1.1. Microbes

Clostridium is a group of obligate, Gram positive, endospore-forming anaerobes. There are lots of strains used for ABE fermentation in different culture collections, such as ATCC (American Type Culture Collection), DSM (German Collection of Microorganisms, or Deut­sche Sammlung Von Mikroorganismen), NCIMB (National Collections of Industrial & Ma­rine Bactria Ltd), and NRRL (Midwest Area National Center for Agriculture Utilization Research, US Department of Agriculture). The different strains share similar phonotype such as main metabolic pathway and end products. Molecular biology technology offers ef­ficient method for classification. The butanol-producing clostridium can be assigned to four groups according to their genetic background, named C. acetobutylicum, C. beijerinckii, C. sac­charoperbutyl acetonicum, and C. saccharobutylicum, respectively. C. acetobutylicum is phyloge — netically distinct from the other three groups.

The common substrate for the solvent production by these strains is soluble starch. The orig­inal starch-fermenting strains belong to C. acetobutylicum. A recently isolated butanol-pro­ducing strain C. saccharobutylicum showed high hemicellulotic activity (Berezina et al., 2009). All of the four group strains can ferment glucose-containing medium to produce solvent. In 4% glucose TYA medium, C. beijerinckii gave the lowest solvent yield (28%), while the sol­vent yield was upper than 30% compared to the other three groups (Shaheen et al., 2000). In standard supplement maize medium (SMM), C. acetobutylicum is the best strain for maize fermentation, and the total solvent concentration can reach 19g/L. The solvent yield was 16, 14, and 11 for that of C. beijerinckii, C. saccharoperbutyl acetonicum, and C. saccharobutylicum respectively. However, C. acetobutylicum can’t ferment molasses well and it produces bright yellow riboflavin in milk, which is different from other groups and easy identified. The best molasses-fermenting strains belong to C. saccharobutylicum and C. beijerinckii (Shaheen et al., 2000). C. saccharoperbutyl acetonicum can utilize sugar, molasses and maize. Comparing to C. acetobutylicum, C. beijerinckii was more tolerant to acetic acid and formic acid (Cho et al., 2012), which suggests the advantage when using lignocellulosic hydrolysate treated with acetic and formic acid as substrate.

There are also some C. beijerinckii strains produce isopropanol instead of acetone (George et al., 1983). Some microorganisms can produce biobutanol from carbon monoxide (CO) and molecular hydrogen (H2), including acetogens, Butyribacterium methylotrophicum, C. autoetha — nogenum, C. ljungdahlii and C. carboxidiworans. The C. carboxidivorans strain P7(T) genome possessed a complete Wood-Ljungdahl pathway gene cluster which is responsible for CO, hydrogen fixation and conversion to acetyl-CoA(Fig.2) (Bruant et al., 2010).