Bacterial phospholipid hydrolysis enhances the destruction of Escherichia coli ingested by rabbit neutrophils. Role of cellular and extracellular phospholipases

Abstract

Escherichia coli ingested by PMN are promptly growth arrested but undergo limited destruction. We have studied bacterial phospholipid hydrolysis as a possible limiting factor in the disassembly of ingested E. coli, comparing the fates, during phagocytosis by rabbit peritoneal exudate PMN, of three isogenic strains, differing in their content of the pldA gene encoding the principal E. coli phospholipase A (PLA), i.e., pldA-, pldA+, pldA (the latter strain bearing the pldA gene in a multicopy plasmid resulting in a 20-fold increase in PLA content). Ingestion and growth inhibition (greater than 99% within 15 min) were the same for the three strains, but phospholipid degradation differed according to bacterial PLA content: pldA up to 60%, pldA+ up to 30%, and pldA- up to 20%. Since the pldA- strain has no activatable PLA, phospholipid degradation in this strain demonstrates the action of a PMN PLA. Added PLA2-rich ascitic fluid (AF) or purified AF PLA2 increased the rate and extent of degradation of the pldA- strain, provided the enzyme was added before ingestion was complete. 125I-AF-PLA2 binds to both E. coli and PMN and thus can enter the vacuole during phagocytosis. Although up to 50-fold more AF-PLA2 than the PLA2 content of the PMN could be loaded into the PMN in this way, degradation of pldA- E. coli did not exceed 30%. Increased phospholipid degradation had no effect on the degradation of bacterial macromolecules. In contrast, bacterial disassembly manifest as structural disorganization, release of bacterial protein derived material, and inhibition of protein synthesis were markedly enhanced when greater than 50% of prelabelled bacterial phospholipids were degraded. These findings reveal a link between envelope phospholipid degradation and overall bacterial destruction, suggesting therefore that factors limiting PLA action limit the destruction of E. coli ingested by PMN.