What causes mrsa

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Last updated: April 4, 2026

Quick Answer: MRSA, or Methicillin-resistant Staphylococcus aureus, is a type of staph bacteria that has developed resistance to certain antibiotics, including methicillin and other common beta-lactam antibiotics. It is typically spread through direct skin-to-skin contact with an infected person or by touching contaminated objects and surfaces.

Key Facts

Staphylococcus aureus bacteria are common and can live on the skin or in the nose of about 30% of healthy adults without causing illness.
MRSA infections occur when the bacteria get into the body, often through cuts or scrapes, and are resistant to common antibiotics.
In healthcare settings, MRSA can spread from patient to patient via the hands of healthcare workers or through contaminated equipment.
In the community, MRSA is often spread through close contact, sharing personal items like towels or razors, and in crowded conditions like locker rooms.
Symptoms of MRSA skin infections can include redness, swelling, pain, and pus-filled bumps that resemble pimples or boils.

Overview

Methicillin-resistant Staphylococcus aureus (MRSA) is a type of bacteria that has become resistant to many common antibiotics used to treat staphylococcal infections. Staph bacteria, in general, are very common and can be found on the skin and in the noses of many healthy people without causing any problems. However, when these bacteria invade the body, they can cause infections. MRSA is a specific strain of staph bacteria that has developed a defense mechanism against a class of antibiotics called beta-lactams, which includes methicillin, penicillin, and oxacillin.

What is Staphylococcus Aureus?

Staphylococcus aureus, often shortened to 'staph', is a bacterium that can cause a range of infections, from minor skin infections like boils and impetigo to more serious conditions such as pneumonia, bloodstream infections, and infections of the bones and joints. In most cases, staph bacteria are harmless and live on the skin or in the nasal passages of about 30% of the population without causing illness. These individuals are referred to as 'colonized'. However, if the bacteria enter the body through a break in the skin, such as a cut, scrape, or surgical wound, they can cause an infection.

How Does MRSA Develop Resistance?

The 'R' in MRSA stands for 'resistant'. This resistance develops over time, often due to the overuse and misuse of antibiotics. When bacteria are exposed to an antibiotic, most of them are killed. However, if a few bacteria have a genetic mutation that allows them to survive the antibiotic, they can then multiply, passing on their resistance. Over time, this can lead to strains of bacteria like MRSA that are no longer susceptible to the antibiotics that were once effective against them. Specifically, MRSA has acquired a gene (the mecA gene) that modifies its cell wall, preventing beta-lactam antibiotics from binding to their target and effectively killing the bacteria.

How is MRSA Spread?

MRSA can spread in two main settings: healthcare facilities and the community.

Healthcare-Associated MRSA (HA-MRSA)

In hospitals, nursing homes, and other healthcare settings, MRSA can spread from an infected patient to other patients. This typically occurs through:

Direct Contact: Touching an infected person's wound or body fluids.
Indirect Contact: Touching objects or surfaces contaminated with MRSA, such as bed linens, medical equipment, or doorknobs.
Healthcare Workers: If healthcare providers do not wash their hands thoroughly between patient contacts or after touching contaminated surfaces, they can inadvertently transfer the bacteria.

HA-MRSA infections are often more serious because patients in healthcare settings may already have weakened immune systems or invasive medical devices like catheters or IV lines, which provide entry points for bacteria.

Community-Associated MRSA (CA-MRSA)

MRSA is also found among people who have not recently been in a healthcare setting. CA-MRSA infections are most common in people who:

Participate in contact sports (e.g., wrestling, football) where there is frequent skin-to-skin contact.
Share personal items such as towels, razors, or athletic equipment.
Live in crowded conditions, such as dormitories or military barracks.
Have poor hygiene.
Have open wounds or skin abrasions.

CA-MRSA infections often start as skin infections, appearing as small red bumps or boils that can quickly develop into painful abscesses. These can spread rapidly if not treated properly.

What are the Symptoms of MRSA Infections?

The symptoms of an MRSA infection depend on where the infection occurs in the body. Common signs of MRSA skin infections include:

Redness and swelling of the skin
Pain or tenderness in the affected area
Warmth to the touch
Pus-filled bumps or boils that resemble pimples
Fever

If MRSA enters the bloodstream or infects other parts of the body, it can cause more severe symptoms, including pneumonia, endocarditis (infection of the heart valves), or bone infections (osteomyelitis). These severe infections can be life-threatening.

Prevention and Control

Preventing the spread of MRSA involves good hygiene practices, especially handwashing. In healthcare settings, strict infection control protocols are crucial. In the community, avoiding the sharing of personal items, keeping wounds clean and covered, and practicing good hygiene can help reduce the risk of transmission.