What Causes MRSA?

Medically reviewed by Jane Kim, MD

Methicillin-resistant Staphylococcus aureus (MRSA) infection is caused by the uncontrolled spread of the bacteria Staphylococcus aureus—which is usually called staph. These bacteria are very common, and many people have them on their skin. Typically, the presence of staph does not cause problems and, if they do, they can be treated with antibiotics. However, MRSA infection results from the spread of a type of staph bacteria that no longer responds to the antibiotic methicillin—and oftentimes other antibiotics as well.

Primarily, the staph bacteria that causes MRSA infection is spread by contact with infected skin through a wound. Hospital workers or those that work in care facilities are more prone to be carriers and are the cause of hospital-acquired (HA) MRSA infections. Wounds, injuries, surgical scars, or catheter use can all lead to this issue.

People outside of a healthcare setting may also be carriers and spread community-acquired (CA) MRSA. Typically, this is spread by doing things like sharing towels or razors or having close contact with a person who has a MRSA infection.

Hospital-Acquired MRSA

HA MRSA infections arise due to resistant staph bacteria that can thrive in hospitals, clinics, senior homes, and related environments. These bacteria have developed an immunity to several types of antibiotics, including methicillin, making them tougher to treat. This is why some refer to MRSA as a "superbug."

HA MRSA bacteria spread through direct contact with wounds on the skin. This can be caused by:

• Exposure from an infected hospital worker's hands

• Infection due to using a contaminated intravenous (IV) line or catheter

• Hemodialysis, a way of cleaning the blood when there's kidney failure

• Infection of a surgery wound

Community-Acquired MRSA

CA MRSA infection also spreads via direct contact with the bacteria in the environment, such as your work, stores, school, or home. By definition, it's picked up outside of the clinical environment. As with HA MRSA, skin infection is often the cause, though it can also affect the lungs, causing pneumonia.

Common ways CA MRSA contamination is spread include:

• Towels

• Clothing

• Athletic equipment

• Person-to-person contact

While HA and CA MRSA infections are very closely related and arise due to the same resistant bacteria, there are some differences. Generally, HA MRSA affects those with weaker immune systems, whereas CA MRSA can affect all people. Studies have found CA MRSA bacteria to be more resistant to antibiotics and difficult to manage.

Is MRSA Hereditary?

MRSA is a bacterial infection, so it is not caused by heredity. However, researchers have found that certain genes in some people may help reduce the severity or fatality of skin infections caused by MRSA. In a small 2019 study, a gene variant called DNMT3A was found to shorten the recovery time for people with HA MRSA. Although more research is needed, genetic variations may affect how well your body fights against MRSA bacteria.

Causes

What can make MRSA infection challenging is that anyone exposed to the bacteria can get it. Most cases are linked to working or being in hospital and clinical environments, though studies suggest those numbers declining slightly, with CA MRSA cases on the rise.

Clinical Environments

Since HA MRSA represents a majority of cases, being a patient of or working in a hospital, medical clinic, nursing home, or other such environment is the primary cause. As such, the condition is more common among:

• Hospital and clinical staff

• Nursing home or long-term care facility residents

• Patients who require frequent hospitalization

• Those who are hospitalized for a long time
  

Weakened Immunity

Generally speaking, susceptibility to HA MRSA infection is closely related to the health of the immune system. Those who have weakened immunity are at more risk. HA MRSA infection is most commonly seen in:

• People who have had kidney dialysis

• People receiving cancer treatments, such as chemotherapy or radiation therapy

• Those taking immunosuppressant medications, such as Humira (adalimumab), Neoral (cyclosporine), and prednisone, among others

• People who have human immunodeficiency virus (HIV/AIDS)

• Those who have open wounds

Medical History

A history of recent medical treatment or disease can also increase your risk of CA MRSA. These risk factors include having recently had influenza (the flu) and surgery within the past year.

Close Contact

CA MRSA can affect those who are otherwise healthy, though poorer health can make you more susceptible. Being in frequent, close contact with others increases your risk. Those at the greatest risk for this type include:

• Athletes, especially if sharing towels, razors, or athletic equipment

• Children who go to daycare

• People living in crowded, densely populated areas

• Military members living in barracks

Lifestyle Factors

Since MRSA typically is a skin infection, there are several additional risks. Many cases are spread among injection drug users, so that's considered a risk factor. In addition, getting a tattoo, especially in unsafe, unsanitary conditions, can increase the chances of contracting this condition.

Demographic Risk Factors

Overall, your chances of infection can rise based on:

• Age: Since the chances of being hospitalized increase as you age, people over 65 are more likely to develop HA MRSA.

• Race and ethnicity: Rates of MRSA have been found to be consistently higher in Black Americans compared to white people, likely due to socioeconomic disparities. CA MRSA was also more common among Native Americans who live in regions with limited access to clean water.

• Economic status: Having a lower income and living in less affluent neighborhoods have also been linked to increased rates of MRSA infection.

• Living conditions: CA MRSA rates were found to be significantly higher for those living in crowded conditions, such as small apartments or military barracks. Limited access to clean water, including being houseless, can also raise the risk.

A Quick Review

Methicillin-resistant Staphylococcus aureus (MRSA) is a type of bacteria that's immune to certain types of antibiotics. Most cases arise in clinical environments like hospitals and nursing homes. However, MRSA can also spread in the community, spreading through close contact with those who are infected. Risk factors for this condition include having a weakened immune system due to HIV or cancer treatment, frequent hospitalization, and living in crowded conditions.

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