Fitness costs of methicillin resistance in Staphylococcus aureus

Staphylococcus aureus is a Gram-positive bacteria, and a common human pathogen. It is a leading cause of healthcare-associated infections worldwide, causing potentially fatal bacteraemia. Infections can be treated using antibiotics, but many strains have quickly developed resistance to the most common antibiotics. Methicillin resistance in Staphylococcus is acquired through the uptake of the Staphylococcal Cassette Chromosome mec (SCCmec). SCCmec is a large mobile genetic element, which can integrate into the Staphylococcus genome. It carries the mecA gene, which confers resistance to broad-spectrum β-lactam antibiotics, including methicillin. Several different SCCmec elements have been described, some of which can carry additional antibiotic resistance genes, alongside mecA. These can include resistance to common antibiotics or resistance to heavy metals. Smaller SCCmec types do not encode any additional antibiotic resistance genes. These have been shown to confer no fitness cost to the bacteria. Because of this, smaller SCCmec types are becoming increasingly more common, particularly in community-associated MRSA infections.