RamA upregulates multidrug resistance efflux pumps AcrAB and OqxAB in Klebsiella pneumoniae.

Overexpression of the acrAB genes regulated by RamA and overexpression of oqxAB regulated by RarA has been reported to mediate multidrug resistance in gram-negative bacilli. In this study, the regulation of acrAB and oqxAB simultaneously by the global regulator RamA was investigated in a multidrug resistant Klebsiella pneumoniae clinical isolate (KP22) resistant to tigecycline and other antimicrobials. KP22 overexpressed ramA due to a ramR mutation, along with an unexpected overexpression of oqxB. Deletion of ramA led to a 16-fold decrease in tigecycline MIC with down-expression of acrB (4.3-fold) and oqxB (7.1-fold) compared to KP22. Trans-complementation of KP22∆ramA with the wild-type ramA gene restored tigecycline MIC and upregulation of the acrB and oqxB genes (acrB, 3.9-fold; oqxB, 4.0-fold compared to KP22). When oqxB was knocked out, MICs of ciprofloxacin, olaquindox and nitrofurantoin were considerably decreased while deletion of acrB led to MIC decreases for cefepime, piperacillin/tazobactam and tigecycline in addition to the above 3 antimicrobials. The results of electrophoretic mobility shift assay showed that RamA could bind the promoter regions of both the acrAB and oqxAB operons. This study demonstrates for the first time that RamA can directly regulate multidrug resistance efflux pumps AcrAB and OqxAB in K. pneumoniae.