In order to establish a successful infection, pathogens must survive host defense systems or else mitigate the activities of PRRs. Consequently, they have evolved to modify the structural components which normally trigger PRR responses. Bacterial PGN is a hetero-polymer consisting of alternating residues of b-1,4-linked N-acetylglucosamine and N-acetylmuramic acid to which a peptide chain is attached [24] . Interestingly, L. monocytogenes modifies its PGN, with fifty per cent of the muropeptide composition being N-deacetylated [25] . We previously reported that a PGN N-deacetylase gene, pgdA, is responsible for this modification [25] . PGN deacetylation confers resistance to the action of lysozyme, one of the most important and widespread antimicrobial agents of the innate defense system, thus preventing degradation and release of immunostimulants. A strain of L. monocytogenes mutated in its ability to alter its PGN, DpgdA, is sensitive to lysozyme and induces an enhanced IFN-b response in macrophages compared to the isogenic parental strain [25] .