Overview
This lecture explains the main bacterial shapes and arrangements, defining key terms and highlighting clinically important examples.
Common Bacterial Shapes
- Coccus: Spherical (round) bacterial cell; plural is cocci.
- Bacillus: Rod-shaped bacterial cell; also used as a genus name (e.g., Bacillus).
- Vibrio: Curved rod-shaped cell resembling a comma; key example is Vibrio cholerae (causes cholera).
- Coccobacillus: Very short rod, may look spherical; examples include Haemophilus influenzae (meningitis) and Bordetella pertussis (whooping cough).
- Spirillum: Large, rigid, spiral or corkscrew-shaped bacteria; not clinically significant.
- Spirochete: Flexible, spiral-shaped bacteria using axial filaments for movement; examples include Treponema pallidum (syphilis) and Borrelia burgdorferi (Lyme disease).
Bacterial Arrangements
- A single coccus or bacillus refers to one cell.
- Diplo-: Pair of cells; diplococcus (pair of spheres), diplobacillus (pair of rods).
- Tetrad: Group of 4 cocci.
- Sarcinae: Group of 8 cocci (less common).
- Strepto-: Chain arrangement; streptococcus (chain of spheres), streptobacillus (chain of rods).
- Staphylo-: Cluster arrangement; staphylococcus (cluster of spheres).
- Note: No "staphylobacillus" because rods do not grow in clusters.
Key Terms & Definitions
- Coccus — Spherical bacterial cell.
- Bacillus — Rod-shaped bacterial cell.
- Vibrio — Curved, comma-shaped bacterial cell.
- Coccobacillus — Short rod-shaped cell that may look round.
- Spirillum — Rigid, spiral-shaped bacterium with flagella at ends.
- Spirochete — Flexible, spiral-shaped bacterium with axial filaments.
- Diplo- — Prefix for a pair arrangement.
- Tetrad — Group of 4 cocci.
- Sarcinae — Group of 8 cocci.
- Strepto- — Prefix for chain arrangement.
- Staphylo- — Prefix for cluster arrangement.
Action Items / Next Steps
- Review examples of clinically important bacteria for each shape.
- Memorize key shape and arrangement terms for upcoming quizzes or exams.