Mupirocin is a unique antibiotic used exclusively on the skin or inside the nose to eliminate certain types of bacterial infections. It is a naturally occurring compound, originally isolated from the bacterium Pseudomonas fluorescens. Chemically distinct from other antibiotics, mupirocin’s primary purpose is to fight skin infections caused by specific organisms.
How Mupirocin Stops Bacterial Growth
The power of mupirocin comes from its highly targeted action within the bacterial cell, focusing on a single enzyme. This antibiotic works by reversibly binding to isoleucyl-tRNA synthetase (IleRS), a specific enzyme necessary for bacteria to build proteins. The enzyme’s normal role is to attach the amino acid isoleucine to its transfer RNA (tRNA) molecule.
When mupirocin attaches to the IleRS enzyme, it acts as a specific blocker, preventing isoleucine from being incorporated into growing protein chains. This interference immediately halts protein synthesis, which is fundamental for bacterial survival and replication. While the drug is bacteriostatic at lower concentrations, the high levels achieved with topical application make it bactericidal, actively killing the bacteria.
This mechanism is highly selective because bacterial IleRS is structurally different from the corresponding enzyme in human cells. This difference allows mupirocin to disrupt the bacterial machinery without negatively affecting host protein synthesis. The unique mechanism also means mupirocin does not share cross-resistance with other major classes of antibiotics, preserving its utility.
The Scope of Mupirocin’s Effectiveness
Mupirocin demonstrates a focused spectrum of activity, primarily targeting Gram-positive organisms responsible for most skin infections. Significant targets include Staphylococcus aureus, encompassing difficult-to-treat strains like Methicillin-Resistant Staphylococcus aureus (MRSA). It is also highly effective against Streptococcus pyogenes, a common cause of skin infections.
The antibiotic is formulated solely for topical use as a cream or ointment because it is rapidly metabolized if taken orally or intravenously. When absorbed into the bloodstream, mupirocin is quickly converted into an inactive compound called monic acid. This rapid deactivation prevents the drug from being used systemically, but ensures high, effective concentrations remain localized at the site of application.
Standard Treatments and Clinical Uses
Mupirocin is widely used as a first-line treatment for common primary skin infections, particularly impetigo. This highly contagious infection, often caused by S. aureus or S. pyogenes, results in characteristic crusty sores that respond well to topical application. It is also prescribed for secondary infected traumatic lesions, such as minor cuts, scrapes, or sutured wounds colonized by susceptible bacteria.
Beyond treating active skin infections, a major application of mupirocin is eradicating S. aureus colonization inside the nose. Many people carry Staphylococcus aureus, including MRSA, harmlessly in their nasal passages, which increases the risk of serious infection, especially in hospital settings. Intranasal mupirocin is applied to temporarily eliminate this nasal carriage, a process known as decolonization.
This decolonization protocol is frequently used before certain surgeries to reduce the risk of surgical site infections in high-risk patients. The treatment involves applying the ointment inside the nostrils, often twice daily for five days. This targeted use is distinct from treating an active skin infection, serving as a preventative measure to lower the overall bacterial load in carriers.
Administration and Safety Profile
Mupirocin is available as a topical cream or ointment, typically in a 2% concentration, for application directly to the affected skin or inside the nose. Most patients tolerate the treatment well, though some local side effects at the application site are commonly reported. These mild reactions can include temporary burning, stinging, itching, or pain.
In rare instances, patients may experience a significant allergic reaction, presenting as a severe rash, hives, or swelling of the throat or face. Any sign of a severe reaction requires immediate medical attention, and the drug should be discontinued. The limited systemic absorption of mupirocin means that body-wide side effects are uncommon.
Resistance Concerns
A primary concern regarding mupirocin use is the development of bacterial resistance, which threatens its long-term effectiveness. Two main types of resistance have emerged due to overuse: low-level and high-level resistance. Low-level resistance is caused by mutations in the bacteria’s native IleRS gene.
High-level resistance, which is concerning for treatment failure, occurs when bacteria acquire a separate gene, often called mupA, that produces a new, resistant version of the enzyme. The potential for resistance means mupirocin use is strictly regulated and should only be used as directed to preserve its utility.