ArmA methyltransferase in a monophasic Salmonella enterica isolate from food

Abstract

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.

Fig. 1.

Plasmid profile of the strains used in the present study. M, lambda ladder molecular marker. The numbers on the left show molecular weights in kilobase pairs. Salmonella enterica 09CEB904SAL and transconjugants BB1063, BB1065, and BB1064 are indicated in lanes 2 to 5, respectively. Each band represents one plasmid. S. enterica 09CEB904SAL bears pB1008, pB1009, and pB1010. The image reveals three types of transconjugants, one with pB1008 alone, one with pB1010 alone, and one with both pB1008 and pB1010.

Fig. 2.

(A) Genetic structure of Tn1548 reported from an animal isolate (pMUR050), a human isolate (pCTX-M-3), and a food isolate of Salmonella enterica 09CEB904SAL (pB1010). Black arrows in the upper part of the panel represent pairs of primers designed for the mapping of Tn1548. (B) PCR fragments obtained in Tn1548 mapping with plasmids from an animal isolate (pMUR050), a human isolate (pCTX-M-3), and a food isolate (pB1010). Lanes 1 to 9 represent the corresponding pairs of primers. M, molecular weight marker. The PCR results for lanes 7, 8, and 9 are negative in pB1010.