Bicarbonate is a stimulus in the inter-species induced sporulation of strict anaerobic Syntrophomonas erecta subsp. sporosyntropha.

Previously Syntrophomonas species had been described as the bacteria those did not form spores, however, in our previous studies, a newly isolated S. erecta subsp. sporosyntropha JCM13344(T) was found to form spores in the co-culture with methanogens, while not in mono-culture or in co-culture with sulfate reducer. In this study, we examined the sporulation stimulus conferred by methanogens in the co-culture. By reducing bicarbonate in mono-culture and sulfate-reducing co-culture, we could induce S. erecta subsp. sporosyntropha JCM13344(T) to form spores, so that bicarbonate at lower concentration was determined as another stimulus for sporulation. Based on the substrate degradation by strain JCM13344(T) in different concentration of bicarbonate vs at different pHs, it was suggested that bicarbonate could stimulate the sporulation by mediating a nutrient deprivation but not pH drop. To further confirm the sporulation potential of this group of bacteria, spo0A fragments were amplified from strain JCM13344(T) as well as all the recognized Syntrophomonas species, confirming that they were members of the spore-forming group. Since sporulation is a kind of response of spore-forming bacteria to environmental stresses, the observation in this work implies that stresses can be created even between the mutual beneficial partners, in this case, inducing sporulation.