OXYGEN DEPLETION BY GUT MICROBES AS A REGULATOR OF INTESTINAL INFLAMMATION

Abstract BACKGROUND Oxygen is a critical factor in shaping gut microbial communities and host-microbe interactions. In the healthy colon, the luminal environment is largely anaerobic, favoring strict anaerobes with beneficial functions. Inflammatory bowel disease (IBD) disrupts this balance. It has been proposed that epithelial damage and inflammation elevate local oxygen tension creating conditions that support facultative anaerobes associated with dysbiosis. Despite this, the ability of different gut bacteria to actively modulate oxygen levels remains poorly understood. Aim We aimed to systematically examine the ability of facultative anaerobic bacteria to deplete oxygen and generate localized anaerobic niches that may influence microbial ecology and inflammation in IBD. Methods & Results We cultured >60 gut-associated bacterial isolates, including Streptococcus, Enterococcus, Lactobacillus, Klebsiella, Pseudomonas, Acinetobacter, Enterobacter, Staphylococcus, Proteus, Listeria, Escherichia, Morganella, Serratia, Citrobacter, Salmonella, and Lactococcus species in a chemically defined medium ZMB1. Growth was monitored on a Synergy H1 plate reader, while oxygen was measured using a Resipher system. Whereas uninoculated controls maintained stable oxygen at ∼200 μM, multiple species, including Morganella morganii, Salmonella enterica, Pseudomonas fluorescens, P. aeruginosa, and Klebsiella spp. reduced oxygen concentrations to < 30 μM within 1 hr in an aerobic environment. Other strains, including Serratia marcescens, Citrobacter freundii, Enterobacter cloacae, Escherichia coli, Proteus spp., Acinetobacter spp., and Enterococcus faecalis, quenched oxygen within 2 hours. The majority of these bacteria were able to maintain anaerobic conditions over 18 hrs. In contrast, most Streptococcus and Lactobacillus strains displayed little oxygen reduction. Genomic analysis revealed enrichment of cytochrome oxidases and nitrate/nitrite reductases among rapid oxygen consumers. Importantly, we demonstrated that the strict anaerobes Fusobacterium nucleatum and Clostridioides difficile could proliferate under aerobic conditions when co-cultured with oxygen-depleting bacteria. These co-cultures triggered heightened inflammatory responses, modeling how shifts in microbial oxygen metabolism could worsen mucosal inflammation. CONCLUSION We identify distinct gut microbes with the capacity to rapidly generate anaerobic microenvironments. In the context of IBD, where epithelial oxygenation is increased, these facultative anaerobes may play dual roles: promoting restoration of anaerobic niches that favor beneficial strict anaerobes, but also enabling the growth of pro-inflammatory taxa. Defining the oxygen-modulating capacity of gut bacteria provides a mechanistic foundation for understanding dysbiosis in IBD.