What Is MSSA?

Medically reviewed by Kashif J. Piracha, MD

MSSA is an infection caused by methicillin-susceptible Staphylococcus aureus bacteria. The bacteria live on the skin and inside the noses of some people, usually without causing harm. But when the bacteria enter the body through cuts, scrapes, or other open wounds on the skin, an infection can develop. Commonly known as a staph infection, skin infected by MSSA may become red, swollen, and warm. If the bacteria enter the bloodstream, any part of the body can become infected, including bones and vital organs.

There are two types of staph aureus infections: MRSA (methicillin-resistant Staphylococcus aureus) and MSSA. MRSA refers to infections caused by strains of the Staphylococcus aureus bacteria that are resistant to certain antibiotics and difficult to treat. MSSA, on the other hand, is more easily treatable with antibiotics.

This article covers what you should know about MSSA, including types of infections, their symptoms and causes, and how to treat and prevent MSSA infections.

Types of MSSA Infections

MSSA can cause infections in many parts of your body, including:

• Skin infection: Bacteria can enter the body through a cut, scrape, or other open wound on the skin, leading to an infection that affects the surface or deeper layers of the skin.

• Bone infection: A bone infection, known as osteomyelitis, occurs when bacteria reach the bones through the bloodstream or nearby infected tissue.

• Heart infection: Infection of the heart (endocarditis) occurs when bacteria enter the bloodstream and attach to the inner lining and valves of the heart

• Pneumonia: When MSSA bacteria enter the lungs, pneumonia can develop.

• Food poisoning: Eating food contaminated with MSSA can lead to food poisoning, usually within a few hours after consumption.

• Mastitis: Bacteria can enter the breasts through cracked or injured nipples, causing mastitis in chestfeeding parents.

MSSA Symptoms

MSSA can cause many symptoms, depending on where the infection develops. Symptoms vary from person to person and can range from mild to severe.

Skin Infection Symptoms

Skin infections are the most common type of MMSA infection and can cause symptoms such as:

• Pus-filled bump that may look like a pimple or boil

• Redness and swelling around the affected area

• Warm to the touch

• Pain or tenderness

• Fever

Bone Infection (Osteomyelitis) Symptoms

When MSSA infects the bones, it is called osteomyelitis. Symptoms may include:

• Bone pain or pain at the site of infection

• Swelling, redness, and warmth in the affected area

• Fever

• Excessive sweating

• Fever

• Malaise (general discomfort)

Heart Infection (Endocarditis) Symptoms

MSSA-related endocarditis occurs when bacteria infect the inner lining of the heart or heart valves, causing symptoms such as:

• Fever and chills

• Fatigue and weakness

• Muscle and joint pain

• Cough

• Night sweats

• Broken blood vessels on the skin

• Rapid or irregular heartbeat

• Shortness of breath

Pneumonia Symptoms

When MSSA infects the lungs (pneumonia), it can lead to symptoms such as:

• High fever

• Chills

• Chest pain

• Shortness of breath

• Persistent cough

Food Poisoning Symptoms

Eating food contaminated with MSSA can cause food poisoning. Symptoms develop within 30 minutes to 8 hours after eating contaminated items and may include:

• Nausea

• Vomiting

• Diarrhea

• Abdominal cramping

Mastitis Symptoms

Chestfeeding parents with mastitis may experience the following symptoms:

• Breast pain, redness, and swelling

• Warmth and tenderness in the affected breast

• Fever and chills

What Causes MSSA?

About 1 in 3 people have Staphylococcus aureus (staph) bacteria on their skin or inside their nasal passages. The bacteria can spread to others through skin-to-skin contact (e.g., shaking hands) or by touching objects contaminated with staph, such as door handles, towels, clothing, bed linens, and athletic equipment. Improper preparation and handling of foods by a person with staph can also spread the bacteria.

Most of the time, the bacteria don't cause problems. But if the bacteria enters the body through an open wound on the skin, an MSSA infection can develop. While skin infections are the most common type of MSSA infection, the bacteria can enter the bloodstream, causing severe illness.

Risk Factors

MSSA can affect people of all ages. Certain risk factors can increase your chances of developing an infection:

• Open wounds, burns, or sores on the skin

• Chronic health conditions, such as cancer, diabetes, eczema, and lung diseases

• A weakened immune system due to chemotherapy, HIV, or taking medicines that suppress the immune system (e.g., corticosteroids)

• Hospital stays or having surgery

• Injecting illicit drugs

• Participating in contact sports (e.g., wrestling)

• Sharing personal items, such as towels, razors, athletic equipment, or clothing

• Unhygienic food preparation practices (e.g., not washing hands)

• Having a medical device, such as a pacemaker, artificial heart valves, or artificial joints 
  

Diagnosis

To diagnose MSSA, your healthcare provider will begin by reviewing your medical history and asking about your symptoms. They will examine your skin to check for classic signs of infection, including redness, swelling, and a pus-filled bump on the skin.

A skin culture test can help identify the type of bacteria causing the infection and confirm the diagnosis. For this test, your healthcare provider will swab the affected area or take a sample of fluid (pus) from the bump or boil and send the sample to the lab for analysis.

If your healthcare provider suspects you have MSSA in a different part of your body, they may order additional diagnostic tests, such as:

• Blood culture: A blood sample is sent to the lab to confirm the presence of MSSA in the bloodstream. This test can also help determine which antibiotic will most effectively clear the infection.

• Imaging scans: X-rays, CT scans, and MRIs help visualize tissues and structures inside the body and assess whether the infection has reached the bones or organs.

• Bone biopsy: If the infection has reached your bones, a small sample (biopsy) of bone tissue may be removed to confirm MSSA infection and help guide treatment decisions.

• Stool sample: In cases of food poisoning, you may need to provide a stool sample, which is sent to the laboratory to confirm MSSA is causing your symptoms.

Treatments for MSSA

MSSA treatments eliminate or stop the growth of MSSA bacteria to cure the infection and prevent complications. Treatment options for MSSA vary, depending on the location and severity of the infection.

Antibiotics

Antibiotics are the first-line treatment for MSSA. These medicines help kill the bacteria or prevent them from growing and spreading. Diagnostic test results and other factors can be used to determine which antibiotic most effectively treats the infection.

MSSA skin infections may be treated with topical antibiotic ointment applied to the affected area or oral antibiotic therapy. MSSA infections that have spread beyond the skin and into the bloodstream may require oral (taken by mouth) or intravenous (IV) antibiotics. Antibiotics commonly prescribed to treat MSSA include:

• Dicloxacillin

• Erythromycin

• Nafcillin

• Clindamycin

Follow your healthcare provider's guidance and complete the entire course of antibiotics, even if you start to feel better before you've finished your prescription. This helps ensure treatment is effective and prevents repeated infections.

Related: When Would You Need Antibiotics?

Incision & Drainage

For skin infections that have reached deeper layers of the skin, healthcare providers may make a small incision on the wound or affected area to drain fluid and remove infected tissue. You will still require antibiotics to ensure the infection clears.

Surgery

Bone infections may require surgery to clear the infection and remove dead bone in the affected area. In some cases, removing medical devices (e.g., artificial heart valves) may be required to treat the infection and prevent recurrences.

Prevention

Preventative measures can lower your risk of a staph infection or prevent spreading it to others. To prevent MSSA, you can:

• Practice good hygiene, including regular handwashing with soap and water

• Clean cuts, scrapes, and wounds on the skin and cover with bandages

• Avoid sharing towels, bedding, clothing, and athletic equipment with someone with a staph infection

• Do not prepare or touch food for others if you have a staph infection

Complications

If left untreated, MSSA infections can cause serious illness and become life-threatening. See a healthcare provider if you suspect you or a loved one have an MSSA infection. Prompt diagnosis and treatment can help lower your risk of complications. 

Possible complications of MSSA include: 

• Toxic shock syndrome (TSS): A rare but serious infection that can develop suddenly when MSSA enters the bloodstream and produces toxins. Though commonly associated with tampon use, TSS can occur due to burns, skin infections, or after surgery, affecting people of all genders and ages.

• Sepsis: MSSA bacteria entering the bloodstream can lead to sepsis—a life-threatening condition caused by the body's extreme reaction to an infection.

A Quick Review

MSSA, or methicillin-susceptible Staphylococcus aureus, is a bacterial infection that develops when bacteria enter the body through a cut or wound on the skin. MSSA commonly causes skin infections but can infect the bones, heart, lungs, and other body parts if it enters the bloodstream.

Antibiotics effectively treat most MSSA infections and may be applied topically to treat skin infections. Oral or intravenous (IV) antibiotics are prescribed to treat MSSA infections that affect internal organs or bones. Regular handwashing, covering skin wounds, and avoiding sharing personal items (e.g., towels, razors) can help prevent MSSA.

MSSA is treatable, and most people recover with appropriate treatment. Talk to a healthcare provider if you suspect you have an MSSA infection. Early diagnosis and treatment can help prevent complications.

Frequently Asked Questions

Do I need to quarantine if I have MSSA?

Generally, having MSSA does not require you to quarantine. However, MSSA is highly contagious and can spread easily through direct contact with an infected person or by sharing personal items, such as towels or bed linens. Wash your hands regularly and cover the affected area to prevent spreading it to others. 

Can an MSSA infection come back?

Yes, MSSA infections can come back, especially if the initial infection wasn't treated effectively or if you're exposed to the bacteria again. Finishing your course of antibiotics, regular handwashing, and covering wounds on your skin can lower the risk of repeat infections.

What is the difference between MSSA and MRSA?

MSSA (methicillin-susceptible Staphylococcus aureus) and MRSA (methicillin-resistant Staphylococcus aureus) are strains of the same bacteria. The key difference is that MSSA can be treated with certain antibiotics, while MRSA is resistant to many antibiotics, making it harder to treat.

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