Extractions and measurements were done as previously described 33 (link),34 (link). Log-phase A. baumannii culture was incubated 0.79 or 0.38 μg/mL RBT in the presence or absence of amino acid mixture at 37 °C. Bacteria were harvested 0, 1, 8, and 24 hours and CFUs were determined by plating serial dilutions on agar media. The cell free supernatant was collected by filtration through a 0.22 μm filter. RBT were extracted by adding LC-MS grade acetonitrile:methanol:water (40:40:20) solution that was precooled to − 40 °C. Liquid chromatography mass spectrometry (LC-MS) differentiation and detection of RBT was performed using a Cogent Diamond Hydride Type C column (Microsolve Technologies) with an Agilent Accurate Mass 6230 TOF coupled with an Agilent 1290 Liquid Chromatography system as previously published.33 (link),35 (link) An isocratic pump was used for continuous infusion of a reference mass solution to allow mass axis calibration. Detected RBT ion was validated based on unique accurate mass-retention time identifiers for masses. RBT level was analyzed using Agilent Qualitative Analysis B.08.00 (Agilent Technologies) with a mass tolerance of <0.005 Da. The intracellular RBT was calculated as the [RBT]drug only control – [RBT]filtrate. 3 biological replicates were tested per group.