Role of the BaeSR two-component system in the regulation of Acinetobacter baumannii adeAB genes and its correlation with tigecycline susceptibility

Abstract

BackgroundTigecycline resistance in Acinetobacter baumannii is primarily acquired through overexpression of the AdeABC efflux pump. Besides AdeRS, other two-component regulatory systems (TCSs) involving the regulation of this transporter have not been clarified.ResultsIn this study, we found that the TCS genes baeR and baeS are co-transcribed and function as stress responders under high osmotic conditions. The baeSR and adeAB genes showed increased transcription in both the laboratory-induced and clinical tigecycline-resistant strains compared with the wild-type strain. The deletion of baeR in the ATCC 17978 strain led to 67–73% and 68% reduction in adeA and adeB expression, respectively, with a resultant 2-fold decrease in the tigecycline minimal inhibition concentration (MIC). In contrast, the overexpression of baeR resulted in a doubled tigecycline MIC, with a more than 2-fold increase in adeA and adeB expression. The influence of baeR knockout on adeAB gene expression can also be observed in the laboratory-induced tigecycline-resistant strain. A time-kill assay showed that the baeR deletion mutant showed an approximate 1-log10 reduction in colony forming units (CFUs) relative to the wild-type strain when the tigecycline concentration was 0.25 μg/mL throughout the assay period. The wild-type phenotype could be restored by trans-complementation with pWH1266-kan r -baeR. Increasing the tigecycline concentration to 0.5 μg/mL produced an even more marked 4.7-log10 reduction in CFUs of the baeR deletion mutant at 8 h, while only a 2.1-log10 reduction was observed for the wild-type strain.ConclusionsTaken together, these data show for the first time that the BaeSR TCS influences the tigecycline susceptibility of A. baumannii through the positive regulation of the resistance-nodulation-division efflux pump genes adeA and adeB.

The Challenge of Efflux-Mediated Antibiotic Resistance in Gram-Negative Bacteria

“…AdeA-AdeA2-AdeB (containing a pair of AdeAs) is a newly identified AdeAB homolog involved in tigecycline resistance (548). Another putative RND pump, AdeT, possibly involved in aminoglycoside resistance (16), was further confirmed to be present in the genomes of two multidrug-resistant isolates (500).…”

“…The BaeSR regulatory system was found to positively influence the expression of the AdeA-AdeA2-AdeB system (548). Transcriptional upregulation of the MFS-type transporters MdfA and Tet(A) was observed among Ͼ200 up-or downregulated genes in a multidrug-resistant mutant after its exposure to a subinhibitory concentration of tigecycline (862).…”

Distribution of different efflux pump genes in clinical isolates of multidrug-resistant Acinetobacter baumannii and their correlation with antimicrobial resistance

“…Each isolate was sampled from a different patient and classified as genus Acinetobacter using the Vitek system (Biomerieux Vitek, Inc., Hazelwood, MO, USA). A. baumannii was further identified by one-tube multiplex polymerase chain reaction (PCR) based on the method of Chen et al 24 Induction of resistance for amikacin, IPM, or TGC The induced TGC-resistant A. baumannii strain was from our previous study, 25 whereas the induced amikacin (AMK)-resistant and IPM-resistant A. baumannii strains were from a study by Kuo et al 26 Serial passaging was performed as previously described by Li at al 27 with several modifications.…”

“…25 The cDNAs were used in PCR reactions with different primers for qRT-PCR (Table 1), which was carried out with a StepOne Real-Time PCR System (Life Technologies, Grand Island, NY, USA). Briefly, each 20-mL reaction mixture contained 25 ng cDNA, 10 mL Power SYBR green PCR master mix (Life Technologies), and 300nM each forward and reverse primer.…”

A Novel Genome-Editing Platform for Drug-Resistant Acinetobacter baumannii Reveals an AdeR-Unrelated Tigecycline Resistance Mechanism

“…However, the presence of an alternative activating mechanism for AdeABC mediated by the TCS BaeSR was recently suggested (34). In order to determine if overexpression of AdeABC was mediating tigecycline resistance despite the deletion of adeR, we quantified the expression level of adeB, which is the main component of the RND efflux pump.…”