José Manuel Fernández-Real
Head of the Nutrition, Eumetabolism and Health Group at IDIBGI and CIBEROBN, Professor of Medicine at the University of Girona and Head of the Endocrinology Section at the Dr. Josep Trueta Hospital
Both studies analyse relatively large populations of subjects, and this is one of their main strengths. One of them has a multi-ethnic population, which helps to analyse the consistency of observations in different populations.
The main limitation of these two studies is that they only characterise the bacterial ecosystem by analysing 16S (a very small portion of the bacterial genome that is highly variable and serves to attribute taxonomy) instead of performing shotgun, which sequences all bacterial DNA. In this way they can only describe families or, at most, genera of bacteria associated with depressive symptoms. Not bacterial species.
It is becoming increasingly important to know the functionality of bacteria, rather than the name of the bacteria. Different bacteria, whatever they are called, may share a functionality and that is what matters when inferring their metabolism. The analysis of this functionality is missing.