Staphylococcus epidermidis is one of the most common bacteria found on human skin. Laboratories frequently use specialized media, such as Mannitol Salt Agar (MSA), to isolate and identify this bacterium. MSA is designed to select for specific organisms while also providing a visual distinction between similar species. S. epidermidis does grow on this medium, and its resulting appearance aids in presumptive identification.
Components of Mannitol Salt Agar
Mannitol Salt Agar functions as both a selective and a differential medium. Its selective capability is due to a high concentration of sodium chloride, typically around 7.5%, which restricts the growth of most bacterial species. Only organisms capable of tolerating this high salt level are able to thrive on the agar surface.
The medium’s differential function relies on the sugar alcohol mannitol and the pH indicator dye, Phenol Red. Mannitol is the primary carbohydrate source that certain bacteria can metabolize. Phenol Red changes color in response to shifts in acidity, allowing researchers to visibly detect if an organism has fermented the mannitol. The medium starts at a neutral pH, giving it a characteristic pinkish-red color before inoculation.
Growth on MSA: The Role of High Salinity
The ability of Staphylococcus epidermidis to grow on MSA is directly linked to its natural environment on human skin. This bacterium is classified as halotolerant, meaning it can survive and proliferate in environments with higher salt concentrations than most other microorganisms. The 7.5% sodium chloride concentration in the agar mimics conditions like salty sweat, which inhibits the growth of the majority of non-staphylococcal bacteria.
This high-salinity environment ensures MSA primarily selects for members of the genus Staphylococcus and other salt-tolerant organisms. The growth of S. epidermidis confirms its resistance to osmotic stress, completing the selective part of the identification process.
Colony Appearance: Testing for Mannitol Fermentation
Once growth is confirmed, the appearance of Staphylococcus epidermidis colonies on MSA provides the differential result. S. epidermidis is characterized as a non-fermenter of mannitol, meaning it cannot metabolize the sugar alcohol present in the medium. Because it does not break down mannitol, the bacterium does not produce acidic byproducts.
The Phenol Red indicator therefore remains its original pink or red color, as the pH of the agar around the colonies does not drop significantly. S. epidermidis colonies typically appear small and white or pink/red, with no color change in the surrounding medium. This confirms that while the organism is salt-tolerant, it lacks the metabolic pathway required to utilize mannitol. The maintenance of the original medium color is the definitive visual characteristic for this species on MSA.
Distinguishing S. epidermidis from Pathogenic Strains
The primary laboratory application of Mannitol Salt Agar is to distinguish between S. epidermidis and the potentially pathogenic Staphylococcus aureus. The contrasting results provide a rapid, presumptive identification. S. epidermidis is a non-fermenter, resulting in red or pink colonies and surrounding medium.
In contrast, Staphylococcus aureus is a mannitol-fermenter, breaking down the sugar alcohol to produce organic acids. This acid production lowers the pH of the surrounding agar. The Phenol Red indicator responds by changing color from red to yellow, creating a visible yellow halo around the S. aureus colonies. This difference—red colonies and red agar for S. epidermidis versus yellow colonies and yellow agar for S. aureus—allows microbiologists to quickly separate these two species.