Recent research indicates that the development of psychological disorders such as depression is linked to an increase in inflammation and activation of pro-inflammatory cytokines such as interleukin-6 (IL-6), IL-1β, and tumour necrosis factor α (TNF-α), which indirectly affect the composition of the intestinal microbiome (Lotrich 2015; Maeng and Hong 2019). Here, IL6 is linked to major depressive disorder.