Box 3 Ecological role of the Christensenellaceae in the human gut

Based on Morotomi’s observations, C. minuta ferments glucose to acetate and butyrate under anaerobic conditions [7], which indicates it ferments sugars in the gut to short chain fatty acids and other fermentation products such as H₂ and CO₂. Goodrich et al. reported that the Christensenellaceae form the hub of a co-occurrence network with other microbiota, including methanogens (archaea of the family Methanobacteriaceae) [21]. Co-occurrence of Christensenellaceae and Methanobacteriaceae across individuals has been reported elsewhere [22, 23]. The Methanobacteriaceae include Methanobrevibacter smithii, the predominant methanogen in the human gut. Given that M. smithii uses fermentation products (e.g., H₂ and CO₂) to produce methane, the co-occurrence with Christensenellaceae may represent a H₂-based syntrophy.