Tn6198, a novel transposon containing the trimethoprim resistance gene dfrG embedded into a Tn916 element in Listeria monocytogenes

Bertsch, David; Uruty, Anaïs; Anderegg, Janine; Lacroix, Christophe; Perreten, Vincent; Meile, Leo (2013). Tn6198, a novel transposon containing the trimethoprim resistance gene dfrG embedded into a Tn916 element in Listeria monocytogenes. Journal of antimicrobial chemotherapy, 68(5), pp. 986-991. Oxford University Press 10.1093/jac/dks531

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OBJECTIVES:
To characterize Tn6198, a novel conjugative transposon from the clinical Listeria monocytogenes strain TTH-2007, which contains the tetracycline and trimethoprim resistance genes tet(M) and dfrG, respectively, and to assess its transferability in vitro and in situ.

METHODS:
The complete sequence of Tn6198 was determined using a primer walking strategy. Horizontal gene transfer studies were performed by filter matings, as well as on the surface of smear-ripened cheese and smoked salmon. The presence of Tn916-like circular intermediates was determined by PCR. Antibiotic resistance was determined by the broth microdilution method and microarray hybridization.

RESULTS:
Sequencing of Tn6198 revealed that a 3.3 kb fragment containing dfrG was integrated between open reading frames 23 and 24 of Tn916. Furthermore, an additional copy of Tn916 was present in L. monocytogenes TTH-2007. Both elements were transferred simultaneously and separately in vitro to recipients L. monocytogenes 10403S and Enterococcus faecalis JH2-2 by conjugation, resulting in either tetracycline- and trimethoprim-resistant or solely tetracycline-resistant transconjugants. On the surface of cheese and salmon, only L. monocytogenes 10403S transconjugants were detected.

CONCLUSIONS:
This study reports the first Tn916-like element associated with a trimethoprim resistance gene, as well as the first fully characterized transposon conferring multidrug resistance in L. monocytogenes. This is of concern, as trimethoprim is administered to listeriosis patients with β-lactam allergy and as Tn6198 has a large potential for dissemination, indicated by both intra-species and inter-genus transfer.

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