Clostridium acetobutylicum

Acetone Butanol fermentation is an example of anaerobic fermentation as well as a mixed fermentation where different products are obtained by using different species of Clostridium. Cham Weizmann in 1919 isolated Clostridium acetobutylicum from various fermented starchy substrates including ground corn. Many other Clostridium spp were later isolated which gave high solvent yields on other carbon sources such as molasses media (eg., Clostridium saccharoacetobutylicum).

Clostridium species are anaerobic, motile, spore forming rods, heat resistant spores, located at or near one end of the vegetative cell. Acetone-butanol producing Clostridia first synthesize butyric and acetic acids which are then converted to butanol and acetone.

Different Clostridia species are isolated such as

1)      Clostridum butyricum - produce acetic and butyric acids along with gases (CO_{2 &} H₂) but no further conversion of the acids to neutral solvents.

2)       Clostridium acetobutylicum - produce acetic and butyric acids along with gases (CO_{2 &} H₂) and further reduce the acids to neutral solvents

 Two species of Clostridium, Clostridium acetobutylicum and Cl. saccharoacetobutylicum are generally employed for acetone-butanol fermentation. They differ slightly in their nutritional requirements and fermentation factors. For Clostridium acetobutylicum, potato/corn medium is used and for Cl. saccharoacetobutylicum, molasses medium. Inoculum growth and fermentative production of the solvents are carried out at 31° to 32°C for Cl. saccharoacetobutylicum and at approximately 37°C for Cl. acetobutylicum.

 Cultures are maintained as soil stocks – spores are not sensitive to O₂ unlike vegetative cells, can be maintained as soil stocks for as long as 30 years. Prolonged storage may sometimes reduce solvent producing ability (degeneration of cultures).

Sluggish growth during fermentation and poor conversion to of acids to corresponding solvents is seen if degenerated.

Rejuvenation done by heat-shock, followed by rapid cooling.  This kills less heat resistance spores and vegetative cells. Weizmann suggested 100-150 successive heat shock treatments. This selects the most heat resistant spores, which can be used for fermentation.