Bacterial Morphology
Bacterial morphology is the study of the shapes and arrangements of bacterial cells.
Size of Bacteria
Bacteria are microscopic, typically ranging in size from 0.2 μm to 3 μm in diameter.
The smallest bacteria, belonging to the genus Mycoplasma, can be as small as 0.2μm.
Larger bacteria, such as Bacillus anthracis, can reach up to 3 μm in diameter.
Studying Bacterial Morphology
Microscopy: The primary tool for observing bacterial morphology is microscopy.
- Light microscopy is used to visualise stained bacteria, revealing their size, shape, arrangement, and staining characteristics.
- Dark-field and phase-contrast microscopy allow observation of live, unstained bacteria, particularly their motility.
Smear Preparation: Bacteria are typically spread on a glass slide to create a thin layer for microscopic examination.
Basic Bacterial Shapes
There are three fundamental shapes that bacteria commonly exhibit:
- 1. Coccus (spherical):
- Cocci - individual spherical bacteria.
- Diplococci - pairs of cocci.
- Streptococci - chains of cocci, resembling beads on a string.
- Staphylococci - grape-like clusters.
- Tetrads - groups of four cocci
- 2. Bacillus (rod-shaped):
- Bacilli - individual rod-shaped bacteria.
- Diplobacilli - pairs of bacilli arranged side-by-side.
- Streptobacilli - chains of bacilli.
- Coccobacilli - short, oval rods that resemble cocci.
- 3. Spirillum (spiral-shaped):
- Spiral - twisted shape bacteria, with two or more turns along their axis.
- Spirilla - rigid spiral shape bacteria , typically with flagella at both ends.
- Spirochetes - flexible spiral shape and are motile, often using axial filaments for movement.
- Vibrios - curved, comma-shaped bacteria, often with a single flagellum at one end.
Additional Morphological Categories
Pleomorphic: Some bacteria do not have a defined shape and can exhibit a variety of forms. For example, the genus Bacteroides are pleomorphic.
Filamentous: Certain bacteria, such as those in the genus Actinomyces, grow in long, branching filaments.
Gram Staining and Morphology
Gram Staining: This differential staining technique divides bacteria into two major groups – Gram-positive and Gram-negative – based on differences in their cell wall structure.
Gram-Positive Bacteria: Retain the crystal violet stain and appear purple or blue under the microscope. This is due to their thick peptidoglycan layer in the cell wall.
Gram-Negative Bacteria: Do not retain the crystal violet stain and are counterstained with safranin, appearing pink or red under the microscope. Their cell wall has a thinner peptidoglycan layer and an outer membrane.