Background: Firefighting is a high-stress occupation associated with an elevated risk of post-traumatic stress disorder (PTSD), with reported prevalence rates ranging from 19% to 57%. Emerging evidence highlights the gut microbiome as a key regulator of mental health via the gut-brain axis. This complex microbial ecosystem plays a critical role in both physical and psychological well-being and may be disrupted by traumatic stress, resulting in gut dysbiosis—an imbalance in the normal composition of the gut microbiota. Despite growing interest in the microbiome–mental health connection, few studies have examined the association between PTSD symptom severity and gut dysbiosis in high-risk populations such as firefighters. Given their repeated exposure to trauma and heightened vulnerability to PTSD, investigating this relationship is essential to inform microbiome-targeted preventive and therapeutic strategies. This study aimed to investigate the association between stress, PTSD symptoms, and gut microbiome composition in firefighters.

Methods: We enrolled 102 healthy male participants aged 21–50 years, including 52 firefighters and 50 nonfirefighters (control group) from South Florida and East Tennessee. This subgroup was derived from the parent Firefighters Health Promotion Study, which initially recruited 102 American and 101 South Korean participants. To minimize potential confounding factors, individuals with specific medical conditions, a current smoker, or recent probiotic use were excluded. Gut bacterial profiles were analyzed using 16S rRNA gene sequencing. PTSD symptoms were assessed using the PTSD Checklist–Civilian Version (PCL-C), with a cutoff score of >30, a commonly used threshold for PTSD screening in primary care settings, to compare gut microbiome composition between individuals with and without elevated PTSD symptoms. Perceived stress was measured using the Perceived Stress Scale (PSS) to evaluate participants’ subjective appraisal of stress in their daily lives. The study protocol was approved by the Institutional Review Boards (IRBs) of the University of South Florida and the University of Tennessee, Knoxville.

Results: Firefighters reported significantly higher PTSD symptoms (p = 0.022) and perceived stress (p=0.024) than the control group. A strong positive correlation was observed between perceived stress and PTSD symptoms (r = 0.698, p < 0.001), indicating that individuals reporting higher stress also reported more traumarelated symptoms. The gut microbial alpha diversity (ASV levels measured by the Shannon index) did not significantly differ between firefighters and the control group (p = 0.05). However, at the genus level, alpha diversity showed a statistically significant difference (p = 0.048). The gut microbial beta diversity (Bray-Curtis dissimilarity) was significantly different between the two groups (p = 0.01). No significant differences in gut microbiome alpha or beta diversity were observed between individuals with high vs. low PTSD scores or PSS scores. Despite the lack of global diversity changes, specific microbial taxa were found to be significantly associated with elevated PTSD symptoms and perceived stress scores.

Conclusions: Firefighters reported significantly higher levels of both PTSD symptoms and perceived stress compared to controls. A strong positive correlation between perceived stress and PTSD symptoms suggests that individuals experiencing greater stress are more likely to report trauma-related symptoms. While alpha diversity did not differ overall between groups, genus-level diversity and beta diversity analyses revealed significant differences in gut microbiome composition between firefighters and controls. These findings indicate that occupational stress may be linked to alterations in the gut microbiome. Although no significant diversity differences were observed in relation to PTSD symptoms or stress scores alone, the identification of specific microbial taxa associated with elevated PTSD symptoms and stress highlights a potential microbiome-based signature of psychological stress.