus) ¹.
• Bacillus: These are rod-shaped bacteria that can be single, in pairs, or in chains. Some common arrangements of bacilli are:
  • Single bacillus: One rod-shaped bacterium (e.g. Escherichia coli) ².
  • Streptobacillus: A chain of bacilli (e.g. Bacillus anthracis) ².
  • Coccobacillus: A short and oval bacillus that resembles a coccus (e.g. Haemophilus influenzae) ².
• Spiral: These are bacteria that have a curved or twisted shape. Some common types of spiral bacteria are:
  • Vibrio: A comma-shaped bacterium (e.g. Vibrio cholerae) ¹².
  • Spirillum: A rigid and helical bacterium with flagella at both ends (e.g. Spirillum volutans) ².
  • Spirochete: A flexible and corkscrew-shaped bacterium that moves by axial filaments (e.g. Treponema pallidum) ²³.

An average coccus is about 0.5-1.0 micrometer (µm) in diameter, while an average bacillus is about 0.5-1.0 µm wide and 1-4 µm long¹. Spiral bacteria can vary in size and shape depending on the species³.
(1) Morphology, Different shapes of bacterial cell – BYJU’S. https://byjus.com/neet/important-notes-of-biology-for-neet-shapes-of-bacteria/.
(2) Bacteria – Definition, Shapes, Characteristics, Types & Examples. https://biologydictionary.net/bacteria/.
(3) 2.1: Sizes, Shapes, and Arrangements of Bacteria. https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_%28Kaiser%29/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/2%3A_The_Prokaryotic_Cell_-_Bacteria/2.1%3A_Sizes_Shapes_and_Arrangements_of_Bacteria.
(4) 3 Common Bacteria Shapes – ThoughtCo. https://www.thoughtco.com/bacteria-shapes-373278.