Germ-free (GF) mice are deficient of any microbes and raised in a sterile envirnment. To analyse the stress response of GF mice, researcher from Ireland performed a stress test called "novel-environment stress". Therefore, they simply transfered individual mice into a novel cage for 30 mins and collected the plasma to check for corticosterone (mouse equivalent to cortisol). They found two interesting things:
First, GF mice showed an almost double as high stres response than normal mice.
Second, increased baseline stress in GF mice, meaning the stress levels without any stimulation were already increased in GF mice!
- Clarke et al., The microbiome-gut-brain axis during early life regulates the hippocampal serotonergic system in a sex-dependent manner, Molecular Psychiatry, 2013