Abstract
Acid tolerance of the lactic acid bacterium (LAB) is crucially important for the production of free lactic acid as a chemical monomer by simplified purification steps. This study conducts both metabolic modification and adaptive evolution approaches on increasing the acid tolerance of an engineered Pediococcus acidilactici strain. The overexpression of the genes encoding lactate dehydrogenase, recombinase, chaperone, glutathione and ATPase did not show the observable changes in acid tolerance. On the other hand, the low pH adaptive evolution showed clear improvement. The L-lactic acid generation and cell viability of the adaptively evolved P. acidilactici were doubled at low pH up to 4.0 when wheat straw was used as carbohydrate feedstock. However, the further decrease in pH value close to the pKa (3.86) of lactic acid led to a dramatic reduction in L-lactic acid generation. This result shows a partially successful approach on improving the acid tolerance of the lactic acid bacterium P. acidilactici.
Highlights
Pediococcus acidilactici for L-LacticIndustrial lactic acid fermentation by lactic acid bacterium (LAB) is generally at the pH of 5.5~6.5 by adding Ca(OH)2, NaOH or their alkaline compounds forming calcium lactate or sodium lactate [1]
When free lactic acid is required as a chemical monomer for the production of polylactic acid (PLA), sulfuric acid or electrodialysis are used to recover calcium lactate or sodium lactate into free lactic acid; this is followed by complicated downstream purification steps [2,3,4,5]
Amorphotheca resinae ZN1 (CGMCC 7452) used for biodetoxification was isolated in our lab and cultured at 28 ◦ C on a potato dextrose agar (PDA) slant prepared by 200 g of potato juice with addition of 20 g glucose and 15 g of agar in 1 L of deionized water, according to previous studies [17,18]
Summary
Industrial lactic acid fermentation by lactic acid bacterium (LAB) is generally at the pH of 5.5~6.5 by adding Ca(OH) , NaOH or their alkaline compounds forming calcium lactate or sodium lactate [1]. The fermentability of lactic acid bacteria is strongly inhibited by the high titer of lactic acid generated with very low pH values [6,7,8]. Two approaches have been attempted to improve the acid tolerance of lactic acid bacteria: one is metabolic modification [9,10] and the other is adaptive evolution [11,12]. The expression of histidine carboxylation in LAB strains enabled the strain to increase intracellular pH and improved acid tolerance [9]
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