MSSA and MRSA are Staphylococcus aureus (S. aureus) bacteria with different resistance patterns. MRSA is resistant to many antibiotics and is more difficult to treat. Both types of bacteria can cause severe infections if left untreated.

Methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) can be found in the environment and are part of our microbiome. These bacteria colonize the noses of about 30% of the population.

MSSA and MRSA can both cause the same types of infections and result in the same symptoms. Where they differ is in their level of antibiotic resistance, which affects how they’re treated.

Learn more about the differences between MSSA and MRSA, including their infection symptoms, causes, and how they’re both diagnosed and treated.

MSSA and MRSA both result in the same symptoms and are often associated with a skin infection.

Potential signs of a skin infection from S. aureus include:

MSSA and MRSA can cause other types of infections as well, especially in hospital or healthcare settings. This includes:

MSSA and MRSA bacteria can colonize the skin or nose. They typically do this without causing any health problems.

However, if these bacteria enter the body, an infection can result. For example, MSSA or MRSA can lead to a skin infection when they enter your body through a cut or scrape. These bacteria can also enter the body via:

S. aureus bacteria can be spread through direct or indirect contact, such as on surfaces.

As a cause of infection, MRSA differs from MSSA in that it’s resistant to methicillin and other related antibiotics like penicillin and oxacillin.

In order to diagnose MSSA or MRSA, a healthcare professional will first take a look at your medical history and then may perform one of the following tests:

Sample testing

During this time, they’ll take a sample to send to a lab for testing. This sample can come from:

  • a wound or skin lesion
  • blood
  • urine
  • sputum, which is mucus coughed up from your lungs

Growing the bacteria in a lab can tell a healthcare professional if S. aureus is causing your infection. Testing the bacterial culture’s sensitivity to antibiotics like oxacillin can help a healthcare professional determine if MRSA is present.

Molecular tests

It can take a day or two for the bacteria to grow, and in severe infections, a healthcare professional may want results sooner. Because of this, molecular tests may be done instead.

An example of a molecular test is a polymerase chain reaction (PCR), which will look for specific sequences of bacterial DNA. PCR tests can help detect if MRSA is likely the cause of the infection.

Both MSSA and MRSA are treated with antibiotics. However, the specific type of antibiotic that’s used will depend on the infection that you have.

If you have a mild to moderate infection, you’ll likely receive oral antibiotics, which come in the form of a tablet or capsule. Severe infections are often treated with intravenous antibiotics, which are given directly into your bloodstream.

Treating MSSA

If you have MSSA, anti-staphylococcal penicillin or related antibiotics will likely be used. These are the types of antibiotics that MRSA is resistant to.

Treating MRSA

A variety of different antibiotics can be used to treat MRSA. These include:

However, it’s important to point out that, in addition to methicillin and related antibiotics, MRSA can also have resistance to other classes of antibiotics as well. This can make treatment more challenging.

VRSA

The emergence of vancomycin-resistant S. aureus (VRSA) can further complicate treatment. Although VRSA is still very rare, it’s concerning to healthcare professionals because vancomycin is the antibiotic of choice for severe MRSA infections.

According to the Centers for Disease Control and Prevention (CDC), all VRSA infections in the United States to date have also been resistant to methicillin, meaning finding an effective treatment for VRSA can be challenging.

Both MSSA and MRSA can cause serious illness if they’re left untreated. Bloodstream infections due to S. aureus can be deadly.

For example, the CDC estimated that in 2017, 120,000 S. aureus bloodstream infections and 20,000 deaths were associated with these infections.

Both MSSA and MRSA continue to be a public health challenge. The same study also noted that declines in MRSA bloodstream infections slowed during the study period. Meanwhile, the number of community-acquired MSSA bloodstream infections increased.

There’s no way to tell if a skin infection is caused by MSSA or MRSA just by looking at it. This is why it’s important to speak with a healthcare professional if you have an area of skin that’s:

  • discolored
  • swollen
  • tender
  • warm to the touch

You should seek immediate care if you have symptoms of a serious S. aureus infection, such as:

  • fever
  • chills
  • shortness of breath
  • rapid heart rate
  • confusion

Is MSSA considered sepsis?

MSSA is not the same as sepsis. However, untreated MSSA can lead to the development of sepsis.

Is MSSA highly contagious?

MSSA can be highly contagious and can pass from person to person, as well as contaminated surfaces and objects.

What does MSSA or MRSA positive mean?

If you’re MSSA positive, it means you have the S. aureus bacterium in your bloodstream. The infection has a high risk of complications and needs to be treated promptly.

MSSA and MRSA are two types of S. aureus bacteria. MRSA is resistant to antibiotics and, therefore, more challenging to treat.

S. aureus can cause infections in various areas of the body, including the skin, lungs, heart, bones, and bloodstream. The antibiotics used to treat an infection depend on whether MSSA or MRSA is causing it.

Both MSSA and MRSA can lead to severe or life threatening illness if they’re not treated. It’s impossible to tell if an infection is due to MSSA or MRSA without testing, which is why it’s important to speak with a healthcare professional if you think you have an S. aureus infection.