Abstract
Bacterial two-component regulatory system (TCS) plays an important role in the regulation of adaptation to and signal transduction of environmental stimuli. BaeSR, a TCS, which detects environmental signals and responds by altering the bacterial envelope, is one of the five extracytoplasmic response pathways. In this study, we found that the TCS genes baeR and baeS in Acinetobacter baumannii 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 decrease in the tigecycline minimal inhibition concentration (MIC). The influence of baeR knockout on adeAB gene expression can also be observed in the laboratory-induced tigecycline-resistant strains. The 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. 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. Efflux pump AdeIJK and MacAB-TolC are also demonstrated under the regulation of BaeSR. Of them, MacAB-TolC had higher gene expression in both the laboratory induced and clinical tigecycline-resistant A. baumannii strains, implicating its importance to tigecycline resistance in addition to AdeABC. Phenotypic microarray results showed that A. baumannii is vulnerable to certain chemicals, especially tannic acid, after deleting baeR, which was confirmed using the spot assay. The reference strain A. baumannii also exhibited increased gene expression of adeJ and macB in the medium with 100 µg/mL tannic acid, but the increase was fully inhibited by baeR deletion. An electrophoretic motility shift assay based on an interaction between His-BaeR and the adeA, adeI and macA promoter regions did not demonstrate direct binding, which implies that their mutual interaction was indirect. In conclusion, A. baumannii can make use of TCS BaeSR system to sense and respond in disposing chemicals, such as tannic acid and tigecycline, through regulating indirectly the efflux pumps. Our finding provides a promising target to develop future antimicrobials against drug- resistant A. baumannii.