Spore-forming properties and enhanced oxygen tolerance of butyrate-producing Anaerostipes spp.

•Three Anaerostipes spp. tested formed spores, but other 3 did not.•The spores of A. caccae were tolerant to oxygen and survived for 15 weeks.•Bile salts are essential for germination of spores in Anaerostipes spp.•All three sporulating Anaerostipes spp. had conserved putative spore formation genes.