Phase Variable Surface Polysaccharides Modulate Akkermansia muciniphila Intestinal Mucus Colonization and Host Immune Responses

Abstract

Akkermansia muciniphila is a mucin-degrading bacterium commonly found in the human gastrointestinal tract. The prevalence and abundance of this species correlate with improved metabolic and immune health, generating substantial interest in its therapeutic potential. Although A. muciniphila is not a dominant member of the gut microbiota, the bacterial factors that enable mucus layer colonization and interaction with the host remain incompletely understood despite. I hypothesize that capsular polysaccharides govern A. muciniphila colonization strategies and host interactions, potentially explaining how this bacterium establishes itself as a stable, health-associated member of the gut microbiota.To establish a foundation for investigating capsule function, I first examined capsule biosynthesis genes across the Akkermansia genus. Comparative genomic analysis of 58 genomes spanning six species revealed that capsular polysaccharide loci are universally conserved, with all species encoding a Wzy-dependent production pathway. Phylogenetic analysis identified species-specific and strain-level diversity in these sequences, suggesting the capacity to produce structurally distinct capsules, and transmission electron microscopy confirmed capsule production in representative strains. Given this universal conservation, I next investigated how capsule expression is regulated. Transcriptional analysis demonstrated that capsule biosynthesis genes are upregulated during growth in mucin, linking capsule production to the bacterium's primary nutrient source. Superimposed on this environmental regulation, I identified phase variation mediated by a conserved invertible DNA element. Both regulatory orientations coexist in laboratory cultures, colonized mice, and human fecal samples, with capsule ON cells predominating in our tested populations. Having established that capsule is both conserved and dynamically regulated, I sought to determine its functional significance during intestinal colonization. Using genetically defined capsule-deficient mutants in specific-pathogen-free (SPF) mice I found that capsule is critical for spatial organization but dispensable for overall colonization efficiency. Capsule-producing bacteria associate with the inner mucus layer near the epithelial surface, while capsule-deficient bacteria are excluded to the outer mucus and lumen. This spatial phenotype correlates with differential antimicrobial susceptibility with capsule-deficient bacteria show heightened sensitivity to LL-37, indicating that capsule enables survival in the antimicrobial rich inner mucus environment. Additionally, capsule-deficient bacteria form large multicellular aggregates in vivo and produce robust biofilms in vitro, establishing that capsule suppresses biofilm formation and maintains a dispersed cellular state. These divergent phenotypes raised the question of whether capsule state also determines host immune recognition. I found that mice colonized with capsule-deficient A. muciniphila exhibit elevated systemic IgG responses and increased mucosal IgA output, accompanied by expansion of plasma cells in the intestinal lamina propria. These findings support a model in which capsule masks immunogenic surface epitopes while also protecting against antimicrobial-mediated cell death. Overall, my findings establish capsular polysaccharides as master regulators of A. muciniphila ecology, coordinating spatial positioning, community organization, and immune visibility through phase-variable expression. The data support a two-state colonization model enabling strategies that may optimize survival in the dynamic gut environment, with implications for rational design of probiotic therapeutics.