🔬 CHAPTER 3 STUDY NOTES: MICROSCOPY & STAINING

1. 📏 Units of Measurement

Microorganisms are measured in:

  • Micrometers (μm) = bacteria, protozoa

  • Nanometers (nm) = viruses, molecules

2. 🔬 Microscopy Overview

Types of microscopes

  • Simple microscope → 1 lens

  • Compound microscope → multiple lenses (most common in microbiology)

3. 💡 Light Microscopy (LM)

Key facts

  • Uses visible light

  • Uses glass lenses

  • Most commonly used: compound light microscope

Magnification

  • Total magnification =
    ocular lens × objective lens

Resolution (VERY IMPORTANT)

  • Resolution = ability to distinguish 2 close points

  • Compound light microscope:

    • Resolution: ~0.2 μm

    • Max magnification: ~1000–1500×

Oil immersion lens

  • Used at 100× objective

  • Increases resolution by reducing light refraction

Types of light microscopy

1. Brightfield

  • Uses stained specimens

  • Dark specimen on light background

2. Darkfield

  • Bright organism on dark background

  • Good for very small/unstained organisms

3. Phase-contrast

  • Enhances internal details of living cells

  • No staining needed

4. DIC (Differential Interference Contrast)

  • Produces 3D-like colored image of live cells

5. Fluorescence microscopy

  • Uses fluorescent dyes (fluorochromes)

  • Organisms glow under UV light

  • Used in immunofluorescence (FA technique)

6. Confocal microscopy

  • Uses laser scanning + fluorescent dyes

  • Produces sharp 3D images

7. Two-photon microscopy

  • Uses long-wavelength light (less damage to live cells)

8. Super-resolution microscopy

  • Breaks normal light resolution limits

  • Produces highly detailed images

9. Scanning acoustic microscopy (SAM)

  • Uses sound waves

  • Used for biofilms and surface-attached cells

4. ⚡ Electron Microscopy

Key idea

  • Uses electrons instead of light

  • Much higher resolution than light microscopes

Types

1. TEM (Transmission Electron Microscope)

  • Thin slices of specimen

  • Internal structure

  • Very high resolution (~0.2 nm)

2. SEM (Scanning Electron Microscope)

  • 3D surface image

  • Lower resolution than TEM but better surface detail

5. 🧫 Scanned-Probe Microscopy

  • STM (Scanning Tunneling Microscope)

  • AFM (Atomic Force Microscope)

  • Produces atomic-level 3D surface images

6. 🧪 Specimen Preparation

Smears

  • Thin layer of microbes on slide

Fixing

  • Uses heat or methanol

  • Kills and attaches cells to slide

Staining purpose

  • Increases contrast

  • Makes structures visible

Cell charge rule

  • Bacteria are negatively charged

  • Basic dyes (positive) stain cells

  • Acidic dyes stain background

7. 🎨 Types of Stains

Simple stain

  • One basic dye

  • Shows shape, size, arrangement

Negative stain

  • Acidic dye stains background

  • Cells remain clear

  • Used for capsules

Differential stains

Gram stain

Steps:

  1. Crystal violet (purple)

  2. Iodine (mordant)

  3. Alcohol (decolorizer)

  4. Safranin (counterstain)

Results:

  • Gram + → purple

  • Gram − → pink/red

Acid-fast stain

  • Used for Mycobacterium, Nocardia

  • Carbolfuchsin retained → red

  • Non–acid-fast → blue

Special stains

Endospore stain

  • Malachite green + heat

  • Endospores = green

Flagella stain

  • Uses mordant to thicken flagella for visibility

8. 🧠 KEY CONCEPTS FROM QUESTIONS

🔬 Microscope selection

Situation

Best microscope

Stained bacteria

Brightfield

Unstained small cells

Darkfield

Live internal detail

Phase contrast

Fluorescent sample

Fluorescence

3D live cells

Confocal

Surface structure

SEM

🔍 Magnification rule

Total magnification =
ocular × objective

Example:

  • 10× ocular × 100× oil immersion = 1000×

🎨 Gram stain interpretation

Step

Gram +

Gram −

Crystal violet

Purple

Purple

Iodine

Purple

Purple

Alcohol

Purple

Colorless

Safranin

Purple

Pink

⚠ Why mordant is used

  • Gram stain → iodine traps dye inside peptidoglycan

  • Flagella stain → thickens flagella so they become visible

🧫 Counterstain purpose

  • Colors cells that lost primary stain (acid-fast stain)

🧬 Decolorizer purpose

  • Removes stain from cells that don’t retain it

9. 🧪 HIGH-YIELD CONCEPTS

Must memorize:

  • Resolution vs magnification

  • Gram stain steps + outcomes

  • Acid-fast bacteria (Mycobacterium)

  • Types of microscopes and what they show

  • Endospore staining

  • Difference between SEM vs TEM

10. 🧠 COMMON EXAM TRICKS

  • Gram-negative bacteria → outer membrane + endotoxin

  • Endospores = NOT visible in Gram stain (need special stain)

  • Acid-fast bacteria resist Gram stain → waxy mycolic acid layer

  • Phase contrast = BEST for living cells

  • Electron microscopes = highest resolution

If you want, I can also:
✔ combine Chapter 3 + 4 into one mega review sheet
✔ make a practice exam (MCQs like your test)
✔ or turn this into flashcards for memorization