The spread of community acquired, methicillin resistant (CA-MRSA) is an increasing problem seen outside the healthcare setting. One such strain, CA-MRSA USA300, is epidemic in the United States. USA300 shows a heightened resistance to the innate immune system, in particular to macrophage engulfment. Two horizontally acquired genes, encoding an efflux pump (CopX) and lipoprotein (CopL), were discovered in 2 different lineages of USA300, representing CA-MRSA epidemics in North and South America. Removal of either of these genes resulted in elevated copper concentrations in the cytoplasm of , implying a function in copper hyper-resistance. While copper is an essential part of metabolic machinery, it is toxic at high concentrations and is utilised by macrophages to kill bacteria in the phagosome. Supporting this, USA300 with functional copXL genes showed increased survival in macrophages compared to their copXL negative counterparts. Although the role of CopX as an efflux pump explains the rise in intracellular copper concentration upon its mutation, the role of the CopL lipoprotein is still unknown. Therefore, to better understand the function of CopL and how it might influence host interaction, transcriptomic analysis is underway to identify downstream targets. This has the potential to uncover an exciting mechanism linking metal resistance to host virulence.

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