Characterization and Fitness Cost of Tn, a Novel Integrative and Conjugative Element Conferring Multidrug Resistance in .

A multidrug-resistant (MDR) strain of , Hi-228, with phenotypic resistance toward ampicillin, cefotaxime, chloramphenicol, gentamicin, and azithromycin, was isolated in Oslo, Norway. The strain was part of a clonal outbreak (2016-2017) comprising five ST143 strains with identical resistotypes. Hi-228 carries a novel integrative and conjugative element (ICE), Tn, contributing to this remarkable and previously unreported MDR profile. Tn contains the following resistance genes: , '-Im, ″-Ib, (E), and . The latter four are previously unreported or rarely reported in . In this study, we investigated the genetic environment, mechanisms of transfer, impact on phenotypic susceptibility, and fitness cost of this ICE. We found that Tn has an overall structure similar to the previously described ICE Tn, with and carried by Tn and Tn, respectively. The major difference between Tn and Tn is that Tn lacks (B) but carries the resistance gene pairs '-Im and ″-Ib and (E) and . The gene pairs are located on the novel transposable elements Tn and Tn, which have high sequence identities to a plasmid in and an ICE in streptococcal species, respectively. Tn does circularize and is transferable, however, at a low frequency. Head-to-head competition experiments showed that uptake of Tn reduces bacterial fitness. Our study shows that MDR strains are capable of clonal spread and that the supragenome comprises an increasingly wide range of transferable resistance genes, with evidence of transfer from unrelated genera. The findings offer a glimpse into the genome dynamics of , highlighting the importance of rational antibiotic usage to contain antimicrobial resistance and the emergence of MDR strains in this important pathogen.