Mutations in ampG or ampD affect peptidoglycan fragment release from Neisseria gonorrhoeae

Neisseria gonorrhoeae releases peptidoglycan fragments during growth. The majority of the fragments released are peptidoglycan monomers, molecules known to increase pathogenesis through the induction of proinflammatory cytokines and responsible for the killing of ciliated epithelial cells. In other gram-negative bacteria such as Escherichia coli, these peptidoglycan fragments are efficiently degraded and recycled. Peptidoglycan fragments enter the cytoplasm from the periplasm via the AmpG permease. The amidase AmpD degrades peptidoglycan monomers by removing the disaccharide from the peptide. The disaccharide and the peptide are further degraded and are then used for new peptidoglycan synthesis or general metabolism. We examined the possibility that peptidoglycan fragment release by N. gonorrhoeae results from defects in peptidoglycan recycling. The deletion of ampG caused a large increase in peptidoglycan monomer release. Analysis of cytoplasmic material showed peptidoglycan fragments as recycling intermediates in the wild-type strain but absent from the ampG mutant. An ampD deletion reduced the release of all peptidoglycan fragments and nearly eliminated the release of free disaccharide. The ampD mutant also showed a large buildup of peptidoglycan monomers in the cytoplasm. The introduction of an ampG mutation in the ampD background restored peptidoglycan fragment release, indicating that events in the cytoplasm (metabolic or transcriptional regulation) affect peptidoglycan fragment release. The ampD mutant showed increased metabolism of exogenously added free disaccharide derived from peptidoglycan. These results demonstrate that N. gonorrhoeae has an active peptidoglycan recycling pathway and can regulate peptidoglycan fragment metabolism, dependent on the intracellular concentration of peptidoglycan fragments.