A new weapon for resistant bacteria

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Methicillin-resistant Staphylococcus aureus (MRSA), a longtime bane of hospitals, thwarts the antibiotic by integrating a mobile genetic element, staphylococcal cassette chromosome mec (SCCmec). More than 10 years ago, Keiichi Hiramatsu's group at Juntendo University in Tokyo started to notice variations of SCCmec, with different combinations of recombinases to transfer the element and one or more resistance genes.

In a 2004 paper, the researchers found a fifth variation of SCCmec - the second to arise outside hospital walls - among Australian aboriginals. ¹ Unlike the previous four versions described, type V has only one recombinase, ccrC, which is surprisingly sufficient for integration, says first author Teruyo Ito. Smaller than the other four, and containing genes for a restriction-modification system, SCCmec V is more stably integrated, says Hiramatsu.

Such classifications help scientists and clinicians understand the evolution of resistance. While types already identified account for 90% of MRSA strains found globally, Hiramatsu says it remains to be seen how future antibiotic development will drive staph evolution. In the meantime, scientists are still trying to discover where these elements come from, and how they "move from one strain to another," says Susan Boyle-Vavra at the University of Chicago.