Megasphaera elsdenii: from gastrointestinal functional symbiont to industrial biomanufacturing chassis.

Anaerobic gastrointestinal commensals are critical regulators of host health and key chassis resources for sustainable green biomanufacturing. Megasphaera elsdenii is a Gram-negative, strictly anaerobic coccus that widely colonizes the gastrointestinal tract of mammals, especially the rumen of ruminants. Its defining metabolic feature is the highly efficient catabolism of lactate via the acrylate pathway, coupled with the synthesis of short- and medium-chain fatty acids (SCFAs/MCFAs), biohydrogen, and high-value metabolic intermediates. This review systematically compiles the fundamental biological characteristics, core metabolic networks, and host-specific physiological functions of M. elsdenii across ruminants, humans, and non-ruminant mammals. We highlight its dual nature: as a promising probiotic candidate for preventing ruminal acidosis and maintaining intestinal homeostasis, and as a high-potential anaerobic chassis strain for biomanufacturing organic acids, bioenergy, and bioplastic precursors. Meanwhile, we comprehensively discuss its biosafety risks, including conditional pathogenicity, antibiotic resistance gene dissemination, and gut microecological disturbance, as well as unresolved scientific controversies. Finally, we identify critical research gaps and propose future research priorities, providing a systematic framework for the rational development and safe application of this microbe.