Fig. 1.

Principle of PC labeling in bacteria to detect PLs. A: Critical step of PC biosynthesis via Pcs in bacteria (34) suitable for metabolic labeling. B: Chemical structures of Cho, PCho, and AECho. C: Principle of metabolic labeling with AECho: I, intact OM of E. coli with PLs on the inner leaflet and LPS on the outer leaflet; II, AEPC is in the OM after cells are incubated with AECho; III, compromised OM (genetic mutation or chemical inhibition of LPS pathways) have PLs on the outer leaflet; and IV, exposed AEPC can be labeled with fluorescent alkyne reagents to allow fluorescent detection. Note that the LPS is represented without core sugars and O-antigens for simplicity.