Monday, August 13, 2012
Columbia Hall, Terrace Level (Washington Hilton)
Lactic acid is an important industrial chemical commonly produced through microbial fermentation. The efficiency of acid extraction from fermentation media is increased at lower pH, which is limited to the pH at which microbial cultures can live and produce lactic acid. To explore mechanisms related to bacterial growth and acid production at low pH, Lactobacillus casei ATCC334 cells were grown in MRS medium containing 3% glucose with and without 2% Tween 80, a compound known to influence cell envelope lipid composition. The cultures were allowed to acidify the media until a pH of 4.5, 4.0 or 3.8 was reached, then the pH was maintained at each setpoint by automatic addition of NH4OH. Tween addition increased the yields of lactic acid at all 3 pH levels, but only increased the rate of acid production at pH 3.8. To determine the role of membrane lipid composition in acid production at low pH, a mutant lacking cyclopropane fatty acid synthase (cfa) was constructed and analyzed at pH 4.0. Loss of cfa dramatically reduced the rate of lactic acid production and, in contrast to the control, also reduced the lactic acid yield at pH 4.0 in medium with Tween. Membrane lipid analysis showed cfa deletion abolished accumulation of cyclic fatty acids, and skewed overall lipid composition. Results indicate increased rate and yield of lactic acid production obtained at low pH by Tween addition is partly based on the ability of L. casei to increase the cyclic fatty acid content of its cell envelope.