Staphylococcus aureus is well adapted to its human host and the health-care environment. S Aureus is both a frequent commensal and a leading cause of endocarditis, bacteraemia, osteomyelitis and skin and soft tissue infections. The first semi-synthetic anti-staphylococcal penicillins were developed around 1960 and methicillin-resistant S. aureus (MRSA) was observed within 1 year of their first clinical use. MRSA was first observed among clinical isolates from patients hospitalized in the 1960s. Methicillin resistance is mediated by mecA and acquired by horizontal transfer of a mobile genetic element.
MRSA colonization increases the risk of infection, and infecting strains match colonizing strains in as many as 50–80% of cases. Colonization can persist for long periods of time. MRSA may also persist within the home environment, complicating attempts at eradication
MGEs carrying antibiotic resistance genes have been acquired by MRSA on multiple independent occasions. Resistance to penicillin (blaZ), trimethoprim (dfrA and dfrK), erythromycin (ermC), clindamycin (constitutively expressed ermC) and tetracyclines (tetK and tetL) have all been identified on insertion sequences, transposons and sometimes plasmids in both MRSA and methicillin-susceptible Staphylococcus aureus (MSSA). S. aureus expresses a wide range of virulence factors, including toxins.
Risk of MRSA infection is elevated among children, elderly individuals, athletes, military personnel, individuals who inject drugs, persons with an indigenous background or in urban, underserved areas, individuals with HIV or cystic fibrosis.
S. aureus colonizes the nares of 28–32% of the US population. MRSA nasal colonization rates range from 0.9% to 1.5% in the United States.
Approved antibiotics for MRSA vary by clinical indication. Despite the prevalence and severity of MRSA infections, there is a relative paucity of high-quality randomized controlled trials (RCTs) to guide therapy for all indications except acute bacterial skin and skin structure infections..
Current American Thoracic Society and IDSA guidelines recommend vancomycin, linezolid or clindamycin for the treatment of MRSA pneumonia.
The propensity for S. aureus to form biofilms complicates the treatment of bone, joint and prosthetic-related infections. Vancomycin remains the first-line therapy
Incision and drainage should be performed whenever possible for purulent abscess and skin infection. A recent large, placebo-controlled trial confirmed that antibiotic therapy reduces the likelihood of recurrent abscesses or treatment failure following incision and drainage.
MRSA is formidable, versatile and unpredictable. Its capacity for genetic adaptation and the serial emergence of successful epidemic strains cause it to remain a major threat to human health. The persistently high mortality associated with invasive MRSA infection highlights the need for high-quality trials to determine optimal management for these patients.