1-49: Homofermentative L-lactate production by Bacillus subtilis and its capability to ferment different sugars

The production of the biodegradable bioplastic polylactate is predicted to increase constantly; hence lactic acid demand is increasing substantially. In this work a strain derived from B. subtilis 168 trp⁺ was constructed in order to obtain a homofermentative L-lactate strain, and its capability to ferment different carbon substrates was assessed. The alsS gene, which encodes for the acetolactate synthase enzyme, was interrupted to eliminate the conversion of pyruvate to acetolactate and butanediol. The strain B. subtilis ΔalsS, called ER382, showed a specific rate for glucose consumption two-fold higher when compared to its parental strain. The ability of strain ER382 to use fructose as a sole carbon source was as efficient as for glucose. However catabolism of cellobiose, sucrose and arabinose was very slow, when compared to that of glucose. Strain ER382 is unable to ferment glycerol, starch or lactose. The efficiency of conversion of sugars to L-Lactate was very close to the maximum theoretical value, except for arabinose, in which the yield was 80% of the maximum theoretical value. This fact suggests that the pentose pathway of B. subtilis is inefficient under anaerobic conditions. B. subtilis has only one gene that encodes for a L-lactate deshydrogenase, therefore a 99.5% of optical purity of L-lactate was found in the product.