Menacing Methicillin

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Science  10 Sep 2010:
Vol. 329, Issue 5997, pp. 1261
DOI: 10.1126/science.329.5997.1261-a

Almost since methicillin was first introduced as an antibiotic in the 1960s, resistant bacteria were detected, but no one has been quite sure where this form of defense originated. Resistance is encoded by mec on a transmissible cassette chromosome that spreads among staphylococcal bacteria, including the sometime hospital resident, Staphylococcus aureus. Tsubakishita et al. investigated, by genetic means, the wild relatives of S. aureus. They found in S. fleurettii (a commensal bacterium of domesticated animals) the original chromosomal locus that served as the template for the cassette; it appears that the cassette was formed by the combination of mec with the mobile element. mec genes comprise four classes, with mecA in S. fleurettii being the prototype and sharing almost complete nucleotide identity with mecA of methicillin-resistant S. aureus. Two scenarios are posited: that new resistant cassettes are continuously generated in staphylococci or that animal commensals act as a reservoir for human resistance.

Antimicrob. Agents Chemother. 54, 10.1128/AAC.00356-10 (2010).

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