Emergence of Tn <i>1999.7</i> , a New Transposon in <i>bla</i> _OXA-48 -Harboring Plasmids Associated with Increased Plasmid Stability

Abstract

OXA-48 is the most common carbapenemase in <i>Enterobacterales</i> in Germany and many other European countries. Depending on the genomic location of <i>bla</i>_OXA-48, OXA-48-producing isolates vary in phenotype and intra- and interspecies transferability of <i>bla</i>_OXA-48. In most bacterial isolates, <i>bla</i>_OXA-48 is located on one of seven variants of Tn<i>1999</i> (Tn<i>1999.1</i> to Tn<i>1999.6</i> and invTn<i>1999.2</i>). Here, a novel Tn<i>1999</i> variant, Tn<i>1999.7</i>, is described, which was identified in 11 clinical isolates from 2016 to 2020. Tn<i>1999.7</i> differs from Tn<i>1999.1</i> by the insertion of the 8,349-bp Tn<i>3</i> family transposon Tn<i>7442</i> between the <i>lysR</i> gene and <i>bla</i>_OXA-48 open reading frame. Tn<i>7442</i> carries genes coding for a restriction endonuclease and a DNA methyltransferase as cargo, forming a type III restriction modification system. Tn<i>1999.7</i> was carried on an ~71-kb IncL plasmid in 9/11 isolates. In one isolate, Tn<i>1999.7</i> was situated on an ~76-kb plasmid, harboring an additional insertion sequence in the plasmid backbone. In one isolate, the plasmid size is only ~63 kb due to a deletion adjacent to Tn<i>7442</i> that extends into the plasmid backbone. Mean conjugation rates of the Tn<i>1999.7</i>-harboring plasmids in J53 ranged from 4.47 × 10^-5 to 2.03 × 10^-2, similar to conjugation rates of other pOXA-48-type IncL plasmids. The stability of plasmids with Tn<i>1999.7</i> was significantly higher than that of a Tn<i>1999.2</i>-harboring plasmid <i>in vitro</i>. This increase in stability could be related to the insertion of a restriction-modification system, which can promote postsegregational killing. The increased plasmid stability associated with Tn<i>1999.7</i> could contribute to the further spread of OXA-48.