Mannitol Salt Agar (MSA) Quiz

Mannitol Salt Agar (MSA)

• The main selective ingredient in Mannitol Salt Agar (MSA) is a high concentration of salt (7.5-10% NaCl).
• The high salinity of MSA promotes the growth of halophilic microorganisms, besides Staphylococci, such as certain species of Micrococcus and Bacillus.
• MSA differentiates between microorganisms based on their ability to ferment mannitol, which is a sugar, and their tolerance to high salt concentrations.
• Mannitol fermentation is indicated by a yellow color change in the agar, while non-fermenters produce a pink or red color.

Staphylococcus aureus

• The strain of Staphylococcus aureus often implicated in infections obtained in hospital settings is Methicillin-Resistant Staphylococcus aureus (MRSA).
• Staphylococcus aureus can acquire resistance to antibiotics through horizontal gene transfer, mutation, and gene expression.
• Resistance to antibiotics can be acquired through the production of enzymes that degrade antibiotics, alteration of target sites, and active efflux of antibiotics.

Detection of Antibiotic-Resistant Strains

• Detection of MRSA or other antibiotic-resistant strains can be performed using Mannitol Salt Agar, which selects for Staphylococci and allows for the detection of mannitol fermentation.
• After isolating Staphylococci spp. on MSA, a Kirby-Bauer disk diffusion test is typically performed to determine antibiotic susceptibility.