Mannitol Salt Agar (MSA) I Purpose and Composition I Selective Media I Bacterial Culture Media

Описание: Mannitol Salt Agar (MSA) is a selective and differential culture medium, composed of mannitol, 7.5% NaCl, phenol red, peptone, beef extract, and agar, mainly used for the isolation and differentiation of Staphylococcus species, especially S. aureus.
Mannitol Salt Agar (MSA) is a selective and differential medium.
It is selective due to its high salt concentration (7.5% NaCl), which inhibits most bacteria except halotolerant organisms like Staphylococcus.
It is differential due to the presence of mannitol (sugar) and phenol red (pH indicator), which help distinguish between mannitol fermenters and non-fermenters.
2. Purpose / Uses
Selective isolation of Staphylococcus species (especially S. aureus).
Differentiation between mannitol-fermenting (pathogenic) and non-fermenting (non-pathogenic) Staphylococci.
Widely used in clinical microbiology labs to identify S. aureus.
Common in food, dairy, and environmental microbiology for detecting Staphylococcus contamination.
3. Composition (per liter, typical)
Peptone – 10 g (provides nitrogen, amino acids).
Beef extract – 1 g (vitamins, growth factors).
Sodium chloride (NaCl, 7.5%) – 75 g (selective agent, inhibits most bacteria).
Mannitol – 10 g (fermentable carbohydrate).
Phenol red – 0.025 g (pH indicator: yellow = acidic, red = neutral, pink = alkaline).
Agar – 15 g (solidifying agent).
Distilled water – 1 L
Final pH – 7.4 ± 0.2 at 25 °C
4. Principle
High salt concentration (7.5% NaCl): Inhibits most Gram-positive and Gram-negative bacteria, allowing growth of Staphylococcus spp.
Mannitol + phenol red:
S. aureus ferments mannitol → acid production → phenol red turns yellow.
Non-pathogenic Staphylococcus (e.g., S. epidermidis) does not ferment mannitol → medium remains red or pink.
5. Summary
Mannitol Salt Agar (MSA):
Purpose: Selective for Staphylococcus spp.; differentiates mannitol fermenters (S. aureus) from non-fermenters.
Composition: Peptone, beef extract, mannitol, 7.5% NaCl, phenol red, agar.
Key Point: Selective + differential medium used in clinical, food, and environmental microbiology.
6. Key Points
MSA = selective + differential.
Selects for Staphylococci due to high salt concentration.
Differentiates S. aureus (yellow colonies, mannitol fermenter) vs S. epidermidis (pink/red colonies, non-fermenter).
Common in pathogen detection (skin, wounds, food contamination).