Understanding the rapid spread of antimicrobial resistance genes mediated by IS.

Insertion sequences (ISs) promote the transmission of antimicrobial resistance genes (ARGs) across bacterial populations. However, their contributions and dynamics during the transmission of resistance remain unclear. In this study, we selected IS as a representative transposable element to decipher the relationship between ISs and ARGs and to investigate their transfer features and transmission trends. We retrieved 2656  translocatable  IS -bounded  units with  ARGs (tIS-bUs-ARGs) in complete bacterial genomes from the NCBI RefSeq database. In total, 124 ARGs spanning 12 classes of antibiotics were detected, and the average contribution rate of IS to these genes was 41.2%. We found that  IS -bounded  units (IS-bUs) mediated extensive ARG dissemination within the bacteria of the  class, showing strong transfer potential between strains, species, and even phyla. The IS-bUs expanded in bacterial populations over time, and their temporal expansion trend was significantly correlated with antibiotic usage. This wide dissemination could be due to the nonspecific target site preference of IS. Finally, we experimentally confirmed that the introduction of a single copy of IS could lead to the formation of a composite transposon mediating the transmission of "passenger" genes. These observations extend our knowledge of the IS and provide new insights into the mediating role of ISs in the dissemination of antibiotic resistance.