# EMB (Eosin Methylene Blue) Agar
**Purpose:** Selective and differential medium.
* **Selective:** Inhibits Gram-positive bacteria.
* **Differential:** Differentiates lactose fermenters.
### Green metallic sheen
* Strong lactose fermentation
* Classic organism: **E. coli**
### Colony colors
* **Green metallic sheen:** Strong lactose fermenter (E. coli)
* **Purple/black colonies:** Lactose fermenter (Enterobacter, Klebsiella)
* **Colorless colonies:** Non-lactose fermenter (Salmonella, Shigella)
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# MacConkey Agar
### Contains
* Lactose
* Neutral red indicator
* Crystal violet
* Bile salts
### Purpose
* Selects for Gram-negative bacteria
* Differentiates lactose fermenters
### Results
**Pink/red colonies**
* Lactose fermenter
**Colorless colonies**
* Non-lactose fermenter
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# TSIA (Triple Sugar Iron Agar)
### Tests for
* Glucose fermentation
* Lactose fermentation
* Sucrose fermentation
* Gas production
* Hydrogen sulfide (H₂S)
### Sugars present
* Glucose (0.1%)
* Lactose (1%)
* Sucrose (1%)
### Color meanings
Yellow = Acid (A)
Red = Alkaline (K)
Black = H₂S production
Cracks/lifting = Gas production
### Interpretations
**A/A**
* Yellow slant
* Yellow butt
* Ferments glucose + lactose and/or sucrose
**K/A**
* Red slant
* Yellow butt
* Ferments glucose only
**K/K**
* Red slant
* Red butt
* No sugar fermentation
### Proper inoculation
* Stab the butt once.
* Streak the slant as you remove the needle.
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# Mannitol Salt Agar (MSA)
### Purpose
Selective and differential.
### Selective
* 7.5% salt selects for **Staphylococcus**.
### Differential
* Mannitol fermentation
### Results
Yellow media
* Mannitol fermented
* Acid produced
Red/Pink media
* No mannitol fermentation
### Species
**Staphylococcus aureus**
* Yellow
* Pathogenic
**Staphylococcus epidermidis**
* Pink/red
* Usually non-pathogenic
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# Blood Agar
### Classification
**Both enriched and differential**
### Hemolysis
**Beta**
* Complete hemolysis
* Clear zone
**Alpha**
* Partial hemolysis
* Green zone
**Gamma**
* No hemolysis
* No color change
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# Kirby-Bauer Test
Purpose:
Determine antibiotic susceptibility.
### Measure
Diameter of the **zone of inhibition** (mm).
Large zone
* Sensitive
Small/no zone
* Resistant
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# UV Light
### Mechanism
UV damages DNA by forming **thymine dimers**.
### Most harmful wavelength
**254 nm (UV-C)**
### Interpretation
Less bacterial growth = UV more effective.
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# Urease Test
### Purpose
Detect urease enzyme.
### Reaction
Urea → Ammonia + CO₂
Ammonia raises pH.
### Color
Pink = Positive
Yellow/orange = Negative
### Strong urease-positive genus
**Proteus**
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# Catalase Test
### Reagent
Hydrogen peroxide (3%)
### Positive
Bubbles immediately
### Negative
No bubbles
Reaction:
2H₂O₂ → 2H₂O + O₂
The bubbles are oxygen gas.
---
# Passive (Indirect) Agglutination
### Definition
Antibodies bind to antigen-coated particles, causing visible clumping.
Positive = Clumping
Negative = Smooth suspension
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## High-Yield Organisms to Memorize
| Organism | Key Feature |
| ------------------------------ | ------------------------------------------------- |
| **E. coli** | Green metallic sheen on EMB |
| **Proteus** | Strong urease positive |
| **Staphylococcus aureus** | MSA turns yellow (mannitol fermenter) |
| **Staphylococcus epidermidis** | MSA stays pink |
| **Salmonella** | Non-lactose fermenter; often H₂S positive on TSIA |
| **Shigella** | Non-lactose fermenter; H₂S negative |
## Last-minute memorization
* **EMB** → Green sheen = **E. coli**
* **MacConkey** → Pink = lactose fermenter
* **MSA** → Yellow = mannitol fermenter (**S. aureus**)
* **Blood agar** → Beta = clear, Alpha = green, Gamma = none
* **TSIA** → Yellow = acid, Red = alkaline, Black = H₂S
* **Kirby-Bauer** → Bigger zone = more susceptible
* **Urease** → Pink = positive (**Proteus**)
* **Catalase** → Bubbles = positive
* **UV** → 254 nm causes thymine dimers
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