基因组改组（genome－shuffling）提高

首先采用紫外线与亚硝基胍两种传统微生物诱变方法对干酪乳杆菌进行诱变，经低pH平板、碳酸钙平板和摇瓶试验获得了5株耐酸性提高的突变菌株。以获得的突变菌株为出发菌株，应用灭活双亲原生质体融合后致死损伤得到互补获得活性融合子的方法，对其进行基因组改组，经过低pH平板、碳酸钙平板和摇瓶筛选，获得4株可以在pH3.8平板上旺盛生长且产酸量较高的改组菌株。将改组菌株与原始菌株分别于pH 3.8和3.4的YE液体培养基中培养，改组菌株能够在原始菌株无法生存的pH条件（pH 3.4）下生长。在pH 3.8的条件下，对改组菌株与原始菌株的发酵特征进行比较，37℃发酵48小时后，改组菌株产酸量为原始菌株的2.4倍，表明基因组改组技术能有效提高多基因调控表型的进化。

Genome-shuffling Improved Acid Tolerance of

Lactic acid fermentation is a typical product inhibited bioconversion. The strains with improved performance of growing at low pH are desirable because low pH of the medium could increase the proportion of product in the free acid form and contributes to the economics of the lactic acid purification process. The use of genome shuffling to improve the acid tolerance of an industrial strain of Labtobacillus casei was described. Two populations were obtained with subtle improvements by UV and NTG mutagenesis and were taken as the starters for genome shuffling. Parental protoplasts were inactivated and fused by polythyiene glycol 6000. The fusants grew at the RM plates were the shuffled strains. The shuffled strains after screening on low pH plate and CaCO3 plate were identified. The mutant strains were incubated in YE liquid medium at pH 3.8 and 3.4 as well as the wild type. The mutant strains grew obviously better than the wild type strain. Only the shuffled strains can grow under the condition of pH 3.4. The mutant strains and wild type were both fermented at 37℃ for 48 h at pH 38. The production of L lactic acid of the mutant strain was 2.4 folds of the wild type.