Taxonomic relationships among strains of Clostridium acetobutylicum and other phenotypically similar organisms

Abstract

Most of the presently studied acetone-butanol (solvent)-producing bacteria are labelled as Clostridium acetobutylicum. This situation contrasts what was experienced by investigators of the 1940s who faced a plurality of names for solvent-producing bacteria. Significant phenotypic differences, however, exist among the presently studied strains of C. acetobutylicum, which raised the question of whether or not these organisms can truly be considered as members of one species. Furthermore, two cultures (ATCC 824 and NCIMB 8052) that are thought to be equivalent in serving as the type strain of C. acetobutylicum have significantly different properties. To assess the relatedness of these bacteria as members of a species, a comparison of similarity of their genomic DNA is most effective. DNAs from cultures of clostridia labelled as C. acetobutylicum, ‘ C. butylicum’, and C. saccharoperbutylacetonicum from several collections have been compared with DNAs from reference strains, including the type strain of C. acetobutylicum and C. beijerinckii. Based on DNA reassociation, which measures sequence similarities, four distinct groups or species (with inter-group similarities below 30%) were identified: (i) those having > 80% DNA sequence similarity with the type strain of C. acetobutylicum; (ii) those, including NCIMB 8052, having > 70% DNA sequence similarity with the type strain of C. beijerinckii; (iii) two cultures (NRRL B643 and NCP 262) having 94% similarity between them; and (iv) C. saccharoperbutylacetonicum. Identification of four species from these solvent-producing clostridia explains the discrepancies reported by different laboratories, and classification of these bacteria on the basis of their genomic relatedness should facilitate future genetic experiments. It is noteworthy that after the carbon source was switched from starch (corn mash) to sugars (molasses), the industrial solvent fermentation indeed utilized organisms (represented by groups 2, 3, and 4) genetically distinct from C. acetobutylicum.