Figure 1 Host factors influencing gut bacterial growth Figure 1 | Host factors influencing gut bacterial growth. a | Typical growth dynamics of a large versus small bacterial population illustrate the different durations of the exponential growth phase (Exp). b | Key host-related factors influencing the balance between stimulation and inhibition of the bacterial cell number in the gut. The role of the immune system is not shown, but it contributes by stabilizing the autochthonic community and neutralizing pathogenic invaders. c | Presence of chemical and digestive factors and the transit time along the gastrointestinal tract might underlie the increasing rostro–caudal gradient of bacterial content225. In the upper gut, the transit time is, apparently, shorter than the time necessary for the bacterial population to reach the stationary growth phase (Stat), as calculated using the formula: t (min) = G (generation time, 20 min, assumed based on in vitro experiments and in vivo infusions) × 3.3 log (minimal bacterial number in the Stat phase, that is, 109) / bacterial number before multiplication (for example, 103 in the duodenum). Figure 1a modified from Cell Metab. 23 (2), Breton, J.et al. Gut commensal E. coli proteins activate host satiety pathways following nutrient-induced bacterial growth. 324–334 © (2016), with permission from Elsevier. Figure 1a modified from Cell Metab. 23 (2), Breton, J. et al. Gut commensal E. coli proteins activate host satiety pathways following nutrient-induced bacterial growth. 324–334 © (2016), with permission from Elsevier. Fetissov, S. O. (2016) Role of the gut microbiota in host appetite control: bacterial growth to animal feeding behaviour Nat. Rev. Endocrinol. doi:10.1038/nrendo.2016.150