Heterologous Production of Isopropanol Using Metabolically Engineered Acetobacterium woodii Strains

The depletion of fossil fuel resources and the CO2 emissions coupled with petroleum-based industrial processes present a relevant issue for the whole of society. An alternative to the fossil-based production of chemicals is microbial fermentation using acetogens. Acetogenic bacteria are able to metabolize CO or CO2 (+H2) via the Wood-Ljungdahl pathway. As isopropanol is widely used in a variety of industrial branches, it is advantageous to find a fossil-independent production process. In this study, Acetobacterium woodii was employed to produce isopropanol via plasmid-based expression of the enzymes thiolase A, CoA-transferase, acetoacetate decarboxylase and secondary alcohol dehydrogenase. An examination of the enzymes originating from different organisms led to a maximum isopropanol production of 5.64 ± 1.08 mM using CO2 + H2 as the carbon and energy source. To this end, the genes thlA (encoding thiolase A) and ctfA/ctfB (encoding CoA-transferase) of Clostridium scatologenes, adc (encoding acetoacetate decarboxylase) originating from C. acetobutylicum and sadH (encoding secondary alcohol dehydrogenase) of C. beijerinckii DSM 6423 were employed. Since bottlenecks in the isopropanol production pathway are known, optimization of the strain was investigated, resulting in a 2.5-fold increase in isopropanol concentration.

Reclassification of Clostridium aurantibutyricum Hellinger 1944 and Clostridium roseum (ex McCoy and McClung 1935) Cato et al. 1988

Clostridium aurantibutyricum, Clostridium felsineum and Clostridium roseum share a very high similarity based on multi-locus sequence analysis. In this study, their correct taxonomic status was determined using genomic and phenotypic investigations. Average nucleotide identity based on MUMmer alignment of the genomes and in silico DNA-DNA hybridization resulted in values of 98.55-100 and 78.7-100 %, respectively, strongly indicating that all strains are members of the same species. In addition, morphological investigations, fatty acid analyses and substrate utilization tests revealed no striking differences between the strains. Therefore, we propose the reclassification of C. aurantibutyricum and C. roseum as later heterotypic synonyms of C. felsineum. The type strain is lodged in several culture collections (ATCC 17788^T=DSM 794^T=NCIMB 10690^T).