Preparation of AHL extract

A. baumannii and A. tumefaciens were stored at −80°C in bacteria stock solution (Beyotime Biotechnology, Shanghai, China). A. baumannii were inoculated on LB agarose plates and incubated overnight at 37°C. Individual colonies (1×10⁸ colony-forming units (CFU)/ml) were selected and cultured in 15 ml LB medium at 37°C.

For HPLC-MS, bacteria were cultured in 500 ml LB medium from the overnight LB agarose plates, and 500 ml bacterial liquid was collected at 8 h, as determined by the AHL activity curve. Bacterial samples were centrifuged (4,500 × g for 20 min) and supernatants were passed through a 0.22 µm filter. An equal volume of 100% ethyl acetate was added to the filtrate, and the ethyl acetate phase was collected for AHL extraction and dried in a vacuum centrifuge. The residue was the AHL extract and was then re-dissolved in 50 µl ethyl acetate.