ArmA methyltransferase in a monophasic Salmonella enterica isolate from food

The 16S rRNA methyltransferase ArmA is a worldwide emerging determinant that confers high-level resistance to most clinically relevant aminoglycosides. We report here the identification and characterization of a multidrug-resistant Salmonella enterica subspecies I.4,12:i:- isolate recovered from chicken meat sampled in a supermarket on February 2009 in La Reunion, a French island in the Indian Ocean. Susceptibility testing showed an unusually high-level resistance to gentamicin, as well as to ampicillin, expanded-spectrum cephalosporins and amoxicillin-clavulanate. Molecular analysis of the 16S rRNA methyltransferases revealed presence of the armA gene, together with bla(TEM-1), bla(CMY-2), and bla(CTX-M-3). All of these genes could be transferred en bloc through conjugation into Escherichia coli at a frequency of 10(-5) CFU/donor. Replicon typing and S1 pulsed-field gel electrophoresis revealed that the armA gene was borne on an ~150-kb broad-host-range IncP plasmid, pB1010. To elucidate how armA had integrated in pB1010, a PCR mapping strategy was developed for Tn1548, the genetic platform for armA. The gene was embedded in a Tn1548-like structure, albeit with a deletion of the macrolide resistance genes, and an IS26 was inserted within the mel gene. To our knowledge, this is the first report of ArmA methyltransferase in food, showing a novel route of transmission for this resistance determinant. Further surveillance in food-borne bacteria will be crucial to determine the role of food in the spread of 16S rRNA methyltransferase genes worldwide.