MRSA

MRSA
Caption	Scanning electron micrograph of methicillin-resistant Staphylococcus aureus
Field	Infectious disease, Microbiology
Causes	Staphylococcus aureus

MRSA. Methicillin-resistant *Staphylococcus aureus* is a significant pathogen responsible for a range of infections, from minor skin conditions to life-threatening systemic diseases. Its defining characteristic is resistance to beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems, which complicates treatment. The emergence and global spread of these resistant strains represent a major challenge in modern clinical medicine and public health.

Overview

The first strains were identified shortly after the introduction of methicillin in the early 1960s, primarily within hospital settings. Over subsequent decades, distinct community-associated lineages emerged, spreading among healthy individuals without traditional healthcare exposures. This pathogen is a prime example of the broader crisis of antimicrobial resistance, driven by genetic adaptation and selective pressure from antibiotic use. Its impact is felt across healthcare systems worldwide, necessitating coordinated surveillance and infection control strategies.

Microbiology and classification

The bacterium is a gram-positive, facultative anaerobe from the family Staphylococcaceae. Resistance is primarily mediated by the *mecA* or *mecC* genes, which code for an altered penicillin-binding protein (PBP2a) with low affinity for beta-lactam drugs. These genes are carried on a mobile genetic element called the staphylococcal cassette chromosome *mec* (SCC*mec*). Strains are broadly categorized as healthcare-associated (HA-MRSA) or community-associated (CA-MRSA), with the latter often carrying smaller SCC*mec* types and virulence factors like the Panton-Valentine leukocidin (PVL) toxin. Typing methods include pulsed-field gel electrophoresis, multilocus sequence typing, and spa typing.

Transmission and risk factors

Transmission occurs primarily through direct contact with colonized individuals or contaminated surfaces. In the hospital environment, key risk factors include recent surgery, indwelling devices like central venous catheters, prolonged hospitalization, and exposure to broad-spectrum antibiotics. Community transmission is often associated with close physical contact, such as in athletic teams, military barracks, correctional facilities, and childcare settings. Shared personal items, crowded living conditions, and compromised skin integrity also elevate risk. Colonization of the anterior nares or skin is a common precursor to active infection.

Clinical manifestations

Infections range from mild to severe. Common presentations include furuncles, carbuncles, cellulitis, and abscesses. More invasive diseases can lead to pneumonia, bacteremia, endocarditis, osteomyelitis, and septic arthritis. CA-MRSA strains are frequently associated with severe, necrotizing skin and soft tissue infections and necrotizing pneumonia. Toxic shock syndrome and sepsis are life-threatening complications. The clinical course can be rapid and severe, particularly with strains producing potent toxins.

Diagnosis

Definitive diagnosis requires culture and antimicrobial susceptibility testing from a clinical specimen, such as pus, blood, or sputum. Chromogenic agar media are often used for rapid preliminary identification. Molecular techniques, including polymerase chain reaction (PCR) assays for the *mecA* gene, provide faster results. The Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing establish interpretive criteria for resistance. Active surveillance screening of high-risk patients using nasal swabs is employed in many hospitals to guide infection control measures.

Treatment and prevention

Treatment depends on infection severity, site, and local susceptibility patterns. For serious infections, vancomycin has been a cornerstone, though strains with reduced susceptibility exist. Other agents include daptomycin, linezolid, ceftaroline, tedizolid, and telavancin. Incision and drainage are critical for abscesses. Prevention in healthcare relies on strict hand hygiene compliance, contact precautions, environmental cleaning, and antimicrobial stewardship programs. In the community, emphasis is on personal hygiene, avoiding shared personal items, and proper wound care. Decolonization protocols may use mupirocin and chlorhexidine in specific scenarios.

Epidemiology

It is a global pathogen. In the United States, significant outbreaks have been reported by the Centers for Disease Control and Prevention. The European Centre for Disease Prevention and Control monitors its spread across the European Union. While HA-MRSA rates have stabilized or declined in some regions due to infection control, CA-MRSA clones, like USA300, have become endemic. High prevalence is also noted in Asia, Latin America, and parts of Africa. The World Health Organization lists it as a high-priority pathogen for research and development of new antibiotics.

Category:Antimicrobial resistance Category:Bacterial diseases Category:Infectious diseases