Prof. Paul Péringer, Dr Jean-Paul Schwitzguébel, Christophe Collet
COST Action 841
April 2001 – March 2004
Prof. Kornel Kovacs, University of Szeged, Hungary; Dr Philippe Soucaille, INSA, Toulouse, France.
Lactose, the main milk sugar, is available in large amounts in milk and whey permeates rejected by dairy and cheese industry. By screening various thermophilic anaerobic bacteria, Clostridium thermolacticum has been found to be appropriate for the production of acetate from lactose, to be used eventually as calcium-magnesium acetate, a non-corrosive salt for roadway de-icing. However, ethanol and lactate are also produced from lactose, whereas the gas phase is composed of CO2 and hydrogen, a promising energy resource. Key metabolic pathways and enzymes have been studied, when the bacterium is grown alone or in the presence of other micro-organisms, under batch or continuous conditions, with the aim to optimize the production of acetate and/or hydrogen.
To test the effect of H2 partial pressure on the metabolism of lactose and on the acetate productivity, the hydrogen consuming species Methanothermobacter thermoautotrophicus (methanogenic Archaea) has been added to the continuous culture of C. thermolacticum. As a consequence, H2 partial pressure was decreased and methane was the major gas generated. Acetate became the major product of the liquid phase, ethanol production was dramatically reduced, but lactate was still produced at low dilution rates. Moorella thermoautotrophica (homoacetogenic bacterium) was thus added to the coculture, to convert lactate into acetate. As a result, acetate was almost the only organic product in the liquid phase. In the consortium, through efficient in situ hydrogen scavenging, the metabolic pattern was modified in favor of acetate production, instead of reduced by-products. A general strategy to produce the desired metabolite by the heterofermentative bacterium has been established. By co-cultivation with one or two other appropriate strains, the metabolism was redirected to increase the yield and the productivity of the desired product. Alternatively, the removal of hydrogen by physical methods should also be efficient to optimize the production of both H2 and acetate.