Not all stress is equal: Examination stress and cortisol reactivity are associated with exploratory species-level variation but not broad restructuring of the gut microbiome.

Background: Psychological stress has been implicated in gut microbiome variation, but human evidence remains inconsistent and often does not distinguish exposure to stressful contexts from individual differences in physiological stress reactivity. This repeated-measures study examined whether naturalistic examination stress and post-awakening cortisol reactivity were associated with gut microbiome diversity, community composition, and species-level differential abundance. Methods: Thirty-nine healthy university students were assessed during examination and non-examination periods. At each assessment, participants completed validated measures of perceived stress, anxiety, affect, self-esteem, sleep, diet, and physical activity; provided salivary cortisol samples at awakening, 30 min, and 45 min post-awakening; and collected a stool sample for whole-metagenome shotgun sequencing. Post-awakening cortisol reactivity was indexed using area under the curve with respect to increase. Results: Examination stress was associated with increased perceived stress, state and trait anxiety, negative affect, poorer subjective sleep, and amplified post-awakening cortisol reactivity. Alpha and beta diversity of the gut microbiome did not differ between examination and non-examination periods, and within-subject changes in community composition were not associated with changes in cortisol reactivity. However, exploratory species-level analyses identified nominal selective microbial differences. Examination stress was associated with higher relative abundance of Conclusions: Short-term examination stress was associated with marked psychological and endocrine responses but was not accompanied by broad restructuring of the adult gut microbiome. Instead, exploratory findings suggest that examination stress and cortisol reactivity may be reflected in selective species-level microbial variation, supporting further longitudinal and functional investigation of stress-related microbiome dynamics.