The chapter discusses the microbial degradation of complex carbohydrates in the human gut, highlighting the significant role of gut microbiota in breaking down dietary and host-derived glycans. Key points include: 1. **Microbial Enzymes and Metabolism**: Bacteria in the human gut possess a diverse array of degradative enzymes and metabolic capabilities, enabling them to utilize complex non-digestible dietary carbohydrates and host-derived glycans. Certain dominant species, particularly Bacteroidetes, have a large number of genes encoding carbohydrate-active enzymes and can switch between different energy sources. 2. **Plant Cell Wall Degradation**: The degradation of plant cell walls, a major source of dietary carbohydrates, is facilitated by specific bacteria. These bacteria, such as *Ruminococcus flavefaciens* and *Prevotella bryantii*, have evolved complex enzyme systems, including cellulosomes, to degrade cellulose and hemicellulose. 3. **Dietary Carbohydrates and Health**: The impact of dietary carbohydrates, including prebiotics, on human health is influenced by the complex relationship between diet composition, gut microbiota, and metabolic outputs. Prebiotics, such as fructans and galacto-oligosaccharides, can enhance the growth of beneficial bacteria like *Bifidobacterium* spp. 4. **Human Colonic Bacteroides**: *Bacteroides* species, particularly *B. thetaiotaomicron*, have evolved sophisticated systems for starch utilization, including the Sus system, which efficiently captures, sequesters, and degrades starch. These systems are regulated by transcriptional regulators and involve multiple protein interactions. 5. **Actinobacteria and Firmicutes**: *Bifidobacterium* spp and *Lachnospiraceae* and *Ruminococcaceae* bacteria play crucial roles in polysaccharide degradation. *Bifidobacterium* spp are effective degraders of high amylose starches and can utilize host-derived carbohydrates like human milk oligosaccharides (HMOs). 6. **Regulation and Horizontal Gene Transfer**: The regulation of polysaccharide utilization is complex, involving hybrid two-component systems and ECF sigma factors. Horizontal gene transfer (HGT) has played a significant role in the diversification of degrading activities among gut bacteria. Overall, the chapter emphasizes the importance of understanding the interactions between diet, gut microbiota, and health, particularly in the context of complex carbohydrate degradation.The chapter discusses the microbial degradation of complex carbohydrates in the human gut, highlighting the significant role of gut microbiota in breaking down dietary and host-derived glycans. Key points include: 1. **Microbial Enzymes and Metabolism**: Bacteria in the human gut possess a diverse array of degradative enzymes and metabolic capabilities, enabling them to utilize complex non-digestible dietary carbohydrates and host-derived glycans. Certain dominant species, particularly Bacteroidetes, have a large number of genes encoding carbohydrate-active enzymes and can switch between different energy sources. 2. **Plant Cell Wall Degradation**: The degradation of plant cell walls, a major source of dietary carbohydrates, is facilitated by specific bacteria. These bacteria, such as *Ruminococcus flavefaciens* and *Prevotella bryantii*, have evolved complex enzyme systems, including cellulosomes, to degrade cellulose and hemicellulose. 3. **Dietary Carbohydrates and Health**: The impact of dietary carbohydrates, including prebiotics, on human health is influenced by the complex relationship between diet composition, gut microbiota, and metabolic outputs. Prebiotics, such as fructans and galacto-oligosaccharides, can enhance the growth of beneficial bacteria like *Bifidobacterium* spp. 4. **Human Colonic Bacteroides**: *Bacteroides* species, particularly *B. thetaiotaomicron*, have evolved sophisticated systems for starch utilization, including the Sus system, which efficiently captures, sequesters, and degrades starch. These systems are regulated by transcriptional regulators and involve multiple protein interactions. 5. **Actinobacteria and Firmicutes**: *Bifidobacterium* spp and *Lachnospiraceae* and *Ruminococcaceae* bacteria play crucial roles in polysaccharide degradation. *Bifidobacterium* spp are effective degraders of high amylose starches and can utilize host-derived carbohydrates like human milk oligosaccharides (HMOs). 6. **Regulation and Horizontal Gene Transfer**: The regulation of polysaccharide utilization is complex, involving hybrid two-component systems and ECF sigma factors. Horizontal gene transfer (HGT) has played a significant role in the diversification of degrading activities among gut bacteria. Overall, the chapter emphasizes the importance of understanding the interactions between diet, gut microbiota, and health, particularly in the context of complex carbohydrate degradation.