Muramidases constitute a superfamily of enzymes that hydrolyse peptidoglycan (PGN) from bacterial cell walls. Recently, a fungal muramidase derived from Acremonium alcalophilum has been shown to increase broiler performance when added as a feed additive. However, the underlying mechanisms of action are not yet identified. Here, we investigated the hypothesis that this muramidase can cleave PGN to muramyl dipeptide (MDP), activating nucleotide-binding oligomerisation domain-containing protein 2 (NOD2) receptors in eukaryotic cells, potentially inducing anti-inflammatory host responses. Using Micrococcus luteus as a test bacterium, it was shown that muramidase from A. alcalophilum did not display antimicrobial activity, while it could cleave fluorescently labelled PGN. It was shown that the muramidase could degrade PGN down to its minimal bioactive structure MDP by using UPLC-MS/MS. Using HEK-Blue™-hNOD2 reporter cells, it was shown that the muramidase-treated PGN degradation mixture could activate NOD2. Muramidase supplementation to broiler feed increased the duodenal goblet cell and intraepithelial lymphocyte abundance while reducing duodenal wall CD3+ T lymphocyte levels. Muramidase supplementation to broiler feed only had moderate effects on the duodenal, ileal and caecal microbiome. It was shown that the newly discovered muramidase hydrolysed PGN, resulting in MDP that activates NOD2, potentially steering the host response for improved intestinal health.

Bacterial samples were obtained from the tonsillar surfaces of seven patients (four males, three females; median age 18 years, range 15 to 21 years) suffering from acute infectious mononucleosis with concomitant pharyngotonsillitis, and from five healthy controls. By using gold-labelled antiserum to human lysozyme and lactoferrin, micro-organisms on the tonsillar surfaces coated with these antibacterial substances could be identified by tracing the gold particles in the transmission electron microscope. In healthy individuals, most of the bacteria were coated with lysozyme and significantly more bacteria were coated with lysozyme than with lactoferrin (p < 0.01). In patients there was a non-significant reduction in lyoszyme-coating of the bacteria, whereas lactoferrin-coating was significantly increased (p < 0.01). Changes in the lysozyme and/or lactoferrin coating of the tonsillar surface bacteria on the palatine tonsils during infectious mononucleosis cannot explain the tendency to immense local bacterial colonization with commensals and proneness to bacterial penetration into the epithelial cells.