“…However, it must also be noted that the microbialneuroendocrine relationship is bi-directional as stress can change the composition of the gut microbiota. This is true of early-life stress (Bailey and Coe, 1999;O'Mahony et al, 2009) prenatal stress (Golubeva et al, 2015a;Jasarevic et al, 2015;Zijlmans et al, 2015) and psychological stress (Bailey et al, 2011;Bharwani et al, 2016;Galley et al, 2014;Reber et al, 2016) As outlined above and elsewhere in this issue, glucocorticoids modulate the expression of TDO (O'Farrell and Harkin, 2015;O'Mahony et al, 2015). As such, TDO activity may at least partly be contingent on a microbial-neuroendocrine interplay with significant implications for brain function and behaviour.…”