Mannitol Salt Agar (MSA)
Mannitol Salt Agar (MSA)
Selective and Differential Medium for Staphylococcus aureus
Catalog Number: AS-1288
Overview
Mannitol Salt Agar (MSA) is the international gold-standard selective and differential solid medium for the isolation and presumptive identification of Staphylococcus aureus from clinical specimens, foods, cosmetics, and environmental samples. It is the official reference medium specified in ISO 6888-1, FDA-BAM Chapter 12, USP <61>/<62>, and other major microbiological standards.
The high salt concentration selectively inhibits most bacteria other than staphylococci, while mannitol fermentation allows differentiation of S. aureus from other staphylococcal species.
Applications
• Primary isolation of coagulase-positive staphylococci (Staphylococcus aureus) from skin, nasal swabs, wounds, and clinical specimens
• Enumeration of S. aureus in foods, particularly meat, dairy, and salted products (ISO 6888-1)
• Detection of S. aureus in cosmetics and pharmaceutical products
• Screening of nasal carriers in clinical and hygiene monitoring
Key Features & Benefits
• 7.5% sodium chloride inhibits most Gram-negative bacteria and many Gram-positives
• Mannitol with phenol red enables clear differentiation of mannitol-fermenting colonies
• S. aureus produces yellow colonies with a yellow halo
• Coagulase-negative staphylococci produce red or pink colonies without halo
• Widely accepted reference medium for clinical and food microbiology
Principle of the Medium
The high sodium chloride concentration (7.5%) creates a selective environment that favours staphylococci. Mannitol serves as the fermentable carbohydrate, and phenol red acts as the pH indicator. Staphylococcus aureus ferments mannitol, producing acid and turning the medium yellow, while most coagulase-negative staphylococci do not ferment mannitol and retain a red or pink colour.
Typical Composition (per litre)
Peptone / Meat Extract – 10.0 g
Sodium Chloride – 75.0 g
D-Mannitol – 10.0 g
Phenol Red – 0.025 g
Agar – 15.0 g
Final pH: 7.4 ± 0.2 at 25 °C
Preparation
Suspend 110.025 g of dehydrated medium in 1 L of purified water.
Heat with frequent agitation and boil for 1 minute to dissolve completely.
Do not autoclave, as phenol red may degrade under high-salt autoclaving conditions.
Cool to 45–50 °C, mix well, and pour plates.
Incubation
Incubate at 35–37 °C for 24–48 hours.
Examine plates at both 24 hours and 48 hours.
Typical Colony Appearance
Staphylococcus aureus: yellow to golden-yellow colonies (1–3 mm) with a yellow halo (mannitol positive)
Coagulase-negative staphylococci: red or pink colonies without halo
Other organisms: completely inhibited or very small red colonies
Storage & Stability
Prepared plates: Store at 2–8 °C, protected from light
Use within 4 weeks for optimal performance
Intended Use
For clinical, food, cosmetic, and pharmaceutical microbiology use only.
Not for human or veterinary therapeutic use.
Quality & Compliance
Manufactured under controlled conditions to ensure batch-to-batch consistency.
Meets performance expectations of ISO 6888-1, FDA-BAM, and USP standards.
Performance comparable to leading international reference media suppliers.
Customs & Trade Information
HS / AHECC Code: 3821.00.00
Prepared culture media for the development or maintenance of microorganisms