Lecture Notes: Mannitol Salt Agar (MSA)

Introduction

Mannitol Salt Agar (MSA): A selective and differential bacterial growth medium.
Purpose: Used to isolate and identify specific types of bacteria.

Selective Nature: Allows growth of some organisms while inhibiting others.
Comparison to Non-Selective Media:
- Non-selective: LB agar, nutrient agar (supports a wide variety of bacteria).
- Selective: MSA supports specific types.
Specific for Staphylococcus & Micrococcus:
- Staphylococcus species: E.g., Staphylococcus aureus, Staphylococcus epidermidis.
- Micrococcus: E.g., Micrococcus luteus.
High Salt Concentration:
- Concentration is 8-10% salt.
- Inhibits organisms unable to tolerate high salt, e.g., Streptococcus and gram-negative bacteria.

Differential Nature: Contains chemicals to visually distinguish different species based on biochemical characteristics.
Components:
- Mannitol (sugar alcohol): Carbohydrate that can be fermented by some bacteria.
- Phenol Red (pH indicator): Changes color based on acidity.
Color Change Mechanism:
- If bacteria ferment mannitol, acid is produced, lowering pH.
- Phenol red changes from pink to yellow in acidic conditions.
Visual Indicator: Yellow zone around colonies that can ferment mannitol.

Streptococcus pyogenes:
- Inhibited due to inability to survive high salt.
- No growth observed.
Staphylococcus epidermidis, Micrococcus luteus, Staphylococcus aureus:
- Can grow due to salt tolerance.

Staphylococcus epidermidis & Micrococcus luteus:
- Grow without changing medium color.
- Do not ferment mannitol.
Staphylococcus aureus:
- Grows and causes medium to turn yellow.
- Ferments mannitol, resulting in acid production.

Gram Positive vs. Gram Negative Bacteria: Reference available video for more detail.

MSA: Useful for identifying bacteria capable of growing in high saline environments and distinguishing those that ferment mannitol.

For visual examples, refer to online images of MSA plates showing yellow zones indicating mannitol fermentation.