Further, it is also known that patients suffering from depression may have abundance of Streptococcus, Rothia, Veillonella, Erysipelothrix, Clostridium, and Actinomyces species in their gut microbiota (Amirkhanzadeh Barandouzi et al., 2020) and it is established that an increased growth of Streptococcus species would raise the levels of IL-6 and TNF-α in humans (Jiang et al., 2015). Here, TNF is linked to major depressive disorder.