A non-solventogenic Clostridium beijerinckii strain lacking acetoacetate decarboxylase assimilates acetate and accumulates butyrate

Abstract

Clostridium beijerinckii is a remarkable biocatalyst that can perform Acetone-Butanol-Ethanol (ABE) fermentation. Nevertheless, some C. beijerinckii strains lose the capacity to produce butanol. Here, we compare xylose fermentation in the presence of acetic acid and butyric acid by two C. beijerinckii strains: Br21 and ATCC 35702. The former strain is non-solventogenic (it lacks the adc gene for acetoacetate decarboxylase), whereas the latter is a known solventogenic strain capable of ABE fermentation. Fermentation of xylose alone and with acetic acid, butyric acid, or acetic and butyric acid supplementation revealed differences between strains Br21 and ATCC 35702 in terms of product profile. Strain Br21 consumed six times more acetic acid than strain ATCC 35702. External butyric acid addition helped strain ATCC 35702 to enhance butanol generation. However, butyric acid supplementation did not affect strain Br21, which was not able to reassimilate butyric acid to produce butanol. Expression of acidogenic enzymes revealed that acetoacetyl-CoA-acetate/butyrate-CoA-transferase was a key enzyme for butyric acid evolution by strain Br21. Besides xylose fermentation, strain Br21 was able to use acetic acid, a biomass hydrolysis derivative, to form butyric acid as the only fermentation product. Although C. beijerinckii Br21 was unable to form butanol, it is a promising biocatalyst for obtaining butyric acid through an unconventional pathway involving xylose fermentation and acetate assimilation.