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A mutant of  Listeria monocytogenes  ScottA with a transposon in the 5' untranslated region of the  asnB  gene was identified to be hypersensitive to the antimicrobial  t -cinnamaldehyde. Here, we report the functional characterization of AsnB in peptidoglycan (PG) modification and intracellular infection. While AsnB of  Listeria  is annotated as a glutamine-dependent asparagine synthase, sequence alignment showed that this protein is closely related to a subset of homologs that catalyze the amidation of  meso -diaminopimelic acid ( m DAP) residues in the peptidoglycan of other bacterial species. Structural analysis of peptidoglycan from an  asnB  mutant, compared to that of isogenic wild-type (WT) and complemented mutant strains, confirmed that AsnB mediates  m DAP amidation in  L. monocytogenes . Deficiency in  m DAP amidation caused several peptidoglycan- and cell surface-related phenotypes in the  asnB  mutant, including formation of shorter but thicker cells, susceptibility to lysozyme, loss of flagellation and motility, and a strong reduction in biofilm formation. In addition, the mutant showed reduced invasion of human epithelial JEG-3 and Caco-2 cells. Analysis by immunofluorescence microscopy revealed that  asnB  inactivation abrogated the proper display at the listerial surface of the invasion protein InlA, which normally gets cross-linked to  m DAP  via  its LPXTG motif. Together, this work shows that AsnB of  L. monocytogenes , like several of its homologs in related Gram-positive bacteria, mediates the amidation of  m DAP residues in the peptidoglycan and, in this way, affects several cell wall and cell surface-related properties. It also for the first time implicates the amidation of peptidoglycan  m DAP residues in cell wall anchoring of InlA and in bacterial virulence.

frontiers in microbiology

AsnB Mediates Amidation of Meso-Diaminopimelic Acid Residues in the Peptidoglycan of Listeria monocytogenes and Affects Bacterial Surface Properties and Host Cell Invasion