Emergence of Tn 1999.7 , a New Transposon in bla OXA-48 -Harboring Plasmids Associated with Increased Plasmid Stability.

OXA-48 is the most common carbapenemase in Enterobacterales in Germany and many other European countries. Depending on the genomic location of bla OXA-48 , OXA-48-producing isolates vary in phenotype and intra- and interspecies transferability of bla OXA-48 . In most bacterial isolates, bla OXA-48 is located on one of seven variants of Tn 1999 (Tn 1999.1 to Tn 1999.6 and invTn 1999.2 ). Here, a novel Tn 1999 variant, Tn 1999.7 , is described, which was identified in 11 clinical isolates from 2016 to 2020. Tn 1999.7 differs from Tn 1999.1 by the insertion of the 8,349-bp Tn 3 family transposon Tn 7442 between the lysR gene and bla OXA-48 open reading frame. Tn 7442 carries genes coding for a restriction endonuclease and a DNA methyltransferase as cargo, forming a type III restriction modification system. Tn 1999.7 was carried on an ~71-kb IncL plasmid in 9/11 isolates. In one isolate, Tn 1999.7 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 7442 that extends into the plasmid backbone. Mean conjugation rates of the Tn 1999.7 -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 1999.7 was significantly higher than that of a Tn 1999.2 -harboring plasmid in vitro . 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 1999.7 could contribute to the further spread of OXA-48.