Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of Staphylococcus aureus (S. aureus) bacteria that has developed resistance to several common antibiotics, including methicillin and penicillin. While this resistance makes infections more challenging to treat, MRSA often exists harmlessly on the body, a condition known as colonization. The nasal cavity, or nares, is the primary body site where S. aureus naturally resides, making it the most common reservoir for MRSA colonization in humans. The presence of MRSA in the nares does not typically cause symptoms, but it is a significant factor in the spread and subsequent development of infection, especially in healthcare settings.
Understanding MRSA Nasal Colonization
MRSA colonization occurs when the bacteria live on a person’s skin or in their nasal passages without causing illness. This is distinct from an MRSA infection, where the bacteria actively invade the body, multiply, and cause symptoms such as painful sores or serious conditions like pneumonia or bloodstream infections. Approximately one in three people carry S. aureus in their nose or on their skin without issue, and about one in 100 people carry the MRSA strain specifically.
The anterior nares, or the inner surface of the nostrils, offer an ideal habitat for S. aureus due to their moisture, warmth, and the presence of nasal epithelial cells. This colonization is typically asymptomatic, meaning the carrier feels healthy and unaware of the bacteria’s presence. Scientists classify carriers into three patterns: persistent carriers, intermittent carriers, and non-carriers.
For colonized individuals, the nares act as a continuous source of the organism, which can spread to other body sites like the skin or throat. While localized carriage is considered a normal state for many people, it is a significant risk factor for later developing an actual MRSA infection. The existence of MRSA in the nares is a state of passive carriage that increases the potential for future problems.
How Nasal MRSA is Identified
Identification of nasal MRSA is typically performed using a simple, non-invasive screening process involving a nasal swab. A healthcare provider gently inserts a specialized swab into both anterior nares and rotates it to collect a sample of the nasal secretions. This screening is frequently performed on patients before high-risk surgeries, those admitted to intensive care units, or individuals with recurrent skin infections.
Once collected, the sample is sent to a laboratory for analysis using two primary methods. The traditional method is a bacteria culture, where the sample is placed in a nutrient-rich dish to allow growth, taking 24 to 48 hours for a definitive result. A much faster method is the molecular test, often a Polymerase Chain Reaction (PCR) assay, which directly detects the MRSA bacteria DNA. The PCR test is highly sensitive and can provide results in as little as five hours, which is beneficial when rapid identification is necessary for patient care decisions.
Strategies for Decolonization
Decolonization refers to the clinical strategy aimed at eradicating MRSA from the nasal passages and other body sites to reduce the risk of subsequent infection. The primary treatment for nasal colonization is the application of mupirocin 2% ointment, a topical antibiotic. This ointment is applied directly to the inside of both anterior nares, typically twice daily for five to ten days. Mupirocin works by interfering with the bacteria’s ability to synthesize proteins, preventing growth and eliminating it from the colonization site.
For a more comprehensive reduction of the bacterial load, the nasal treatment is often combined with antiseptic body washes. A common complementary measure is the use of 4% chlorhexidine gluconate (CHG) soap for daily full-body washing during the five-day nasal treatment period. This combination therapy targets colonization on both the skin and in the nares, showing superior efficacy in reducing the overall risk of MRSA infection.
Patients are also advised to implement simple hygiene practices to support decolonization and prevent re-colonization. These measures include frequent hand washing, especially after touching the nose, and avoiding the sharing of personal items like towels or washcloths. Completing the full prescribed course of treatment without interruption is important for maximizing successful MRSA eradication.
Risks of Transmission and Progression to Infection
While nasal colonization is asymptomatic, it significantly increases the chance that the carrier will develop a future MRSA infection. Colonized individuals have an elevated risk of subsequent MRSA infection compared to non-carriers. This risk is pronounced in patients who are immunocompromised, have existing wounds, or are undergoing invasive procedures like surgery or dialysis.
The bacteria can progress from the nares to an infection site through self-inoculation. The organism is transferred from the nose to the hands, which then touch an open wound, catheter site, or other compromised skin barrier, allowing the bacteria to enter the body. Colonized individuals can also transmit MRSA to others through direct skin-to-skin contact or indirectly via contaminated surfaces.
Transmission is a concern in healthcare settings, where high-risk patients are concentrated. Proper hand hygiene, including frequent washing, serves as the most important preventative measure to limit the spread of MRSA from the nasal reservoir to other body sites or people. Reducing the bacterial burden in the nares through decolonization substantially decreases the risk of both self-infection and transmission to others.