Relationships between indigenous bacteria and host energy homeostasis, longevity and population structure. Indigenous bacteria and their products—as well as pathogens, albeit for shorter periods—are involved in energy homeostasis, through intermediary host metabolic processes, which are modelled here with insulin-like growth factor-1 (IGF-1) and insulin. In a closed system, host population number and density affect microbial transmission and concentrations, and vice versa, through endocrine effects on fertility. Diminished levels of particular interactive bacteria, which can occur, for example, after antibiotic treatment, can lead to increased energy storage, leading to fat accumulation and accelerated ageing.

Hypothetical relationship of Helicobacter pylori with human health and disease. There is now considerable evidence that H. pylori has colonized the stomach of humans for more than 100,000 years. Its persistence and ubiquity before modern times suggest selection for its presence, possibly based on enhanced protection against diarrhoeal diseases and improved energy homeostasis (for example, leptin/ghrelin-mediated; in parallel with Fig 1). H. pylori colonization of the stomach induces inflammation, and affects gastric hormones and digestive physiology. In turn, the interaction of particular constellations of H. pylori strains with certain hosts affects the risk of specific diseases. Epidemiological and pathophysiological data indicate that the presence of H. pylori increases lifetime risk for peptic ulcer disease (PUD) and gastric cancer (GC), but protects against gastroesophageal reflux disease (GERD) and its consequences, including oesophageal adenocarcinoma. In response to changing hygiene demographics and antibiotic usage, H. pylori is disappearing, and reduced incidences of PUD and GC, along with increasing incidences of GERD, have been observed (Peek & Blaser, 2002).