Recent advances inn-butanol and butyrate production using engineeredClostridium tyrobutyricum

Acidogenic clostridia naturally producing acetic and butyric acids has attracted high interest as a novel host for butyrate andn-butanol production. Among them,Clostridium tyrobutyricumis a hyper butyrate-producing bacterium, which re-assimilates acetate for butyrate biosynthesis by butyryl-CoA/acetate CoA transferase (CoAT), rather than the phosphotransbutyrylase-butyrate kinase (PTB-BK) pathway widely found in clostridia and other microbial species. To date,C. tyrobutyricumhas been engineered to overexpress a heterologous alcohol/aldehyde dehydrogenase, which converts butyryl-CoA ton-butanol. Compared to conventional solventogenic clostridia, which produce acetone, ethanol, and butanol in a biphasic fermentation process, the engineeredC. tyrobutyricumwith a high metabolic flux toward butyryl-CoA producedn-butanol at a high yield of > 0.30 g/g and titer of > 20 g/L in glucose fermentation. With no acetone production and a high C4/C2 ratio, butanol was the only major fermentation product by the recombinantC. tyrobutyricum, allowing simplified downstream processing for product purification. In this review, novel metabolic engineering strategies to improven-butanol and butyrate production byC. tyrobutyricumfrom various substrates, including glucose, xylose, galactose, sucrose, and cellulosic hydrolysates containing the mixture of glucose and xylose, are discussed. Compared to other recombinant hosts such asClostridium acetobutylicumandEscherichia coli, the engineeredC. tyrobutyricumstrains with higher butyrate and butanol titers, yields and productivities are the most promising hosts for potential industrial applications.