The intestinal mucus is composed mainly of mucins that are complex agglomerates of structural glycoproteins with specific O-linked glycans (O-glycans) produced by specialized cells of the host called goblet cells (Forstner, 1995). Mucins can either be secreted and form a gel, or be produced as membrane-bound glycoproteins that are part of the epithelial glycocalyx (Johansson et al., 2008, 2011; Jonckheere et al., 2013; Nilsson et al., 2014). These glycoproteins share a common structure made of tandem repeated amino acids rich in proline, threonine and serine and are call PTS domains. These sequences of amino acid provide sites for the covalent attachment of the polysaccharides and are widely O-glycosylated (Moran et al., 2011). Four different types of polysaccharide core structures are commonly found in mucin glycoproteins. These cores are formed by a combination of three polysaccharides, galactose, N-acetyl-galactosamine and N-acetyl-glucosamine (Larsson et al., 2009; Juge, 2012). Different chains of glycan will be attached to the core. The terminal monosaccharide is usually a fucose or a sialic acid (Larsson et al., 2009; Juge, 2012). Oligosaccharide chains are also sulfated, especially in colonic regions (Rho et al., 2005). The mucin proteins MUC1, MUC5AC, and MUC6 mainly form the mucus layer in the stomach, whereas MUC2 is the most abundant mucin in the small intestine and the colon (Johansson et al., 2009; Moran et al., 2011). The thickness of the mucus layer varies through the gut. The colon, which harbors the highest density of microorganisms, is covered by the thickest mucus layer (Gum et al., 1994). It is composed of an inner layer that is dense and firmly attached to the epithelium and an outer loose layer that is exposed to bacterial proteolytic activity. The numerous O-glycans of the outer layer can serve as adhesion sites and as nutrients for bacteria while the inner layer is less permissive to bacterial penetration in healthy individuals (Johansson et al., 2008, 2011). Most bacterial residents are present in the outer mucus layer and the competition for survival in this niche shapes the composition of the microbiota. The differential resource utilization of bacterial species participates to the establishment of distinct communities that includes non-mucolytic bacteria (Li et al., 2015).

The mucus barrier has an important role in regulating the severity of infectious diseases. It provides protection against many intestinal pathogens, including Yersinia enterocolitica, Shigella flexneri, Salmonella, and Citrobacter rodentium (Mantle and Rombough, 1993; Bergstrom et al., 2010; Arike and Hansson, 2016). MUC2 (Mouse, Muc2) plays a crucial role during infection. Using Muc2-deficient mice, it was shown that the glycoprotein is critical in controlling Salmonella infection (Zarepour et al., 2013). Moreover, Muc2^−/− mice revealed higher susceptibility to attaching and effacing (A/E) Citrobacter rodentium infections (Bergstrom et al., 2010).

An alteration of mucosal integrity is generally associated with health problems, such as inflammatory bowel diseases, including ulcerative colitis and Crohn's disease (Trabucchi et al., 1986; Hanski et al., 1999). During ulcerative colitis, alteration of mucus integrity results in a thinner mucus layer due to goblet cell depletion (Pullan et al., 1994) and a reduced O-glycosylation and sulfation of mucins (Raouf et al., 1992; Larsson et al., 2011). During Crohn's disease, the mucus layer is essentially continuous and comparable to healthy mucosa (Strugala et al., 2008) although there is evidence of abnormal expression and glycosylation of the mucin (Buisine et al., 2001; Moehle et al., 2006; Dorofeyev et al., 2013). These changes in the mucosal environment could also be linked to dysbiosis, an abnormal change in the composition of the intestinal microbiota due to Crohn's disease. Once impaired, the mucus barrier becomes permeable to bacteria that are able to access the epithelium and therefore cause inflammation (Antoni et al., 2014; Johansson et al., 2014), which is why the integrity of the mucus layer is critical for the upkeep of a homeostatic relationship between the intestinal microbiota and its host.

A distinct subset of intestinal bacteria possesses the enzymatic activity, such as glycosidases, necessary for the degradation of mucin oligosaccharides, which can be further metabolized by resident microbiota (Koropatkin et al., 2012; Ouwerkerk et al., 2013). Indeed, various anaerobic bacteria species of gut microbiota, such as Akkermansia muciniphila (Derrien et al., 2004; Png et al., 2010), Bacteroides thetaiotaomicron (Xu et al., 2003; Sonnenburg et al., 2005), Bifidobacterium bifidum (Crociani et al., 1994; Png et al., 2010; Garrido et al., 2011), Bacteroides fragilis (Macfarlane and Gibson, 1991; Swidsinski et al., 2005a; Huang et al., 2011), Ruminococcus gnavus (Png et al., 2010; Crost et al., 2013), and Ruminococcus torques (Hoskins et al., 1985; Png et al., 2010) are now known as mucin-degrading specialists. These bacteria will use their specific enzymatic activities to release monosaccharides attached to the mucin glycoproteins. Some mucolytic bacteria, such as B. thetaiotaomicron, that possess an important variety of glycosidases, are better suited for the utilization of a wide range of glycans (Xu et al., 2003; Marcobal et al., 2013). To complete the degradation of mucins, a combination of enzymatic activity of several mucolytic bacteria is needed (Derrien et al., 2010; Marcobal et al., 2013). Therefore, MUC2 glycans act as nutritional sources for bacteria that can utilize the mucus-derived sugars, but lack the enzymes necessary for cleaving sugar linkages (Johansson et al., 2015; Arike and Hansson, 2016). Commonly, several bacteria collaborate in a community and it has been shown that the sulfatase activity of some commensal bacteria on sulfomucin allows glycosidases to access and act on mucins (Rho et al., 2005). Released saccharides, such as N-acetyl-D-glucosamine (GlcNAc also called NAG), N-acetylgalactosamine (GalNAc), galactose, fucose and sialic acid (N-acetylneuraminic acid also called NANA) can then be used by the degrader itself or by other resident bacteria (Bjursell et al., 2006; Martens et al., 2008; Sonnenburg et al., 2010). As example, commensal E. coli that are limited to growth on mono- or disaccharides, are unable to degrade the complex polysaccharides that constitute mucin (Hoskins et al., 1985) and therefore use such carbohydrate sources (Chang et al., 2004; Png et al., 2010; Bertin et al., 2013). Another example is vancomycin-resistant Enterococcus that can grow on mucin pre-digested with extracts from human stools, but not on purified mucin. This suggests that Enterococcus can benefit of the microbiota activity on mucin and uses released mucus-derived products (Pultz et al., 2006). In this way, mucolytic bacteria make mucus O-glycan derived products also available for other bacterial residents.

Intestinal pathogens have developed strategies to compete with commensal microflora for nutrients, such as carbohydrates and these strategies have been reviewed in Conway and Cohen (2015), Vogt et al. (2015), and Baumler and Sperandio (2016). Pathogenic and commensal E. coli strains displayed considerable catabolic diversity when colonizing streptomycin-treated mice, indicating that nutrient availability can influence their colonization success and their niche adaptation (Maltby et al., 2013). For example, pathogenic E. coli such as enterohemorrhagic E. coli (EHEC) strain EDL933 efficiently utilizes some mucus-derived monosaccharides. This can provide competitive growth compared to that of commensal E. coli (Fabich et al., 2008). Moreover, the metabolic flexibility of some pathogenic strains to use both glycolytic and gluconeogenic nutrients may be advantageous (Bertin et al., 2013). The pathogen Vibrio cholerae's preferential use of mucus-derived monosaccharides, such as GlcNAc and sialic acid confers an advantage in the infant mouse model of infection (Almagro-Moreno et al., 2015). C. jejuni also possess the ability to metabolize fucose. Its growth is enhanced in culture medium supplemented with it (Alemka et al., 2012). In addition, antibiotic treatment also perturbs the microbiota and therefore affects the availability of mucin carbohydrates. The concentration of free fucose and sialic acid reaching high levels during antibiotic treatment facilitates expansion of pathogens such as Salmonella enterica serotype Typhimurium and Clostridium difficile (Ng et al., 2013). In addition, Salmonella serotype Typhimurium is known both to bind glycoprotein containing sialic acids (Vimal et al., 2000) and to have the ability to release the carbohydrate using its sialidase (Hoyer et al., 1992). Thereby, to colonize specific niches, many pathogens have evolved in a way to use mucus-derived sugars as a carbon source.

To adhere to mucus, commensal and pathogenic bacteria use different strategies. First, they can produce proteins that specifically bind the mucus. Mucus-binding proteins (MUB) are cell-surface proteins mainly described in lactic acid bacteria (LAB) (Boekhorst et al., 2006), especially in Lactobacillus reuteri (Roos and Jonsson, 2002; MacKenzie et al., 2009). MUB contain domains that are similar to the model mucin-binding protein (MucBP) from the Pfam database (Boekhorst et al., 2006). The MucBP domain is found in a variety of bacterial proteins that are known for their capacity to adhere to mucus (Juge, 2012). MUB also share structural and functional homology with pathogenic Gram-positive adhesins that have specificity to sialylated mucin glycans (Etzold et al., 2014). For example, some surface proteins of Listeria monocytogenes contain a MucBP domain similar to those found in Lactobacillus, allowing them to adhere to mucin (Bierne et al., 2007; Mariscotti et al., 2014). The causative agent of cholera, V. cholerae, can also bind to mucin using surface protein called GbpA (chitin-binding protein) that binds specifically to N-acetyl D-glucosamine residues of intestinal mucins (Bhowmick et al., 2008). In addition, C. jejuni is well-known for its ability to interact with different human histoblood group antigens (HBGAs) expressed in mucosa (Naughton et al., 2013). The major outer membrane protein (MOMP) of C. jejuni is involved in these interactions (Mahdavi et al., 2014). This way, C. jejuni can interact with intestinal mucin MUC2 in the intestine (Tu et al., 2008). Furthermore, Bifidobacterium spp. is also known for its specific adhesion to mucus. For example, in a B. bifidum mucin-binding assay, the expression of an extracellular transaldolase correlated with a positive mucin-binding phenotype (Gonzalez-Rodriguez et al., 2012). B. longum subsp. infantis is another species that binds specifically to mucin using family-1 solute binding proteins (Kankainen et al., 2009). Interestingly, a study using gnotobiotic mice colonized by B. fragilis and E. coli revealed that the commensal bacterium B. fragilis was found in the mucus layer while E. coli was only found in the lumen. Further analysis showed that B. fragilis specifically binds to highly purified mucins. This indicated that a direct bond with intestinal mucus could be a mechanism used by B. fragilis for gut colonization (Huang et al., 2011).

In addition to produce specific mucus binding proteins, some bacteria can also use cell-surface appendix, such as pili or fimbriae to bind the mucus. For example, production of pili by LAB was shown to be implicated in mucus-binding activity (Douillard et al., 2013) and moreover, the SpaC pilus protein of L. rhamnosus GG was shown to strongly binds the human mucins (Kankainen et al., 2009). An in vitro study using mucus-secreting HT29-MTX intestinal epithelial cell model showed that the adhesion of Salmonellae enterica serotype Typhimurium to mucus-secreting intestinal epithelial cells was higher than in non- and low-mucus producing cells (Gagnon et al., 2013). Moreover, virulent strains seem to bind more efficiently to mucus than avirulent strains and the binding that preferentially targets the neutral mucin is mannose-dependant (Vimal et al., 2000). As with some uropathogenic E. coli (Wurpel et al., 2014), the adhesion of S. enterica serotype Typhimurium could be the result of interaction between fimbrial adhesin and mucin glycans, more specifically terminal fucose residues (Chessa et al., 2009). The E. coli K88 (F4) fimbriae is also able to bind mucus from the small intestines of 35-day-old piglets with a specificity to the glycolipid galactosylceramide (Blomberg et al., 1993). Hence, pili and fimbriae are involved in specific adhesion to mucus.

Many enteric bacteria also produce flagellum. In addition to their role in motility, flagella are also involved in adhesion. As example, the E. coli probiotic strain Nissle 1917 was shown to be able to interact, via its flagella, with human and porcine mucus but not with murine mucus. Furthermore, the mucus component gluconate has been identified as one receptor for the adhesion of these flagella (Troge et al., 2012). Other studies have revealed the role of the flagella for the binding of mucin glycoproteins by C. difficile (Tasteyre et al., 2001) and pathogenic E. coli (Erdem et al., 2007). Indeed, a mutation of the flagellum element fliC prevents the adhesion of EPEC and EHEC to mucins (Erdem et al., 2007). More recently, the flagella of EPEC (O127:H6) and EHEC (O157:H7) were shown to adhere to mucin-type core 2 O-glycan in MUC2. C. jejuni is another pathogen that uses its flagella to bind mucin. It was showed that the major flagella subunit protein (FlaA) is also involved in the adhesion to HBGA in the mucus. Therefore, flagella can be used in attachment strategies by gut residents.

Pathogens have also adapted mechanisms to modulate mucin secretion to enhance pathogenesis by acting on the mucin-secreting cells, altering or inhibiting mucin production (Table 2). One of them is S. flexneri that alters the mucus layer through a type III secretion system-dependent manner. This pathogen will act on different elements, such as gene expression, mucin glycosylation and secretion, leading to a less effective mucus barrier (Sperandio et al., 2013). C. difficile produces a toxin, ToxA that is responsible for barrier dysfunction and causes severe inflammatory enteritis. ToxA also decreases the mucin exocytosis of colonic mucus-producing cells (Kelly et al., 1994; Branka et al., 1997). The recognition of bacterial components by these cells can also lead to an increased production and secretion of mucin in order to harm the present pathogen. As example, the adhesion of the EHEC O157:H7 to human colon cells HT-29 leads to an increased expression of MUC2 (Xue et al., 2014). Moreover, the cholera toxin of V. cholerae and lysteriolysin O of L. monocytogenes enhance the secretion of mucin by goblet cells and HT29-MTX cells, respectively (Lencer et al., 1990; Epple et al., 1997; Coconnier et al., 1998; Lievin-Le Moal et al., 2002, 2005). Surprisingly, the Pic protein secreted by S. flexneri and enteroaggregative E. coli (Henderson et al., 1999; Harrington et al., 2009) is known for its mucolytic activity, but is also a potent mucus secretagogue that induced hypersecretion of mucus by goblet cells (Navarro-Garcia et al., 2010). These studies show how pathogens can affect the behavior of mucus-producing cells in their advantage.

Pathogens also developed specific mechanisms to subvert and penetrate the mucus barrier. Some bacteria can directly act on the mucin through a mucinase activity. During enterotoxigenic E. coli infections, the autotransporter A (EatA) is involve in mucin degradation and this participate to the delivery of E. coli toxins to the cell surface (Kumar et al., 2014). Another example is the adherent and invasive E. coli strain LF82, associated with Crohn's disease. LF82 possesses a protease called Vat-AIEC that is implicated in the degradation of mucins and therefore decreases mucus viscosity (Gibold et al., 2016). The Pic autotransporter found in enteroaggregative E. coli and Shigella flexneri can also degrade various glycoproteins including mucins (Henderson et al., 1999; Harrington et al., 2009). Moreover, the plasmid-bearing Yersinia enterocolitica, which contain mucin-degrading enzyme(s), will increase the permeability of the mucus gel layer, allowing the bacteria to move more easily through the mucin (Mantle and Rombough, 1993). V. cholerae also produces a secreted protease called TagA that is encoded by the Vibrio pathogenicity island (VPI). TagA specifically cleaves mucin glycoproteins and may directly modify host cell surface molecules during V. cholerae infection (Szabady et al., 2011). Therefore, to facilitate their infection process, pathogens can directly modify the mucus.

Pathogens associated molecular patterns, such as lipopolysaccharide (LPS) and peptidoglycan are also known to stimulate mucin production (Petersson et al., 2011). This stimulation can occur directly on secreting cells, but also be through proinflammatory cytokine production. Recognition of LPS by LPS-binding protein (LBP), CD14, and TLR4 (Toll-Like Receptor) leads to a strong pro-inflammatory response in mammalian cells. LPS has been shown to induce mucin gene expression by binding to TLR4 and LBP (Dohrman et al., 1998; Smirnova et al., 2003). LPS and flagellin from Gram-negative bacteria as well as lipoteichoic acid, a component of the cell wall of Gram-positive bacteria, induce mucin upregulation through the Ras pathway (McNamara and Basbaum, 2001; Theodoropoulos and Carraway, 2007). LPS also increases the production of IL-8 by goblet cells, which leads to secretion of mucin (Smirnova et al., 2003). In addition, pro-inflammatory cytokine IL-6 and TNF-α increase secretion of MUC2, MUC5A, MUC5B, and MUC6 by the intestinal cell line LS180 despite a reduced glycosylation (Enss et al., 2000). Inflammation could be one of the aspects affecting the integrity of the mucus layer in inflammatory bowel diseases. Furthermore, the AIEC strain LF82 is able to alter the expression of the mucin gene and IL-8 of colonic cells T84 that could also lead to a defective mucus layer (Elatrech et al., 2015). Thus, pathogens can also alter the mucus production indirectly, through inflammation.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.