Optimized Sample Handling Strategy for Metabolic Profiling of Human Feces

Fecal metabolites are being increasingly studied to unravel the host-gut microbial metabolic interactions. However, there are currently no guidelines for fecal sample collection and storage based on a systematic evaluation of the effect of time, storage temperature, storage duration, and sampling strategy. Here we derive an optimized protocol for fecal sample handling with the aim of maximizing metabolic stability and minimizing sample degradation. Samples obtained from five healthy individuals were analyzed to assess topographical homogeneity of feces and to evaluate storage duration-, temperature-, and freeze-thaw cycle-induced metabolic changes in crude stool and fecal water using a (1)H NMR spectroscopy-based metabolic profiling approach. Interindividual variation was much greater than that attributable to storage conditions. Individual stool samples were found to be heterogeneous and spot sampling resulted in a high degree of metabolic variation. Crude fecal samples were remarkably unstable over time and exhibited distinct metabolic profiles at different storage temperatures. Microbial fermentation was the dominant driver in time-related changes observed in fecal samples stored at room temperature and this fermentative process was reduced when stored at 4 °C. Crude fecal samples frozen at -20 °C manifested elevated amino acids and nicotinate and depleted short chain fatty acids compared to crude fecal control samples. The relative concentrations of branched-chain and aromatic amino acids significantly increased in the freeze-thawed crude fecal samples, suggesting a release of microbial intracellular contents. The metabolic profiles of fecal water samples were more stable compared to crude samples. Our recommendation is that intact fecal samples should be collected, kept at 4 °C or on ice during transportation, and extracted ideally within 1 h of collection, or a maximum of 24 h. Fecal water samples should be extracted from a representative amount (∼15 g) of homogenized stool sample, aliquoted, and stored at <-20 °C, avoiding further freeze-thaw cycles.