Xer site-specific recombination and IS26 mediated plasmid rearrangements in Acinetobacter baumannii: implications for resistance gene acquisition and evolution.

GIACONE L; VIALE AM; MORAN BARRIO J

Chapadmalal

Congreso; XVIII Congreso Argentino de Microbiología General; 2023

SAMIGE

Acinetobacter baumannii (Ab) represents a major cause of healthcare-associated infections generally affecting immunocompromised and severely injured patients, with the global spread of a number of epidemic clonal complexes (CC) displaying multidrug-resistance (MDR) phenotypes including resistance to last-resort carbapenems (carbR). The most frequent cause of carbR in Ab relies on acquired OXA-type carbapenemases, with the respective blaOXA genes carried by plasmids. Thus, a detailed characterization of MDR Ab plasmids is essential for understanding the evolution and dissemination of these resistance structures.We characterized a number of epidemiologically-related MDR Ab strains belonging to the CC15 prevalent in our geographical region. Two carbR strains (Ab242 and Ab825) housed different iteron plasmids carrying a blaOXA-58 and TnaphA6-containing resistance module (RM) conferring resistance to carbapenems and aminoglycosides, respectively. This RM is bordered by 28-bp sequences recognized by the XerC and XerD tyrosine recombinases (pXerC/D-like sites), suggesting functions of this site-specific recombination (SSR) system in their horizontal mobilization. In Ab825, the RM is present in a 36-kbp multireplicon (pAb825_36) composed of a 27 kbp bireplicon domain (pAb825_27) and a 9-kbp monoreplicon domain (pAb825_9). Interestingly, we found a 7 kbp plasmid (pAb244_7) sharing homology to pAb825_9 in a local carbS strain, Ab244, isolated two years earlier than Ab242 and Ab8251,2. Here, we propose an evolutionary pathway for the genesis of Ab resistance plasmids that involves both Xer-dependent SSR and IS26-mediated mechanisms based on bioinformatics analysis and experimental evidence.BlastN searches of the GenBank database using Ab825 plasmid queries retrieved a series of pAb825_9-like plasmids carried by CC15 Ab strains isolated in Argentina and Chile. These plasmids, including pAb244_7, shared with pAb825_9 the replication and mobilization backbone2. We found in Ab825_9-like plasmids a DNA region containing genes for a toxin-antitoxin system, bordered by directly oriented pXerC/D sites. The identification of a SSR hybrid site in pAb244_7 indicates that a XerC/D-mediated event contributed to the acquisition of this stability module, and our experimental evidence supports this hypothesis. In addition, pAb244_7 contained an IS26 element, which was also part of a composite transposon carrying blaTEM and aacC2 resistance genes present in some pAb825_9-like plasmids. Our results underscore a pivotal role for Xer-SSR and IS26-mediated recombinatorial events in Ab plasmid evolution, aiding in the dissemination of resistance genes and a rapid adaptation of the bacterial host(s) to highly dynamic environments.1) Giacone et al. 2023 Front Microbiol. 9(14) doi:10.3389/fmicb.2023.10576082) Cameranesi et al. 2020 Microb Genom 6(9). doi:10.1099/mgen.0.000360