Staphylococcus epidermidis on mannitol salt agar

Mannitol salt agar is a selective and differential medium that is commonly used to isolate and identify S. epidermidis from clinical specimens. 

In this blog/article, we will discuss the characteristics of S. epidermidis on mannitol salt agar and its significance in clinical microbiology.

What is Staphylococcus epidermidis?

Staphylococcus epidermidis is a common bacterium found on human skin and mucous membranes, usually harmless but capable of causing infections in people with weakened immune systems or medical devices. 

What is Mannitol Salt Agar?

Mannitol Salt Agar (MSA) is a selective and differential medium used to isolate and identify Staphylococcus species, including Staphylococcus epidermidis. 

The medium has a high salt concentration, preventing most other bacteria from growing, except, notably, Staphylococcus. 

The selective property of MSA is due to this high salt concentration that directly inhibits the growth of most other bacteria except staphylococci. 

MSA is differential due to the presence of mannitol, a kind of sugar that can be fermented by some Staphylococcus species but not others. 

Staphylococcus epidermidis on mannitol salt agar

The culturing procedure for Staphylococcus epidermidis on mannitol salt agar is as follows: 

  • Inoculate the MSA plate with a sterile loop or swab by streaking it across the surface of the agar. 
  • Incubate the plate at 35-37°C for 24-48 hours, depending on the growth rate of the bacteria. 
  • After incubation, examine the plate for growth and colony morphology. Staphylococcus epidermidis colonies are typically small, white to pale yellow, and have a convex shape. 
  • To determine if the organism is able to ferment mannitol, observe the color of the agar around the colonies. If the organism is able to ferment mannitol, the acid produced will cause the medium to turn yellow, indicating a positive result. If the organism is unable to ferment mannitol, the medium will remain red, indicating a negative result. 
  • Confirm the identification of the organism using additional biochemical tests or molecular methods if necessary.

Staphylococcus epidermidis mannitol salt agar results

When Staphylococcus epidermidis is grown on MSA, it grows due to its high tolerance for salt. 

The bacterium does not ferment mannitol, so, the medium remains pink without any color change due to acid production. 

In contrast, Staphylococcus aureus, a mannitol fermenter, produces acid during fermentation, causing the medium to turn yellow. 

The colony morphology of Staphylococcus epidermidis on MSA is typically small, white to grayish-white, and slightly raised, with round, opaque colonies and a smooth or slightly rough surface. 

Under a microscope, the morphology and arrangement of Staphylococcus epidermidis can be seen as gram-positive cocci arranged in clusters. 

Does Staphylococcus epidermidis grow on mannitol salt agar? 

Yes, Staphylococcus epidermidis can grow on mannitol salt agar. Mannitol salt agar (MSA) is a selective and differential medium that is commonly used for the isolation and identification of Staphylococcus species.

Is Staphylococcus epidermidis salt tolerant? 

Staphylococcus epidermidis is salt tolerant and can grow on MSA due to its ability to tolerate high salt concentrations. 

MSA contains a high concentration of sodium chloride (7.5%) which inhibits the growth of many other bacteria that are not salt-tolerant. 

Staphylococcus species, including S. epidermidis, are able to grow on MSA due to their ability to tolerate high salt concentrations.

How do you test for Staphylococcus epidermidis? 

To test for Staphylococcus epidermidis, a sample is typically collected from a suspected infection site (e.g. skin, blood, or urine) and cultured on a suitable medium, such as MSA. 

The sample is usually inoculated onto the surface of the agar using a sterile swab or loop, and then incubated at the appropriate temperature and oxygen conditions for the bacteria being tested. 

After incubation, the culture is examined for characteristic growth patterns and further tests may be performed to confirm the identity of the bacteria.

What pH does Staphylococcus epidermidis grow best at? 

Staphylococcus epidermidis thrives best at pH levels of around 7.0 to 7.5, which is near-neutral. This pH range is characteristic of many human tissues, and S. epidermidis is a frequent member of normal human skin flora.

Is Staphylococcus epidermidis aerobic or anaerobic?

Staphylococcus epidermidis is a facultative anaerobe, meaning that it can grow in the presence or absence of oxygen. It is typically cultured aerobically in the laboratory, as this allows for easier observation and identification of growth patterns and characteristics.

What Next?

In conclusion, MSA is beneficial for the isolation and identification of Staphylococcus species, including Staphylococcus epidermidis.

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Last Updated on September 13, 2023 by Our Editorial Team