OXOID Mannitol Salt Agar (MSA)

OXOID Mannitol Salt Agar (MSA) – Selective & Differential Medium for Staphylococci

OXOID Mannitol Salt Agar (MSA) is a widely used selective and differential culture medium designed for the isolation and differentiation of Staphylococcus species, particularly Staphylococcus aureus. Produced by Oxoid, this medium is trusted in clinical, food, and pharmaceutical laboratories for accurate microbial identification.

 What is Mannitol Salt Agar?

Mannitol Salt Agar is both selective and differential:

• Selective due to its high sodium chloride (NaCl) concentration, which inhibits most bacteria except salt-tolerant organisms like Staphylococci.
• Differential because it contains mannitol and a pH indicator (phenol red) to distinguish mannitol fermenters from non-fermenters.

 Key Features

• High Selectivity – 7.5% NaCl inhibits non-halophilic organisms
• Differential Capability – Detects mannitol fermentation
• Clear Visual Results – Color change for easy interpretation
• Reliable Performance – Suitable for routine microbiology labs

 Composition (Typical per Liter)

• Peptone: 10.0 g
• Beef extract: 1.0 g
• Sodium chloride: 75.0 g
• Mannitol: 10.0 g
• Phenol red: 0.025 g
• Agar: 15.0 g
• Final pH: 7.4 ± 0.2 at 25°C

 Principle of the Medium

• Mannitol Fermenters (e.g., Staphylococcus aureus) → produce acid → medium turns yellow
• Non-Fermenters (e.g., Staphylococcus epidermidis) → no acid → medium remains red/pink
• High Salt Concentration → inhibits most Gram-negative and many Gram-positive bacteria

 Applications

• Clinical Diagnostics – Isolation of pathogenic Staphylococci
• Food Testing – Detection of contamination by S. aureus
• Pharmaceutical QC – Microbial limit testing
• Environmental Monitoring – Surface and air testing

 Directions for Use

1. Suspend the required amount of powder in distilled water.
2. Heat gently to dissolve completely.
3. Sterilize by autoclaving at 121°C for 15 minutes.
4. Cool to 45–50°C and pour into sterile Petri plates.
5. Allow to solidify before inoculation.

 Storage & Shelf Life

• Store in a cool, dry place (10–30°C)
• Protect from moisture and direct light
• Keep tightly closed after use

 Precautions

• For laboratory use only
• Maintain aseptic technique
• Avoid overheating, as it may affect indicator performance

 Why Choose OXOID Mannitol Salt Agar?

Backed by the trusted quality of Oxoid, this medium ensures precise differentiation and consistent results, making it a preferred choice for microbiology professionals.