The Mucin Paradox: Decoding the Molecular Strategies of Akkermansia muciniphila in Health and Disease

1. Introduction

Mucin-degrading enzymes (MDEs) hydrolyze specific peptide or glycosidic bonds in mucin glycoproteins, thereby disrupting the intact macromolecules and liberating oligo- or monosaccharide fragments, dipeptides, and amino acids [1]. These enzymes, along with other antimicrobial peptides produced by host cells, help regulate pathogenic bacteria, preventing their overgrowth. The corresponding enzymatic activities occur in the gut, thereby affecting the production of short-chain fatty acids (SCFAs) and other metabolites that serve as signaling molecules in host-microbe interactions [2]. Recently, researchers have shown significant interest in microorganisms with evolved enzymatic systems to investigate mucin as a primary nutrient source.

A. muciniphila has emerged as a great microbe of interest due to its significant association with host metabolic and gastrointestinal health [3,4,5]. The bacterium has been widely studied as a potential probiotic, often focusing on its protective properties against metabolic syndrome, intestinal dysbiosis, and inflammation [6,7]. A. muciniphila is one of the few gut commensals encoding glycoside hydrolases to degrade mucin glycans, such as Mucin 2 (MUC2), the major gel-forming protein of the colon and intestine, creating a protective gel barrier [8,9,10]. MUC2 helps protect the epithelial lining from invaders and promotes self-digestion, playing a significant role in the gut; however, abnormal function has also been associated with conditions such as ulcerative colitis [11]. Surprisingly, colonization by A. muciniphila is not only mucin-dependent but also affects mucus homeostasis [12].

It has been found that A. muciniphila significantly increases MUC2, the colonic mucin glycoprotein secreted by intestinal Goblet cells [13,14,15]. Accordingly, it has been hypothesized that mucin-degrading enzymes (MDEs) promote the growth of these specialized substrates by facilitating their consumption [16]. This paradox of simultaneous mucus degradation and protection is the very embodiment of the MDE-mediated balanced host-microbe interaction.

Within the Akkermansia genus, mucin degradation has been well documented; however, elucidating the substrate specificity of A. muciniphila strains remains an essential step toward fully understanding this process [17]. Several enzymes enhance growth on complex mucin substrates such as MUC2. However, MDEs differ in substrate specificity and regulatory responses. Thus, knowledge of the diversity and function of these enzymes is central to comprehending the ecological success of A. muciniphila within the mucus environment [5,18].

A. muciniphila is a Gram-negative, non-sporulating, rod-shaped bacterium that occupies and thrives in the gastrointestinal tract of numerous species, including humans [15]. It is a highly adapted organism to the colon mucus layer, where it participates in cross-feeding and other beneficial symbiotic interactions. This microorganism has been described as important for gut health; high abundance of A. muciniphila is correlated with the prevention or alleviation of obesity, type 2 diabetes, and other metabolic diseases [19]. What is still not clear is how exactly mucin degradation activity plays a role in these beneficial responses.

A. muciniphila has recently become very popular since it is the only known member of the phylum Verrucomicrobia that has a documented probiotic role [15]. It is indeed highly referred to as such. But the mechanisms by which this organism would exert any beneficial effects are still unknown. The goal of this review is to analyze the diversity, enzymatic specificity, regulatory mechanisms, and structural organization of A. muciniphila glycosidases, proteases, sulfatases, and sialidases as key enzymes involved in mucin degradation. Also, this review analyses the peculiarities of mucin utilization by A. muciniphila that not only enables this microorganism to be an active player in the gut microbiota (GM) but also contributes to the maintenance of mucus and intestinal barrier integrity. Exploring the duality of mucus degradation and mucus protection, the review further discusses the role of enzymes in the probiotic activity of A. muciniphila.

2. A. muciniphila as Probiotic

A. muciniphila is a member of the human gastrointestinal microbiota that promotes health by supporting mucosal barrier richness and integrity. The bacterium adheres to mucins in mucus layers and degrades these glycoproteins into glycan monosaccharide units, which serve as nutrient sources [20]. This has recently been demonstrated by elucidating the intricate regulatory mechanisms governing the crosstalk between A. muciniphila, the host, and the GM [21]. The adhesion of A. muciniphila to the intestinal epithelium and its interaction with the host mucosa have been previously investigated in in vitro models [22]. In addition, the ability of A. muciniphila to grow with mucin as the sole carbon source has been demonstrated in vitro using gut reactors [23]. Beyond serving as a carbon and nitrogen source for bacterial proliferation, degradation of mucins by A. muciniphila generates essential metabolic by-products, namely SCFAs, which are essential for maintaining gut homeostasis and metabolism within the host.

A. muciniphila-derived SCFAs, especially acetate and butyrate types, are known as physiological metabolites for the integrity of the intestinal barrier and maintaining overall intestinal homeostasis and immunity [24,25]. Moreover, the capacity of A. muciniphila to generate SCFAs is intricately connected to its distinctive metabolic features, as the microorganism uses mucin as an energy source and produces beneficial by-products. Lastly, several experimental studies designed to mimic the colonization process have found that mucin supplementation enhances its proliferation and metabolism in response to pH changes [5,6,26]. By being so intimately associated with the mucus layer, A. muciniphila occupies an ecological niche unparalleled at the host–microbe interface.

To produce SCFAs that improve the integrity of the epithelial layer while regulating host-microbe interactions, A. muciniphila adheres to mucins and uses enzymatic action to break them down into beneficial glycans (Figure 1).

A. muciniphila possesses a repertoire of putative bacterial MDEs and/or secreted proteins that impact host–microbe interactions. Current knowledge points to glycoside hydrolases, proteases, sulfatases, and sialidases as the principal enzymes involved in mucin degradation. Biochemical characterization of the purified enzymes from A. muciniphila has revealed the presence of 54 glycoside hydrolases and 11 sulfatases that synergistically work to break down the O-glycans present in mucin [27]. It has also been found that glycoside hydrolases and sulfatase-encoding genes are upregulated in the presence of mucin, while other putative secreted-protein genes show increased expression when mucin is absent [28]. Similarly, targeted mutagenesis studies have also pointed towards the importance of cell-attached sialidases and fucosidases in the ability of the cell to grow from mucin; their inhibition causes an inability to grow from mucin in vitro [9,27], thereby proving the importance of these enzymes in the degradation of glycosylation [29]. Furthermore, the proteome analysis has also emphasized the importance of multiple β-galactosidases with specific affinities to linkages present in mucin glycans [13,30,31]. The bacterium´s capacity to alter its metabolism in response to available substrates is demonstrated by this differential gene expression.

Gene expression analysis suggests that one digestive strategy involves the stepwise degradation of mucins into low-molecular-weight oligosaccharides, thereby facilitating the release of monosaccharides that are subsequently catabolized [32]. Mucin transcriptomic data show significant upregulation of MDE genes in A. muciniphila cultures grown with mucin, confirming substrate-induced expression changes [33]. In vitro mucin array experiments support the glycan recognition as a prerequisite for degradation by A. muciniphila, via specific binding to O-glycan structures on mucins [20]. The sequential breakdown of substances is likely the best way to get nutrients without compromising the renewal of the mucus layer [9,34]. A full-length sialidase has been characterized, and protease homologs may act on mucins and their degradation products [20,28]. All these enzymatic activities emphasize the important role of A. muciniphila in keeping mucus homeostasis and enhancing intestinal barrier integrity. Table 1 summarizes key findings from recent studies.

3. A. muciniphila as an Important Probiotic Producing MDEs

A. muciniphila is a promising new probiotic currently undergoing extensive preclinical and clinical research. It stands out from better-established probiotic candidates because it thrives in the gut mucus layer and utilizes mucin glycans as nutrients, producing mucin-degrading enzymes in its natural habitat [33]. Recent genetic mutagenesis analyses have exemplified particular loci critical to mucin degradation and stable mucinophage colonization in a laboratory setting, thereby establishing the relationship between mucin ability and its ecological fitness in the mucus habitat [9]. These results clearly illustrate the notion that mucin degradation is not a random event but rather an innate and critical ability of A. muciniphila.

Advanced molecular techniques and metabolomics have clarified how this bacterium adapts to the high-mucin environment of the intestine. A recent study of heterologous expression and biochemical analysis of a novel α-galactosidase demonstrated unexpected activity towards mucin-derived glycans, demonstrating the continued discovery of novel enzymes for mucin degradation in this species [41]. Simultaneous metabolomics and lipidomics of A. muciniphila grown under conditions of high mucin availability reflect dramatic changes in carbohydrate metabolic pathways, cell membrane lipid content, and energy-related metabolism, suggesting synchronized metabolic responses to higher mucin availability [39]. These laboratory findings collectively confirm again that mucin degradation is coupled with systemic metabolism.

Based on the available knowledge, the beneficial effects of A. muciniphila in conditions such as obesity and inflammatory bowel disease can be attributed to a combination of mucin degradation and the ability to utilize mucin-derived glycans. In vitro synthesized mucosal communities show that while A. muciniphila degrades mucin; they remain functionally compatible with other mucus-associated microorganisms, underscoring their medically important role [29,39]. Further studies using human colonoid tissue cultures show mucin-mediated gene induction related to uptake, adherence, and pili biogenesis in a human transcriptome [40]. These studies establish that degradation of mucin by A. muciniphila is a regulated process to maintain host-microbe homeostasis.

Although mucin removal from the growth medium markedly altered the transcriptome, proteome, and metabolome, it increased the capacity for carbohydrate degradation and resistance to bile acids. A. muciniphila cultured under high-mucin conditions retained significant probiotic properties [33]. Likewise, its capacity to degrade mucin does not appear to compromise its suitability as a great probiotic [39]. Thus, the MDEs of A. muciniphila are predicted to promote cycles of mucin degradation and replenishment without undermining the overall integrity of the intestinal mucus barrier.

Since it is proven that the bacterium plays a crucial role in ensuring a healthy status of the host cell, its boundary conditions, such as host fiber deprivation and extreme metabolic stress, are unknown, and future research must ensure that it recalibrates its MDE expression in scenarios where nutrients are scarce. This will be crucial for ensuring the integrity of A. muciniphila, particularly within a multi-kingdom ecosystem, where competitive ecological pressures may influence its probiotic activity.

4. Types and Categories of MDEs

MDEs are enzymes that degrade mucin glycans into oligosaccharides or other small molecules. The mucin-degrading capability is demonstrated by a variety of species [42]. In the human gut, A. muciniphila was the first identified bacterium to exhibit mucin-degrading activity [35], indicating its presence within the mucus layer. Intestinal colonization by A. muciniphila allows the bacterium to use mucins not only as a carbon source but also as a nitrogen source to fulfill other nutritional and growth requirements [23,43].

Nearly 5% of the fecal microbiota is composed of A. muciniphila [33], a finding which underscores the medical significance of this bacterium for gut homeostasis. Experimental evidence shows that this bacterium can degrade mucin, producing monosaccharides, oligosaccharides, and/or amino acids during mucin metabolism [44]. These properties indirectly influence other gut bacteria, thereby releasing degradation products of glycoproteins and ultimately controlling inflammation. The exact A. muciniphila–mucin connection remains poorly understood from a functional standpoint, necessitating further studies to fully elucidate the enzymatic activity harbored by this bacterium [27]. Notably, experiments with purified A. muciniphila proteins have demonstrated the enzymatic activity of some of these proteins over mucin [41]. In addition to determining mucin-degrading ability, it is imperative to understand the functional characteristics of the mucinases secreted by A. muciniphila.

Recent biochemical, proteomic, and structural biology studies using recombinant enzymes, targeted gene deletions, and growth on defined mucin glycans [38], and mass spectrometry-based glycan analysis have begun to reveal the exact enzymatic process involved in mucin degradation by A. muciniphila [27]. These studies have shown that mucinolysis is catalyzed by a multi-enzyme complex that acts sequentially on both the glycan and protein backbones of mucin molecules.

4.1. Endocrine vs. Exocrine Nature of MDEs

MDEs can be grouped based on their functions, location of their primary activities, release mechanisms, and biological reactions. In this regard, MDEs are broadly divided into exocrine and endocrine-like agents. This classification does not imply or suggest normal hormone release; rather, it is more closely aligned with the idea that enzymatic activity is either located on the surface of the mucus polymer or operates downstream in signaling cascades through certain biomolecules.

4.1.1. Exocrine Mucin-Degrading Enzymes (Exo-MDEs)

Exocrine mucin-degrading enzymes (Exo-MDEs) target mucin glycoproteins in the mucus layer [45]. These MDEs belong to microbial proteins that can attach to microbial surfaces and target specific bonds within mucin O- and N-glycans [20], facilitating mucin degradation into smaller oligosaccharides and ultimately into monosaccharides [18,27]. Some Exo-MDEs belong to the sialidases, fucosidases, β-galactosidases, hexosaminidases, and sulfatases known as carbohydrate-active enzymes (CAZ enzymes), which target terminal structures and inner mucin bonds [20,28]. In A. muciniphila, the key enzymes include glycoside hydrolases, sialidases (GH33), and fucosidases (GH29 and GH95) that target bacterial cell surfaces. They play vital roles in bacterial growth when mucin is used as the substrate [28].

For instance, ten glycoside hydrolases in A. muciniphila were found to remove GHs sialyl and fucosyl caps from mucin O-glycans, triggering the breakdown of mucin and allowing them to access the sugar chains beneath [28]. The activity of these enzymes is characterized by cell surface binding to mucin and by enabling nutrient sharing with other bacteria [20,28]. Another example of Exo-MDEs is the identification and biochemistry of the secreted β-galactosidases (Amuc_0771, Amuc_0824, and Amuc_1666) of A. muciniphila, which recognize specific galactosidic bonds abundant within mucin structures [31]. These enzymes specifically degrade the glycan moieties of porcine mucin, thus verifying their ability to catalyze mucin degradation outside cells [29,31].

Therefore, future studies need to abandon, once and for all, the futile effort to enumerate CAZymes and instead uncover the hierarchy, or orchestrated command chain, used in Exo-MDEs. We know, for example, that certain glycoside hydrolases, such as GH33 and GH29, have been identified; however, the spatiotemporal logic governing how these enzymes sequentially cleave caps from a complex O-glycan remains unknown. Exploring how these enzymes, bound to or secreted onto a biofilm, compete in a polymicrobial context is essential for understanding nutrient economies. And finally, determining how a host or dietary factor influences these exocrine tools is also needed. In summary, the mechanisms involved in mucin degradation are illustrated in Figure 2 below.

4.1.2. Endocrine Mucin-Degrading Enzymes (Endo-MDEs)

In contrast, endocrine-like mucin-degrading enzymes (Endo-MDEs) are not defined by direct mucin cleavage but by the systemic or regulatory effects of their metabolic products on host physiology. This means that the bacterium does not directly produce any endocrine enzymes; instead, its enzymatic activity regulates the production of these host enzymes [46]. These endocrine-like enzymes enable the bacterium to produce short-chain fatty acids (SCFAs) and other metabolites that directly modulate the host’s endocrine system, metabolic activities, and immune functions [46,47]. In fact, once mucin is degraded by exocrine enzymes, the liberated monosaccharides and SCFAs modulate host pathways, including immune responses, barrier integrity, and metabolic networks [47]. For instance, mucin degradation products can be metabolized into SCFAs, such as acetate, propionate, and butyrate, by commensal microbes [48]. These SCFAs influence host gene expression and immunomodulatory pathways via G protein-coupled receptors and histone deacetylase inhibition, affecting epithelial health and systemic metabolism [36,49,50].

Although A. muciniphila does not produce hormones classically defined as endocrine factors, both intracellular microbial enzymes and metabolic pathways generate signaling metabolites that affect host tissues beyond the mucus niche and can be conceptually classified as an endocrine-like activity [51]. For example, SCFAs produced from mucin degradation circulate and bind host receptors, contributing to mucin secretion, immune modulation, and metabolic regulation. In conclusion, Exo-MDEs play a significant role in the primary breakdown of mucin polymers in the mucus layer. At the same time, the defining feature of endocrine-like enzymes is their ability to produce metabolites that function as messengers in host-microbe crosstalk.

However, the future of the Endo-MDE framework should involve bridging the gap between microbial catabolism and systemic host signaling. Studies need to transcend descriptive taxonomies toward a mechanistic mapping of how specific enzymatic repertoires dictate metabolite fingerprints. By using systems biology to integrate multi-omic data, we can decode the synergistic effects of mucin-derived metabolic soups on host receptors. Quantitative thresholds for these interactions will enable precision therapeutics, in which engineered commensals modulate the endocrine system. The common mucin-degrading enzymes of A. muciniphila and their functions, including representative genes or proteins linked to specific roles, are summarized in Table 2 below.

4.2. MDEs in Extracellular Vesicles

Mucins serve as a first-line defense mechanism, being secreted to reinforce the gastrointestinal barrier against pathogens, toxins, and other harmful agents, thereby forming a protective mucus layer over the intestinal epithelium [42]. A. muciniphila, which is considered a next-generation probiotic candidate, belongs to the Verrucomicrobia phylum and is a mucin-degrading bacterium that can grow on mucin under laboratory conditions as a sole source of carbon and nitrogen [9]. This capacity for mucinolysis is thought to play an important role in the beneficial effects attributed to the bacterium in maintaining host metabolic and immune homeostasis, as has been extensively described in the literature [42].

Complete genomic analysis indicates that A. muciniphila possesses an enormous genome encoding MDEs, including glycoside hydrolases, proteases, sulfatases, and sialidases, which incrementally degrade mucins into peptides and ultimately amino acids that serve as nutrients for bacteria [52,53]. Recent findings suggest that bacterial mucinases may be associated with bacterial outer membrane vesicles [10,35]. This would provide a potential mechanism for delivering enzymes beyond the immediate bacterial cell surface, thereby facilitating localized mucin remodeling within the mucus layer [54].

Characterizing the possible packaging of MDEs into EVs is critical to understanding how A. muciniphila plays its ecological role in turning over mucins with a broader impact on microbiome–host interactions. Such enzymatic activities might be involved in supporting balanced cycles between mucin degradation and renewal, while promoting cross-feeding among microbes and keeping intact the integrity of the intestinal barrier system. Figure 3 above illustrates EVs’ structures in A. muciniphila.

4.3. Characteristics of MDEs

In the process of breaking down mucins, A. muciniphila releases other metabolites known as glycans, which are usually used by other microbes [29]. This process increases the efficiency of metabolic cross-feeding among different microbial pathways, while promoting the establishment of stable microbiomes [29]. Genomic studies also validate this process of functional specialization, as A. muciniphila possesses a relatively large genome with several transporters associated with mucus-related genes [52], which affirms its adaptation at the mucosal site [27]. These highlight the prime importance of MDEs for the metabolic adaptation of A. muciniphila.

A. muciniphila is also a strict anaerobe and a dominant gut symbiont in humans, and it has adopted a specialized metabolic pathway to utilize mucin components as its energy source [35]. The enzymes responsible for mucin degradation are also involved in bacterial catabolic processes, acting sequentially by first cleaving O-glycans, then saccharides, and finally peptidic bonds to obtain monosaccharides, oligosaccharides, and amino acids as energy sources for the bacterium [27]. This degradation requires several sequential steps and encompasses a coordinated microbial strategy for the efficient breakdown of mucin components [27]. Mucin comprises a large sugar-protein complex produced by specialized epithelial cells known as goblet cells, forming a protective barrier between the intestinal epithelium and its lumen [32].

Finally, it has been found that the structure of mucins has a protein backbone heavily modified with O-linked carbohydrate chain [55]. These chains can be an important source of carbon and energy for specific bacteria that live in the gut [35]. A. muciniphila has genes for many distinct types of enzymes that can degrade mucin [56]. These include glycoside hydrolases, sulfatases, sialidases, proteases, and polysaccharide lyases [18]. Together, these enzymes enable the step-by-step breakdown of complex structures found in mucins [57].

5. Mechanisms of MDE Action

The ability of A. muciniphila to colonize the mucosal layer is based on a repertoire of mucin-degrading enzymes, which can be divided into five functional groups: glycoside hydrolases (GHs), proteases, sialidases, sulfatases, and acetylesterases [18,35]. An in-depth critical analysis of these enzymes reveals a highly coordinated, sequential mechanism of action that is critical for nutrient acquisition and gut barrier modulation [2,58]. The basic architecture of mucin, consisting of a protein backbone with a high degree of O-glycosylation and complex, often branched glycan structures, underscores the need for such a multi-enzymatic degradation process [55].

5.1. Glycoside Hydrolases

Members of this category are referred to as backbone degraders. They are the primary enzymes that cleave these complex O-glycan chains, which account for most of the mucin molecules. Rather than being random in their activity, the glycoside hydrolases (GHs) of A. muciniphila have exquisite specificity. For example, the GH33 family of enzymes comprises sialidases, as discussed above. The GH20 and GH84 enzyme families are involved in the cleavage of core chains. Recent studies have shown that some of the GHs of A. muciniphila can cleave the core 3 GlcNAcβ1-3GalNAc and the core 6 GlcNAcβ1-6GalNAc O-glycans, primarily found in intestinal mucins such as MUC2 [13]. Their mode of action also includes an endo-acting mechanism that cleaves glycosidic bonds within the glycan chains, resulting in the release of smaller oligosaccharides. This is different from the mode of action of exo-glycosidases, where they nibble the glycan chains from the non-reducing ends. This dual GHs activity appears to be a disassembly line, in which the endo-cleavage activity creates a variety of ends for the exo-glycosidases to act on, thus maximizing energy extraction from the glycan source [2].

5.2. Sialidases

Sialic acids, especially the O-acetylated types, which are present in large quantities, can also form the terminal ends of mucin glycan structures. They can serve as a physical barrier against bacterial breach, while also providing immune evasion mechanisms through molecular mimicry. However, A. muciniphila uses exo-acting sialidases, which predominantly belong to the GH33 family. These enzymes can specifically degrade terminal sialic acids by unmasking the glycans. The most important aspect of these enzymes is that they are not all the same. Some of them can degrade α2-3-linked sialic acids, while others can degrade α2-6-linked sialic acids. This is because the sialylation of the mucin glycan structures can vary [2]. By removing the sialic acids, these enzymes can perform a gatekeeping role. The underlying glycan structures can then be made accessible to the action of other GHs. The bacterium can also metabolize sialic acids to obtain carbon, nitrogen, and energy.

5.3. Sulfatases

Members of this class have additional sulfate groups attached to the glycan moiety. Typical representatives include GalNAc and GlcNAc, both of which confer a negatively charged group that greatly enhances mucin viscosity and resistance to microbial degradation. They act by neutralizing negatively charged ionic groups through a powerful catalytic mechanism. A. muciniphila harbors sulfatases that hydrolyze these sulfate groups. The mechanism of action of this microbe is through an "exo-sulfatase" mechanism, which removes the sulfate groups from the non-reducing end of the glycan moiety [35,59]. This removal of negatively charged groups is not just an action but a prerequisite. Many GHs cannot accommodate sulfated groups in their active sites. Thus, sulfatases are an essential mechanism for detoxifying or neutralizing the glycan moiety, which can then be acted upon by GHs. This is an important aspect of the hierarchical degradation of mucin.

5.4. Proteases

Although GHs are responsible for glycan degradation, proteases are the primary enzymes that break down the protein backbone of MUC2, which serves as a scaffold, attaching these glycan bristles. A. muciniphila utilizes several types of proteases with different specificity profiles. In fact, it is the first protease to cleave the N-terminal domain of MUC2, which is required for disulfide bond-mediated polymerization [37]. This initial cleavage event is crucial for MUC2 degradation, as it enables the breakdown of large net-like MUC2 multimers into smaller soluble monomers. Other proteases, most likely serine or cysteine proteases, subsequently degrade MUC2 monomers into even smaller peptides and eventually, amino acids. These specific proteases cleave after aromatic amino acids (Phe and Tyr) or polar amino acids [18], thereby fully degrading the MUC2 protein backbone down into small peptides and free amino acids. Thus, members of this group act by disassembling the scaffolds from the glycans in an elegant two-step process.

5.5. Acetylesterases

The final class of enzymes is the acetylesterases. They act on the O-acetyl groups of sialic acid, which are referred to as O-acetylesterases, as well as other carbohydrates such as GalNAc. The mechanism of action of this enzyme class is the hydrolysis of the bond that holds the acetyl group to the carbohydrate, thereby fine-tuning the substrate. The biological importance of this enzyme class is two-fold. Firstly, it is similar to sulfation. It acts as a GH inhibitor. The deacetylation of GH by this enzyme class is a requirement for GH to degrade carbohydrate chains [18,35].

The presence of the five enzyme classes is, however, not coincidental and represents a complex, multi-step approach (Figure 4). This begins with the action of preparatory enzymes, whereby sulfatases and acetylesterases remove charged and bulky groups, and sialidases cleave the terminal caps. This, in turn, allows glycoside hydrolases to cleave the exposed glycan chains. At the same time or subsequently, the proteolytic enzymes depolymerize and degrade the newly exposed or degraded protein scaffold. This multi-step synergistic approach is directed toward the “promiscuous” substrates of MUC2 and MUC5AC [34,57]. This would enable A. muciniphila to completely degrade the mucin polymer and access the full range of carbon, nitrogen, and energy sources it contains.

6. Balancing between the Advantages and Disadvantages of A. muciniphila as a Probiotic

A. muciniphila is epitomized as a promising probiotic. Mucins, which function as protective barriers for epithelial cells and are critical for host–microbe crosstalk, are one of its few carbon sources [21]. Biochemical investigations have shown that this gut commensal secretes mucin-degrading enzymes that could affect host microbiota and health [55,60]. These mucolytic capabilities are assumed to enhance the commensal’s own proliferation, owing to the glycan content. Still, they may also produce mucin oligosaccharides that reach monomeric forms, thereby influencing epithelial cell communication [55,60]. Since mucins are also critical for gut health, elucidating the trade-off between mucin degradation and subsequent beneficial or detrimental effects to the host would be relevant [21,37]. Recent reviews and mechanistic investigations highlight their positive role in metabolic disorders, mucosal barrier maintenance, and immune modulation. Below is a synthesis of key findings, with references to the underlying literature.

Studies in humans and animals have shown that supplementation with a live or pasteurized strain of A. muciniphila improves insulin sensitivity, lowers fasting glucose levels, and reduces metabolic indicators of inflammation and obesity [61,62]. Research has also shown that when administered at appropriate doses, A. muciniphila enhances tight-junction protein expression, increases mucus thickness, and decreases the synthesis of pro-inflammatory cytokines, such as tumor necrosis factor alpha and interleukin 6 (IL-6 in murine colitis and “leaky gut” models [63].

It is now known that GM dysbiosis is implicated in the development of systemic lupus erythematosus (SLE) in humans [64,65]. In this regard, a recent study demonstrated that administration of A. muciniphila decreased inflammatory markers in mouse models of systemic lupus erythematosus (SLE), specifically reducing levels of IL-6 and IL-17 and increasing levels of IL-10, an anti-inflammatory cytokine [66]. In addition, in mice models of immunosuppression, catabolic syndrome, and persistent inflammation, A. muciniphila modified the GM. Beyond increasing levels of Muribaculaceae, Lachnospiraceae, Lachnoclostridium, and Parabacteroides goldsteinni, both live and pasteurized A. muciniphila reduced the abundance of bacteria associated with illness and inflammation [67]. Another investigation found that this bacterium reduces intestinal inflammation by stimulating the generation of regulatory T-cell populations [68]. In certain preclinical studies, pasteurized A. muciniphila has been shown to enhance response to anti-PD1 cancer immunotherapy, indicating immunomodulatory potential [69,70]. In another study, supplementation with A. muciniphila reduced depressive-like behavior in an animal model by modulating gut-brain axis signaling and serotonin pathways [71,72]. A growing body of evidence suggests that A. muciniphila could alleviate symptoms in some neurological diseases, such as Alzheimer´s disease [73], epilepsy [74], Parkinson’s disease [75,76], and autism spectrum disorder [77].

Although A. muciniphila is widely regarded as a beneficial mucin-degrading commensal, accumulating evidence indicates that its mucinolytic activity can become detrimental under specific ecological and dietary conditions [16]. When mucus degradation becomes excessive or dysregulated, it can erode the protective mucus layer, leaving intestinal cells vulnerable to harmful substances such as bacterial toxins, inflammatory compounds, and pathogens, predisposing the host to colitis and infection. The study revealed that a low-fiber diet increased A. muciniphila levels, reduced the thickness of the inner mucus layer, and rendered animals more susceptible to Citrobacter rodentium, a pathogen that mimics human Escherichia coli infection [78]. This paradox reflects a fundamental concept: mucin degradation is beneficial only when tightly balanced with mucus renewal and microbial ecosystem stability (Figure 5).

For example, in murine models, antibiotic administration followed by oral supplementation with A. muciniphila resulted in an “over-colonized” state, leading to a significant reduction in mucus levels and increased inflammatory responses [11]. Other research revealed that when animals were fed a low-fiber diet, A. muciniphila levels were elevated and correlated with a diminished mucus layer, increased allergic sensitivity to food allergens, and increased colon inflammation [79]. An additional investigation found that A. muciniphila specifically increased host susceptibility to C. rodentium infection during fiber deprivation, as its removal from the synthetic community was sufficient to prevent a serious infection. Importantly, when the mice were put back on a fiber-rich diet, the presence of A. muciniphila lowered the pathogen burden, demonstrating the mucin specialist´s context-dependent benefits [80].

In addition to fiber, research has shown that glucose concentration influences Akkermansia levels in vivo. According to a recent study, mice given drinking water with 10% glucose or fructose showed higher levels of A. muciniphila than animals given vehicle control. Analysis of the colonic mucus layer showed that, compared to control animals, gut bacteria were located closer to the epithelial layer, and the inner mucin layer was thinner in glucose-treated mice. When these mice were administered dextran sulfate sodium, they not only had a reduced mucus layer but also developed severe colitis [81].

Changes in Akkermansia levels have also been observed in the context of viral infections. Notably, in neonatal mouse pups, rotavirus infection led to an early increase in Akkermansia abundance, a phenomenon associated with a reduction in mucus-filled Goblet cells [82]. As discussed in the following reviews, rotaviruses bind to intestinal enterocytes through certain glycans [83]. In silico analysis has demonstrated that the VP8* domain of rotavirus binds mucin glycans containing sialic acid and galactose residues [82]. Interestingly, the addition of intestinal mucin was found to inhibit rotavirus replication [82,84]. Furthermore, it was revealed that mucus degraded by A. muciniphila had reduced capacity to suppress rotavirus infection, indicating that mucin may act as a decoy receptor that interferes with rotavirus cell invasion [82].

Akkermansia levels are similarly elevated in COVID-19 patients [85], and oral administration with live A. muciniphila to mice infected with the H7N9 influenza virus attenuated lung damage but did not affect survival rates or weight loss [86]. In addition, Akkermansia levels are elevated in coronavirus infections, such as COVID-19; however, whether this increase is ultimately beneficial or detrimental to the host remains to be established [87]. Interestingly, coronaviruses are known to attach to sialic acids, and one study demonstrated they can bind to MUC2 from porcine intestinal organoids. The infectivity of transmissible gastroenteritis coronavirus and porcine epidemic diarrhea virus was reduced by the addition of organoid mucus [88]. Therefore, it is possible that excessive mucin degradation by A. muciniphila could render the intestinal barrier more permeable to pathogen passage.

7. Conclusions

This narrative review positions the bacterium A. muciniphila as a functional specialist colonizing the gut niche, and the therapeutic potential of such symbionts is critically dependent on the tightly regulated degradation of the host glycan layer, termed mucin. While neither inherently beneficial nor detrimental, the probiotic properties of Akkermansia have been attributed to enzyme-mediated interactions with the intestinal mucus layer, resulting in a homeostatically balanced mucin degradation. In fact, this tightly regulated degradation may limit probiotic benefit, potentially even inducing adverse effects in the host. In recent years, the consumption of different probiotic strains has been widely promoted to improve overall health. However, administering the bacterium alone is not sufficient; if adequate amounts of fiber are not consumed, A. muciniphila excessively degrades the mucin layer to survive. The intestinal barrier weakens, and as a result, allows the passage of toxic substances and pathogens, contributing to the development of various diseases.

Therefore, from a translational perspective, therapeutic manipulation of these delicate host-microbe relationships through Akkermansia-based probiotics, prebiotics, postbiotics, and MDEs may hold considerable promise in developing proactive interventions targeting metabolic and inflammatory disorders. Specifically, in delineating context-specific markers, titers, and culture conditions that maintain a tightly regulated degradation of the host mucus layer for disease management and therapeutic interventions involving this bacterium as a probiotic or postbiotic.