Efflux Pump AdeABC Assessment in Acinetobacter baumannii Strains Isolated in a Teaching Hospital

Over the past twenty years the worldwide clinically impact of Acinetobacter baumannii (A. baumannii) demonstrated its etiopathogenetic relevance. During a previously retrospective study in a teaching hospital, between January 2011 and February 2015, we observed increasingly infections caused by A. baumannii associated with antibiotic multi-resistance. Tigecycline, the first member of the glycylcycline class, is an effective option for the treatment of such infections even if, due to its increased clinical use, tigecycline resistant isolates have recently emerged. In A. baumannii several mechanisms are associated with a tigecycline decrease susceptibility, among these, expression efflux pump AdeABC and the presence of insertion sequence (IS) in the adeRS operon. About that, we decided to analyze adeB and adeS genes in 24 MDR A. baumannii clinical isolates, selected on the different tigecycline phenotype. The study of adeB and adeS genes was performed by an in-house polymerase chain reaction (PCR) and by Sanger sequencing method. According to literature adeB and adeS genes were detected in all MDR A. baumannii isolates tested. Therefore our attention has focused on two resistant tigecycline clinical strains (ACI 2313 and ACI 1213), with a MIC value ≥ 8. In particular the ACI 2313 strains, showed the presence of an IS in the adeS gene. Then, adeS sequence analysis identified ISAba1 insertion. Moreover, adeB gene expression was evaluated by an in-house SYBR Green I-based real-time RT-PCR. We found an over expression of adeB gene in ACI 2313 strain, according to IS presence on adeS gene, while the lack of adeB overexpression in ACI 1213, still resistant to tigecycline, could be due to different resistance mechanisms.