In fact, the first paper indicating that MDD is associated with alterations in the gut microbiota–brain axis was published in 2008, stating that many, but not all, MDD patients exhibit increased translocation of Gram-negative enterobacteria or their LPS as assessed with increased serum levels of IgM/IgA to the LPS of Morganella morganii, Hafnia alvei, Citrobacter Koseri, Pseudomonas Putida, Pseudomonas Aeruginosa, and Klebsiella Pneumoniae [13]. This evidence concerns the gene CD40LG and major depressive disorder.