What is MRSA?

For decades methicillin-resistant Staphylococcus aureus (MRSA) has been considered the prototype of multi-resistant nosocomial pathogens, causing infections in high-risk patients. Changes in the healthcare system, coupled with the evolution of this versatile microorganism, have transformed MRSA into a cause of community-onset infections, in both patients who have contact with the healthcare system and patients without such a risk factor. New lineages of MRSA, defined as community acquired (CA)-MRSA, have emerged that have a propensity to cause infections in young individuals without risk factors. CA-MRSA primarily causes skin infections and, rarely, necrotising pneumonia. In the USA, these strains belong to a single widespread clone, designated USA300, while in Europe they belong to a variety of clones. Most strains carry genes for the Panton-Valentine leukocidin, whose role in diseases is under debate. In subjects living in the community who have contact with the healthcare system, MRSA strains of the nosocomial type are a frequent cause of infection and of pneumonia in particular. The detection of a large MRSA reservoir in pigs and the finding that professionally exposed individuals are colonised, has further shown that it is necessary to closely follow the epidemiology of MRSA if we want to combat it effectively.