Longitudinal analysis of the hand microbiome in response to chlorine-based antiseptic use during a military field exercise.

Hand hygiene is essential for infection control, yet the impact of frequent antiseptic use on the skin microbiome, which is crucial for skin barrier function and pathogen exclusion, remains underexplored, especially in field conditions. In high-risk military settings, there is a need for safe, multipurpose antiseptics that avoid the drawbacks of alcohol-based options, including skin irritation, flammability, and unsuitability for wound application. We conducted a longitudinal study during the Norwegian-led military exercise Cold Response 2022 to assess the effects of repeated use of stabilized hypochlorous acid, a non-alcoholic antiseptic, on the hand microbiome of soldiers in a field environment. Participants used either the chlorine-based antiseptic (n = 20) or standard hygiene practices, mainly alcohol-based sanitizers (n = 19), for 10 days. Skin swabs from hands and untreated forearms were collected at baseline, post-intervention, and three weeks after discontinuation, and analyzed using 16S rRNA sequencing. Overall, our data revealed that field exposure accounted for more variation in bacterial composition than antiseptic use. When comparing alpha and beta diversity trajectories between the two antiseptic groups, there was minimal divergence during active field use, but a significant post-treatment increase in alpha diversity associated with the use of chlorine-based antiseptics. In contrast, untreated forearms showed no such group-level differences. These findings suggest that antiseptic-driven microbiome shifts are most apparent during recovery, with non-alcoholic formulations associated with faster recolonization after treatment. Stabilized hypochlorous acid represents an alternative to standard alcohol-based antiseptics for military personnel.IMPORTANCEMilitary personnel often rely on frequent hand antisepsis in field environments where infection risk is elevated and products may need to be used on both intact and damaged skin. Yet the effects of repeated antiseptics on the skin microbiome are poorly understood. To our knowledge, this is the first longitudinal study comparing hand microbiota trajectories following repeated antiseptic use in a real-world operational setting. Microbiome changes were most evident after treatment cessation than during active use, and the non-alcoholic hypochlorous acid-based antiseptic was associated with greater recovery of bacterial diversity. These findings show the value of longitudinal sampling and may have implications for infection control and antiseptic selection in military and other high-risk settings.