Topic 1: Microbial Morphology & Staining
Introduction
Microbiology is the study of microorganisms — living entities too small to be seen with the naked eye. For FMGE preparation, understanding microbial morphology and staining techniques is foundational, as these form the basis for identifying pathogens in the clinical microbiology laboratory. This chapter covers bacterial cell structure, size, shape, and the major staining procedures you must know.
Bacterial Cell Structure
Size and Shape
Bacteria typically range from 0.2 to 10 micrometers (µm) in their longest dimension. Their small size allows them to survive in diverse environments, from soil to human tissue.
Common bacterial shapes:
- Cocci — spherical (e.g., Staphylococcus aureus)
- Bacilli — rod-shaped (e.g., Escherichia coli)
- Spirilla — helical/spiral (e.g., Helicobacter pylori)
- Vibrios — comma-shaped (e.g., Vibrio cholerae)
- Coccobacilli — intermediate (e.g., Haemophilus influenzae)
Bacteria may also appear in characteristic arrangements based on their plane of division:
- Diplococci — pairs (e.g., Neisseria meningitidis)
- Streptococci — chains
- Staphylococci — grape-like clusters
- Palisades — Chinese-letter pattern (e.g., Corynebacterium diphtheriae)
Cell Wall Structure
The bacterial cell wall is a rigid, protective structure that maintains shape and prevents osmotic lysis. It is the target for many important antibiotics, including penicillins and cephalosporins.
Gram-positive bacteria have a thick peptidoglycan layer (up to 40 layers) with teichoic acids embedded within it. This thick wall retains crystal violet dye, giving them a purple/blue color on Gram staining.
Gram-negative bacteria have a thin peptidoglycan layer (1–2 layers) surrounded by an outer membrane containing lipopolysaccharide (LPS/endotoxin). The outer membrane is disrupted during Gram processing, causing them to lose the primary dye and take up safranin, appearing pink/red.
Key Surface Structures
- Capsule — polysaccharide layer; anti-phagocytic; detected by India ink (negative staining) or Quellung reaction (specific for Streptococcus pneumoniae)
- Flagella — motility organelle; arrangements (peritrichous, polar, lophotrichous) are used for identification
- Pili/Fimbriae — hair-like appendages for adhesion; sex pili assist in genetic transfer (conjugation)
- Plasma membrane — phospholipid bilayer; site of oxidative and transport enzymes
- Cytoplasm — contains ribosomes (70S), nucleoid (circular DNA), and inclusion bodies
- Spores — dormant, resistant structures formed by Bacillus and Clostridium species; heat and chemical resistant
Bacterial Staining Techniques
Staining is the primary tool for preliminary identification of bacteria in clinical specimens.
Simple Stains
Single dyes (methylene blue, crystal violet, safranin) are used to reveal morphology, size, and arrangement. Methylene blue is particularly useful for demonstrating metachromatic granules in Corynebacterium diphtheriae.
Gram Staining (Most Important for FMGE)
This is the single most important stain in clinical microbiology.
Procedure:
- Crystal violet (primary stain) — all bacteria stain purple
- Gram’s iodine (mordant) — fixes crystal violet
- Acetone/alcohol (decolorizer) — removes dye from gram-negative cells
- Safranin (counterstain) — gram-negative bacteria take up pink/red color
| Bacteria | Color | Example |
|---|---|---|
| Gram-positive | Purple/Blue | Staph aureus, Strep pyogenes |
| Gram-negative | Pink/Red | E. coli, Pseudomonas aeruginosa |
Common pitfalls leading to false results: over-decolourization (gram-positive may appear gram-negative), old cultures (gram-positive may lose dye), improper timing of decolorization.
Acid-Fast Staining (Ziehl-Neelsen)
Used primarily for Mycobacterium tuberculosis and other mycobacteria. Mycobacteria resist decolorization by acid-alcohol after staining with carbol fuchsin due to high mycolic acid content in their waxy cell wall. They appear as bright red, beaded rods against a blue background.
Koch’s bacillus (M. tuberculosis) is acid-fast; so is Mycobacterium leprae (from skin slit smears).
Albert Staining
Used to demonstrate metachromatic granules in Corynebacterium diphtheriae. The granules stain dark green/black; the cytoplasm stains light green.
India Ink Negative Staining
Used to detect capsules — the capsule appears as a clear halo against a black background. Used for Cryptococcus neoformans in CSF.
Giemsa Staining
Used for blood parasites (Plasmodium, Babesia), chlamydial inclusions, and Donovan bodies (granuloma inguinale). Also used for malarial parasite identification in thick and thin blood smears.
FMGE High-Yield Points
- The cell wall difference (thick peptidoglycan vs. thin + outer membrane) is the anatomical basis for Gram staining
- Gram-positive cocci in clusters = Staphylococcus; in chains = Streptococcus
- Acid-fast bacteria = Mycobacteria — think TB and leprosy
- Capsule detection (S. pneumoniae, Klebsiella) via India ink or Quellung reaction
- Spores = Bacillus anthracis, Clostridium perfringens, Clostridium tetani, Clostridium botulinum
⚡ Exam tip: Gram-positive rod with metachromatic granules in V-shaped arrangement (Chinese letter) = Corynebacterium diphtheriae → Albert staining.