Nested mobile genetic elements mediating antimicrobial resistance genes mobility within and between <i>Acinetobacter</i> isolates

Mobile genetic elements (MGEs) play a central role in the acquisition and dissemination of antibiotic resistance genes (ARGs). This study analyzed the distribution of MGEs using the whole-genome sequences of 38 <i>Acinetobacter</i> isolates from patient, environmental, and pig waste samples. Pig waste isolates exhibited the highest mean number of plasmids, while prophages were more prevalent in environment-associated isolates. Interestingly, we observed a significant positive correlation between number of plasmids and number of defense systems. Co-localization of multiple ARGs within a single plasmid was observed, with up to 10 distinct ARGs observed within a single p<i>dif</i> module. Additionally, putative genomic resistance islands (GRIs) were identified in non-baumannii <i>Acinetobacter</i> species, representing the first documentation of GRIs outside the <i>Acinetobacter calcoaceticus-baumannii</i> (Acb) complex. This study provides new insights into the mechanisms of ARG dissemination in <i>Acinetobacter</i>, particularly the role of MGEs in facilitating hierarchical gene transfer processes.