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Background  The Gram-negative oral pathogen  Tannerella forsythia  strictly depends on the external supply of the essential bacterial cell wall sugar  N -acetylmuramic acid (MurNAc) for survival because of the lack of the common MurNAc biosynthesis enzymes MurA/MurB. The bacterium thrives in a polymicrobial biofilm consortium and, thus, it is plausible that it procures MurNAc from MurNAc-containing peptidoglycan (PGN) fragments (muropeptides) released from cohabiting bacteria during natural PGN turnover or cell death. There is indirect evidence that in  T. forsythia , an AmpG-like permease ( Tanf_08365 ) is involved in cytoplasmic muropeptide uptake. In  E. coli , AmpG is specific for the import of  N -acetylglucosamine (GlcNAc)-anhydroMurNAc(−peptides) which are common PGN turnover products, with the disaccharide portion as a minimal requirement. Currently, it is unclear which natural, complex MurNAc sources  T. forsythia  can utilize and which role AmpG plays therein.    Results  We performed a screen of various putative MurNAc sources for  T. forsythia  mimicking the situation in the natural habitat and compared bacterial growth and cell morphology of the wild-type and a mutant lacking AmpG ( T. forsythia  Δ ampG ). We showed that supernatants of the oral biofilm bacteria  Porphyromonas gingivalis  and  Fusobacterium nucleatum , and of  E. coli  Δ ampG , as well as isolated PGN and defined PGN fragments obtained after enzymatic digestion, namely GlcNAc-anhydroMurNAc(−peptides) and GlcNAc-MurNAc(−peptides), could sustain growth of  T. forsythia  wild-type, while  T. forsythia  Δ ampG  suffered from growth inhibition. In supernatants of  T. forsythia  Δ ampG , the presence of GlcNAc-anhMurNAc and, unexpectedly, also GlcNAc-MurNAc was revealed by tandem mass spectrometry analysis, indicating that both disaccharides are substrates of AmpG. The importance of AmpG in the utilization of PGN fragments as MurNAc source was substantiated by a significant  ampG  upregulation in  T. forsythia  cells cultivated with PGN, as determined by quantitative real-time PCR. Further, our results indicate that PGN-degrading amidase, lytic transglycosylase and muramidase activities in a  T. forsythia  cell extract are involved in PGN scavenging.    Conclusion   T. forsythia  metabolizes intact PGN as well as muropeptides released from various bacteria and the bacterium ’ s inner membrane transporter AmpG is essential for growth on these MurNAc sources, and, contrary to the situation in  E. coli , imports both, GlcNAc-anhMurNAc and GlcNAc-MurNAc fragments.

Utilization of different MurNAc sources by the oral pathogen Tannerella forsythia and role of the inner membrane transporter AmpG