Normalization of Short-chain Fatty Acid Concentration by Bacterial Count of Stool Samples Improves Discrimination Between Eubiotic and Dysbiotic Gut Microbiota Caused by Infection

Short-chain fatty acids (SCFAs) represent a cornerstone of gut health, serving as critical mediators of immune modulation and overall host homeostasis. Patients with dysbiosis caused by infection (CDI) typically exhibit lower SCFAs levels compared to healthy stool donors and, thus, the concentration of SCFAs has been proposed as a proxy marker of a healthy microbiota. However, there is no consistency in the methods used to quantify SCFAs in stool samples and usually, the results are normalized by the weight of the stool samples, which does not address differences in water and fiber content and ignores bacterial counts in the sample (the main component of stool that contributes to the composition of these metabolites in the sample). Here, we show that normalized SCFAs concentrations by the bacterial count improve discrimination between healthy and dysbiotic samples (patients with CDI), particularly when using acetate and propionate levels. After normalization, butyrate is the metabolite that best discriminates eubiotic and dysbiotic samples according to the area under the receiver operating characteristic (ROC) curve (AUC-ROC = 0.860, [95% CI: 0.786-0.934],  < .0001).