Methicillin-Resistant Staphylococcus aureus (MRSA)

Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of the common Gram-positive bacterium Staphylococcus aureus that resists nearly all beta-lactam antibiotics, including penicillins and most cephalosporins, by producing an altered penicillin-binding protein. It is a leading cause of healthcare-associated and community-associated infections and a defining example of a clinically significant multidrug-resistant pathogen.

Definition

MRSA is a Staphylococcus aureus strain that has acquired the mecA (or related mecC) gene encoding penicillin-binding protein 2a, which has low affinity for beta-lactams and thereby confers resistance to methicillin and essentially all other beta-lactam agents.

Scope

The entry covers the molecular basis of methicillin resistance, the principal mechanism conferring beta-lactam resistance, the distinction between healthcare-associated and community-associated lineages, and the organism's epidemiologic significance. It describes the pathogen and its resistance phenotype for reference and does not prescribe antimicrobial regimens.

Core questions

• What genetic and biochemical change makes Staphylococcus aureus resistant to beta-lactams?
• How do healthcare-associated and community-associated MRSA differ epidemiologically?
• Why does altered penicillin-binding protein 2a confer such broad beta-lactam resistance?

Key concepts

• mecA gene and SCCmec element
• Penicillin-binding protein 2a (PBP2a)
• Beta-lactam resistance
• Healthcare-associated MRSA (HA-MRSA)
• Community-associated MRSA (CA-MRSA)
• Panton-Valentine leukocidin
• Colonization versus infection

Mechanisms

Methicillin resistance is conferred chiefly by the mecA gene, carried on a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec). mecA encodes penicillin-binding protein 2a (PBP2a), a transpeptidase with low affinity for beta-lactam antibiotics; because PBP2a can continue cell-wall synthesis when the native penicillin-binding proteins are inhibited, the organism remains viable across nearly the entire beta-lactam class. A related determinant, mecC, produces an analogous low-affinity protein. Resistance is thus an acquired, target-based mechanism rather than enzymatic drug destruction.

Clinical relevance

MRSA causes skin and soft-tissue infections, bacteraemia, endocarditis, pneumonia, and device-related infections, and its beta-lactam resistance removes a large group of otherwise first-line agents from consideration, which is why it is a focus of infection prevention and antimicrobial stewardship. This entry characterizes the organism and its resistance for educational reference and is not a guide to selecting therapy for an individual patient.

Epidemiology

MRSA exists as distinct healthcare-associated lineages, historically linked to hospital exposure and invasive devices, and community-associated lineages that emerged later and can infect otherwise healthy people, often presenting as skin and soft-tissue infection. It is a major contributor to the global burden of resistant infections and is included among high-priority resistant pathogens.

History

Resistant strains were recognized soon after methicillin was introduced in the early 1960s. Over subsequent decades MRSA spread through hospitals worldwide, and from the late 1990s distinct community-associated clones emerged in people without traditional healthcare risk factors, broadening the epidemiology of the organism.

Debates

How distinct are community-associated and healthcare-associated MRSA?

The two were historically separated by risk factors, genetic background, and resistance patterns, but the lineages have increasingly overlapped in many regions, complicating the once-clear distinction.

Seminal works

• tong-2015
• liu-2011

Frequently asked questions

Why is MRSA resistant to almost all beta-lactam antibiotics, not just methicillin?

The mecA gene produces penicillin-binding protein 2a, which carries out cell-wall synthesis but has low affinity for beta-lactams as a class, so resistance extends across penicillins and most cephalosporins rather than to a single drug.

Does carrying MRSA mean a person is infected?

No. People can be colonized with MRSA on the skin or in the nose without symptoms; colonization is distinct from active infection, though it can be a source for later infection or transmission.