Microbes and diet reshape the intestine via distinct cellular dynamics

BackgroundThe intestine adapts to environmental stimuli by dynamically altering its size. Infections causes shrinkage and subsequent regrowth to original intestinal dimensions, suggesting homeostasis, whereas diet can induce adaptive changes. Whether diet and infection differ in the kinetics, magnitude, or cellular mechanisms that drive intestinal resizing, and whether intestines use a fixed size "memory" versus an adaption to nutrient availability, still remains unclear. ObjectiveTo determine whether intestinal regrowth after infection reflects a fixed "memory" of a target size (homeostasis) or an adaptation to the nutritional state during regrowth, and to identify the cellular modalities that drive infection- and diet-induced resizing. DesignUsing Drosophila melanogaster as a model, we quantified intestinal size, cell size, cell number, and epithelial turnover in response to dietary shifts and oral bacterial infections, both individually and in combination. ResultsInfection-induced atrophy occurred only when the intestine was initially large, and regrowth required nutrient availability. Distinct cellular mechanisms underpinned changes in intestinal size: diet-induced growth in unchallenged intestines was mediated by enterocyte hypertrophy, whereas post-infection regrowth required intestinal stem cell (ISC) proliferation. Consistently, ISC ablation impaired regrowth only after infection. Bacteria functioned both as pathogens and nutrients, triggering intestinal shrinkage when damaging a large organ, but promoting growth when consumed by nutrient-deprived flies. ConclusionIntestinal regrowth after infection reflects a context-dependent, diet-driven adaptation, not a fixed intestinal size memory. Diet and infection engage distinct, stimulus-specific cellular programs to reshape the midgut, revealing a modular logic of organ plasticity with implications for tissue repair and metabolic regulation. Summary boxO_ST_ABSWhat is already known on this topicC_ST_ABS- Intestinal size is regulated by epithelial turnover and responds dynamically to dietary and microbial stimuli. What this study adds- Post-infection intestinal regrowth is governed by dietary input rather than an intrinsic memory of pre-infection intestinal size. - Intestinal resizing is driven by distinct cellular programs: diet-induced intestinal growth occurs independently of stem cells, while post-infection intestinal regrowth requires stem cell proliferation. - Diet, rather than microbes, is the primary driver of intestinal size, while pathogens influence the mode of size change by increasing cellular turnover. - Pathogenic microbes act as both sources of damage and nutrients, influencing organ resizing. How this study might affect research, practice, or policy- Reframes intestinal regeneration as a flexible, context-dependent adaptation rather than a fixed homeostatic process. - Identifies nutritional status as a critical determinant of recovery following intestinal injury. - Offers a conceptual framework for understanding epithelial plasticity in health and disease, emphasizing how the balance of cell size and stem cell activity coordinates adult intestinal size.