Diacylglycerol Kinase A Is Essential for Polymyxin Resistance Provided by EptA, MCR-1, and Other Lipid A Phosphoethanolamine Transferases.

Gram-negative bacteria utilize glycerophospholipids (GPLs) as phospho-form donors to modify various surface structures. These modifications play important roles in bacterial fitness in diverse environments influencing cell motility, recognition by the host during infection, and antimicrobial resistance. A well-known example is the modification of the lipid A component of lipopolysaccharide by the phosphoethanolamine (pEtN) transferase EptA that utilizes phosphatidyethanoalmine (PE) as the phospho-form donor. Addition of pEtN to lipid A promotes resistance to cationic antimicrobial peptides (CAMPs), including the polymyxin antibiotics like colistin. A consequence of pEtN modification is the production of diacylglycerol (DAG) that must be recycled back into GPL synthesis via the diacylglycerol kinase A (DgkA). DgkA phosphorylates DAG forming phosphatidic acid, the precursor for GPL synthesis. Here we report that deletion of dgkA in polymyxin-resistant E. coli results in a severe reduction of pEtN modification and loss of antibiotic resistance. We demonstrate that inhibition of EptA is regulated posttranscriptionally and is not due to EptA degradation during DAG accumulation. We also show that the inhibition of lipid A modification by DAG is a conserved feature of different Gram-negative pEtN transferases. Altogether, our data suggests that inhibition of EptA activity during DAG accumulation likely prevents disruption of GPL synthesis helping to maintain cell envelope homeostasis. IMPORTANCE For Gram-negative bacteria, modification of a key surface structure known as lipopolysaccharide (LPS) is critical for resistance to cationic antimicrobial peptides, including the last-resort antibiotic polymyxin. One key enzyme that is critical for resistance is EptA that adds a positively charged residue to LPS, preventing polymyxin binding. Here we show that EptA can be posttranscriptionally regulated by a key cell envelope lipid leading to changes in antibiotic resistance.