Ruminococcus gnavus is a human gut symbiont wherein the ability to degrade mucins is mediated by an intramolecular trans-sialidase (RgNanH). RgNanH comprises a GH33 catalytic domain and a sialic acid-binding carbohydrate-binding module (CBM40). Here we used glycan arrays, STD NMR, X-ray crystallography, mutagenesis and binding assays to determine the structure and function of RgNanH_CBM40 (RgCBM40). RgCBM40 displays the canonical CBM40 β-sandwich fold and broad specificity towards sialoglycans with millimolar binding affinity towards α2,3- or α2,6-sialyllactose. RgCBM40 binds to mucus produced by goblet cells and to purified mucins, providing direct evidence for a CBM40 as a novel bacterial mucus adhesin. Bioinformatics data show that RgCBM40 canonical type domains are widespread among Firmicutes. Furthermore, binding of R. gnavus ATCC 29149 to intestinal mucus is sialic acid mediated. Together, this study reveals novel features of CBMs which may contribute to the biogeography of symbiotic bacteria in the gut.

Invasive fungal infections are some of the most life-threatening infectious diseases in the hospital setting. In industrialized countries, the most common fungal species isolated from immunocompromised patients are Candida and Aspergillus spp. However, the number of infections due to Mucorales spp. is constantly increasing and little is known about the virulence factors of these fungi. The fungal cell wall is an important structure protecting fungi from the environment. A better knowledge of its composition should improve our understanding of host-pathogen interactions. Cell wall molecules are involved in tissue adherence, immune escape strategies, and stimulation of host defenses including phagocytosis and mediators of humoral immunity. The fungal cell wall is also a target of choice for the development of diagnostic or therapeutic tools. The present review discusses our current knowledge on the cell wall structure of Mucorales in terms of the polysaccharides and glyco-enzymes involved in its biosynthesis and degradation, with an emphasis on the missing gaps in our knowledge.

Intestinal mucins trigger immune responses upon recognition by dendritic cells via protein–carbohydrate interactions. We used a combination of structural, biochemical, biophysical, and cell‐based approaches to decipher the specificity of the interaction between mucin glycans and mammalian lectins expressed in the gut, including galectin (Gal)‐3 and C‐type lectin receptors. Gal‐3 differentially recognized intestinal mucins with different O ‐glycosylation profiles, as determined by mass spectrometry (MS). Modification of mucin glycosylation, via chemical treatment leading to a loss of terminal glycans, promoted the interaction of Gal‐3 to poly‐ N ‐acetyllactosamine. Specific interactions were observed between mucins and mouse dendritic cell‐associated lectin (mDectin)‐2 or specific intercellular adhesion molecule–grabbing nonintegrin‐related‐1 (SIGN‐R1), but not mDectin‐1, using a cell‐reporter assay, as also confirmed by atomic force spectroscopy. We characterized the N‐glycosylation profile of mouse colonic mucin (Muc)‐2 by MS and showed that the interaction with mDectin‐2 was mediated by high‐mannose N ‐glycans. Furthermore, we observed Gal‐3 binding to the 3 C‐type lectins by force spectroscopy. We showed that mDectin‐1, mDectin‐2, and SIGN‐R1 are decorated by N ‐glycan structures that can be recognized by the carbohydrate recognition domain of Gal‐3. These findings provide a structural basis for the role of mucins in mediating immune responses and new insights into the structure and function of major mammalian lectins.—Leclaire, C., Lecointe, K., Gunning, P. A., Tribolo, S., Kavanaugh, D. W., Wittmann, A., Latousakis, D., MacKenzie, D. A., Kawasaki, N., Juge, N. Molecular basis for intestinal mucin recognition by galectin‐3 and C‐type lectins. FASEB J. 32, 3301–3320 (2018). www.fasebj.org

H. pylori is a gram negative bacteria that colonizes the mucus niche of the gastric mucosa and infects more than half of the world's human population. Chronic infection may cause gastritis, duodenal ulcer, intestinal metaplasia or gastric cancer. In the stomach, H. pylori interacts with O-glycans of gastric mucins but the mechanism by which the bacteria succeed in altering the mucosa remains mainly unknown.

Resistance of the opportunistic pathogen Candida albicans to antifungal drugs has increased significantly in recent years. After screening 55 potential antifungal compounds from a chemical library, 2,3-dihydroxy-4-methoxybenzaldehyde (DHMB) was identified as having potential antifungal activity. The properties of DHMB were then assessed in vitro and in vivo against C. albicans overgrowth and intestinal inflammation. Substitution on the aromatic ring of DHMB led to a strong decrease in its biological activity against C. albicans. The MIC of DHMB was highly effective at eliminating C. albicans when compared to that of caspofungin or fluconazole. Additionally, DHMB was also effective against clinically isolated fluconazole- or caspofungin-resistant C. albicans strains. DHMB was administered to animals at high doses. This compound was not cytotoxic and was well-tolerated. In experimental dextran sodium sulphate (DSS)-induced colitis in mice, DHMB reduced the clinical and histological score of inflammation and promoted the elimination of C. albicans from the gut. This finding was supported by a decrease in aerobic bacteria while anaerobic bacteria populations were re-established in mice treated with DHMB. DHMB is a small organic molecule with antifungal properties and anti-inflammatory activity by exerting protective effects on intestinal epithelial cells.

The molecular composition and structural organisation of the cell wall of filamentous fungi underlie the ability of the host to identify them as pathogens. Although the organisation of the fungal cell wall, composed of 90% polysaccharides, is similar from one fungus to another, small variations condition their ability to trigger pattern recognition receptors. Because the incidence of mucormycosis, an emerging life-threatening infection caused by the species of the order Mucorales is increasing worldwide, the precise composition of the cell wall of two strains of Lichtheimia corymbifera was investigated in the early growth stages of germination (spores and germ-tubes) using trimethylsilylation and confocal microscopy. This study also characterises the response of THP-1 cells to Mucorales. The study identified the presence of uncommon monosaccharides (fucose, galactose and glucuronic acid) whose respective proportions vary according to the germination stage, revealing early parietal reorganisation. Immunofluorescence studies confirmed the exposure of β-glucan on the surface of swollen spores and germ-tubes. Both spores and germ-tubes of L. corymbifera promoted an early and strong pro-inflammatory response, through TLR-2. Our results show the singularity of the cell wall of the order Mucorales, opening perspectives for the development of specific diagnostic biomarkers. L. corymbifera is a causative agent of mucormycosis, an emerging invasive fungal infection. Deciphering cell wall composition can lead to the identification of a polysaccharide epitope, which could be use as a biomarker, useful for the diagnosis of mucormycosis.