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How do cocci, bacilli, and spirilla differ in shape?
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Bacteria
A scanning electron micrograph of gram-positive Mycobacterium tuberculosis bacteria, the cause of tuberculosis and scrofula.
Bacteria are categorized into three primary shapes based on their morphology:12
- Cocci (singular: coccus): These are spherical-shaped bacteria. Depending on how they divide and remain attached, they can form pairs (diplococci), long chains (streptococci), random clusters (staphylococci), or organized packets.31
- Bacilli (singular: bacillus): These are rod-shaped bacteria. While they typically occur singly, some strains form long chains. They may have squared or pointed ends, and some variations are bent into a comma shape.14
- Spirilla (singular: spirillum): These are curved or spiral-shaped bacteria. The term is generally used to describe helical, corkscrew-like cells. They differ from other curved bacteria, such as spirochetes, which form a tighter helix wrapped around a central axial filament.15
These shapes are fundamental to microbiology and are often used in the identification of different bacterial species.31