MRSA

Some strains of Staph. aureus are resistant to methicillin (flucloxacillin), and such strains have been the scourge of a number of hospitals in the last few years.

Methicillin-resistant Staphylococcus aureus (MRSA) may also be resistant to other antibiotics such as clindamycin, erythromycin and aminoglycosides

• methicillin resistance was first detected in S. aureus in 1961

Serious S. aureus infections can be caused by strains that are methicillin-resistant or -susceptible and which may or may not express the pathogenic Panton-Valentine leucocidin (PVL) toxin

• overall prevalence of S. aureus strains that carry the gene for PVL production is believed to be <2%, and these are mainly methicillin sensitive Staphylococcal aureus (MSSA)

Staphylococcus aureus is the major bacterial cause of skin, soft tissue and bone infections, and one of the commonest causes of healthcare-associated bacteraemia

• about 25% of healthy people carry one or more strains asymptomatically at any given time and infections are commonly endogenous being caused by the patient’s colonizing strain
• antibiotics and surgical drainage are the basis of treatment of staphylococcal infections, but the emergence of multiple resistance to penicillin, methicillin and other agents has compromised therapy

Community-associated MRSA (CA-MRSA)

• this is the term used to describe MRSA strains isolated from patients in an outpatient or community setting (community onset), or within 48 h of hospital admission (hospital onset)
  • patients also typically have no previous history of MRSA infection or colonization, hospitalization, surgery, dialysis or residence in a long-term care facility within the previous year, and absence of indwelling catheters or percutaneous devices at the time of culture
• CA-MRSA strains are genetically and phenotypically distinct from HA-MRSA (healthcare-associated MRSA)
  • overall prevalence of CA-MRSA is also presently low worldwide (thought to be <0.5% of all MRSA)
  • typically resemble some strains of MSSA in being susceptible to a wider range of anti-staphylococcal antibiotics (some are resistant only to ß-lactams), and often produce PVL, a toxin that destroys white blood cells and is a staphylococcal virulence factor

Healthcare-associated MRSA (HA-MRSA)

• MRSA strains that are transmitted to and circulate between individuals who have had contact with healthcare facilities
  • HA-MRSA infections can present in the hospital or healthcare setting (hospital or healthcare onset) or in the community (community onset), for example after hospital discharge
• note however that the boundaries between HA-MRSA and CA-MRSA are becoming blurred due to the movement of patients and infections between hospitals and the community, and to nosocomial outbreaks of CA-MRSA following admission of colonized or infected patients

Reference:

• (1) Nathwani D et al. Guidelines for UK practice for the diagnosis and management of methicillin-resistant Staphylococcus aureus (MRSA) infections presenting in the community.J Antimicrob Chemother. 2008 May;61(5):976-94.