How colonization factors are linked to outbreaks of methicillin-resistant Staphylococcus aureus: the roles of SasX and ACME.

Methicillin-resistant Staphylococcus aureus (MRSA) is the most frequent cause of hospital-associated morbidity and mortality. One reason why MRSA has remained a serious threat to public health is that new clones of MRSA constantly keep re-emerging. These new clones are better adapted to thrive in the hospital environment or even the community than their predecessors because they have developed increased and diversified antibiotic resistance and/or enhanced virulence. In addition, non-symptomatic colonization has been identified as a risk factor for subsequent MRSA infection; therefore, acquisition of factors promoting colonization has gained increased attention regarding the surge of MRSA outbreak clones. Two specific genes or genetic loci, namely sasX and the arginine catabolic mobile element (ACME), could recently be linked to the epidemiological success of MRSA clones, supporting the notion that colonization factors play a crucial role in MRSA outbreaks. SasX is a surface protein that enhances nasal colonization. ACME contains an arc arginine deiminase gene cluster promoting the survival of MRSA in the acidic skin environment, in addition to a polyamine resistance gene that deals with the increased production of toxic polyamines by the host that is prompted by arc. Notably, a better understanding of MRSA colonization on the molecular level may lead to eradication strategies based on vaccination or bacterial interference, with great promise to decrease MRSA infection rates.