具有胆盐水解酶活性乳酸菌的筛选及影响其活性因素分析

具有胆盐水解酶活性的乳酸菌有潜在的降低血液胆固醇的作用。对分离自酸马奶的10株乳杆菌的胆盐水解酶活性进行了定性及定量分析,在此基础上对影响胆盐水解酶的一些因素进行了研究。结果表明,3株乳杆菌具有胆盐水解酶活性,其中以MG13-3为最高,其次为XB4-1及MG16-3;当pH值为6时,超声波破碎时间在12～15min之间,培养基中加入巯基乙酸钠并在厌氧条件下培养时,3株乳杆菌均体现出较高的胆盐水解酶活性。     

降胆固醇乳酸菌的益生特性及其作用机理

考察了具有降胆固醇乳酸菌(乳酸乳球菌L14-3、鼠李糖乳杆菌118-1、粪肠球菌M53-2、植物乳杆菌117-1、植物乳杆菌23-1)的益生特性,且初探乳酸菌降胆固醇的作用机理。通过模拟胃液、胰液进行乳酸菌的耐酸耐胆盐测定,并评价了乳酸菌对抗生素的抗性。通过胆固醇、胆盐的共沉淀作用和菌体细胞吸收两种方式去除发酵液中的胆固醇,利用邻苯二甲醛法测定胆固醇的含量,确定胆固醇的去除率。结果表明:通过共沉淀作用和菌体吸收,乳酸乳球菌L14-3和鼠李糖乳杆菌118-1分别去除胆固醇的比例最高。在pH=2.0的人工胃液中,粪肠球菌M53-2和乳酸乳球菌L14-3的活菌数显著高于其他3株菌(P0.05)。粪肠球菌M53-2对pH 3.0的人工胃液有较好的耐受性,存活率显著高于其他4株菌(P0.05)。在pH=8.0人工胰液中,鼠李糖乳杆菌118-1和植物乳杆菌117-1的存活率显著高于其他菌株(P0.05)。植物乳杆菌117-1耐药性最好。因此,研究具有降胆固醇的益生菌具有广阔应用前景。     

内蒙古传统酸奶乳酸菌的筛选及体外益生效果评价

对内蒙古传统酸奶中分离出的8株乳酸菌菌株进行形态学及16S rDNA鉴定,确定其中包括鼠李糖乳杆菌、植物乳杆菌、干酪乳杆菌、瑞士乳杆菌和嗜热链球菌。然后进一步对菌株的酸、胆盐、人工胃肠液耐受性、胆固醇降解能力及自由基清除能力进行体外益生效果评价。结果表明,鼠李糖乳杆菌217-3对pH=1.5的强酸和人工胃肠液有较高的耐受性,同时还能耐受1.5%的胆盐。此外,其胆固醇降解和自由基清除能力也具有较高的水平。综上所述,鼠李糖乳杆菌217-3具有显著的益生性能,为高活性乳酸菌菌剂及相关发酵产品的开发奠定了一定的理论基础。     

具有胆固醇降解能力的人源乳酸菌筛选及其特性研究

研究旨在筛选出具有降胆固醇能力的人源乳酸菌，为开发良好的乳酸菌产品提供依据。对公司自建菌库中的28株人源乳酸菌中进行胆固醇降解率的初步测定，筛选出3株降解能力较高的乳酸菌进行耐酸耐胆盐的特性试验，最终确定一株具有胆固醇降解能力及特性良好的人源乳酸菌。结果表明，28株人源乳酸菌中讲解率较高的6株乳酸菌分别为来自婴儿粪便源的格氏乳杆菌F020、F024和来源于母乳源格氏乳杆菌B1-26、B1-29以及母乳源罗伊氏乳杆菌B1-27、B1-28。对这6株乳酸菌进行体外胆固醇降解试验得到F024对胆固醇的同化量最高达到25.92%,F020的沉淀量最高达到了10.49%;B1-28的胆盐水解酶活性最高。将格氏乳杆菌F020、F024以及罗伊氏乳杆菌B1-28进行耐酸耐胆盐试验，pH 1.5时，3株乳酸菌均不耐受，pH 2.0和pH 3.0 3 h时，格氏乳杆菌F020的存活率超过100%；在0.3%的胆盐培养基中，格氏乳杆菌F020 3 h存活率达到102.94%。本实验最终选出胆固醇降解率高并且特性良好的格氏乳杆菌F020，为进一步开发乳酸菌产品提供良好依据。     

分离自内蒙古传统发酵酸马奶中L.casei Zhng潜在益生特性的研究

从内蒙古地区蒙古族家庭制作的16份酸马奶样品中分离得到的50株乳杆菌中，经耐酸性实验和人工胃肠消化液耐受性实验，筛选出1株对酸耐受性强的乳杆菌，为L.casei Zhang。将L.casei Zhang制成发酵乳后，其对人工胃肠消化液的耐受性要高于纯菌体，这是由于乳蛋白对消化液的缓冲作用和对菌体的保护作用所致。L.casei Zhang对介质中胆盐的最大耐受浓度为1．6g／100mL。L．casei Zhang具有一定的胆盐水解酶活性，可以分解培养介质中的牛磺胆酸钠释放出游离胆酸；L.casei Zhang在37℃培养24h可脱除培养介质中49．61％的胆固醇。这一结果表明，L.casei Zhang可以作为潜在的益生菌。     

内蒙古奶豆腐中潜在益生性乳酸菌的筛选

对从内蒙古奶豆腐中分离的16株乳酸菌进行16S rDNA分子鉴定,其中9株鉴定为乳酸片球菌、5株为屎肠球菌、1株为植物乳杆菌、1株为鼠李糖乳杆菌。进一步对分离得到的菌株进行模拟人工胃液和胆盐耐受性、疏水性、体外降胆固醇和抗氧化特性研究。结果表明:植物乳杆菌M1-2和鼠李糖乳杆菌M6-1对模拟人工胃液有较高的耐受性;屎肠球菌M7-1、M8-1和M8-2具有较高的胆盐耐受性;植物乳杆菌M1-2和鼠李糖乳杆菌M6-1对甲苯和十六烷的疏水性显著高于其他菌株。体外功能性实验结果表明,屎肠球菌M1-1、植物乳杆菌M1-2、乳酸片球菌M1-3和M2-1的体外降胆固醇能力较高,胆固醇降解率均超过30%;鼠李糖乳杆菌M6-1对过氧化氢的耐受性、羟自由基(?OH)和1,1-二苯基-2-三硝基苯肼(1,1-diphenyl-2-picrylhydrazyl,DPPH)自由基的清除能力最高。综上所述,植物乳杆菌M1-2和鼠李糖乳杆菌M6-1具有较好的益生特性,可作为今后开发功能性产品的潜在益生菌株。     

植物乳杆菌NDC 75017的降胆固醇作用

植物乳杆菌NDC 75017分离自中国内蒙古传统发酵乳制品。本实验对其体外环境耐受性及其降胆固醇作用进行研究。采用活菌计数法检测植物乳杆菌NDC 75017在pH 2.0～3.0的酸性环境、胆盐质量浓度0.3g/100mL、NaCl质量浓度0～7g/100mL以及模拟人工消化液环境下的耐受情况;同时以邻苯二甲醛法测定其体外降胆固醇能力。结果表明:植物乳杆菌NDC 75017可耐受pH3.0及0.3g/100mL胆盐环境;在7g/100mL的高盐质量浓度条件活菌数仍可达108CFU/mL;可以通过胃进入肠道而保持活性,在肠道处理8h后存活率为58.73%。体外降胆固醇实验测得细菌发酵上清液、菌体洗涤液和细胞破碎液中的胆固醇脱除率分别为16.43%、26.35%和32.87%。植物乳杆菌NDC 75017环境耐受能力良好,具有体外降胆固醇作用,该菌株可以作为降胆固醇的潜在益生菌。     

乳酸菌MR25的筛选、鉴定及降胆固醇益生特性

通过气相色谱法,从萝卜干中共分离到31株具有清除培养基中胆固醇能力的乳酸菌,其中乳酸菌MR25的胆固醇清除率最高(42.48％).通过形态观察、生理生化试验、糖发酵试验等,鉴定MR25为米酒乳杆菌.乳酸菌MR25具有酸耐受性、胆盐耐受性和一定的胆固醇降低能力.在pH 2.0、培养2h条件下菌株浓度仍能达到106 cfu/mL.加胆盐(0.3％～0.4％)对菌株生长量(OD值)的影响很小.热杀死和休眠的细胞能去除很少的胆固醇,分别是3.26 mg/mL和8.42 mg/mL,而生长的细胞去除的胆固醇达到44.06 mg/mL.胆固醇去除与菌体的生长有一定的相关性.乳酸菌MR25去除胆固醇的机理是菌体对胆固醇的吸附作用,以及菌体在生长过程中对胆固醇的吸收利用.     

乳杆菌体外降解胆固醇特性研究

以实验室自主分离纯化的鼠李糖乳杆菌zxr01、嗜酸乳杆菌zrx02、植物乳杆菌zrx03, 3株乳杆菌体外降解胆固醇的特性以及不同金属阳离子对其影响进行了研究。嗜酸乳杆菌、鼠李糖乳杆菌、植物乳杆菌体外胆固醇降解率分别为28.87%、25.61%、20.39%；当pH为3时，3株乳杆菌耐酸性分别为：52.77%、52.57%、59.82%;牛胆盐浓度为0.3%时胆盐耐受性为28.65%、27.40%、26.04%；嗜酸乳杆菌在氯仿中的疏水性最好，达到了19.88%，静置24 h后，嗜酸乳杆菌和植物乳杆菌的自凝聚力都达到了80%以上且3株乳杆菌产生白色沉淀说明能产生胆盐水解酶，在Ca2+浓度为0.02 N时，嗜酸乳杆菌的胆固醇降解率为35.4%和降解效果65.43μg/u。结果显示3株乳杆菌均能在体外降解胆固醇，嗜酸乳杆菌的降解胆固醇的能力要优于其它两株乳杆菌，添加浓度为0.02 N的Ca2+可以促进嗜酸乳杆菌胆固醇降解率和降解效力。本研究结果可为进一步研究胆固醇降解提供了试验依据，为开发降胆固醇食品提供材料。     

酸马奶中干酪乳杆菌发酵特性的研究

研究了从内蒙古锡林浩特传统发酵酸马奶中分离4株(Lb.caseiZL3-1,Lb.caseiZL3-4,Lb.caseiZhang和Lb.caseiXM2-1)具有潜在益生特性的干酪乳杆菌的发酵特性。结果表明,37℃发酵24h结束后,干酪乳杆菌发酵乳滴定酸度为79.75°T,pH值变化在4.52～4.26之间。Lb.caseiZL3-1,Lb.caseiZL3-4,Lb.caseiZhang和Lb.caseiXM2-1在发酵结束的活菌总数分别为8.11×108,8.53×108,9.61×108和5.30×108mL-1,其中Lb.caseiZhang活菌数为最高。24h发酵结束时,Lb.caseiZL3-1,Lb.caseiZL3-4,Lb.caseiZhang和Lb.caseiXM2-1的黏度分别为0.1830,0.2957,0.1167和0.2497Pa.s。发酵过程中质量分数为12%的TCA可溶性氮无明显增加,4株干酪乳杆菌的蛋白水解力无显著性差异(P>0.05)。     

Growth and metabolism of selected strains of probiotic bacteria, in maize porridge with added malted barley

A fermented probiotic maize porridge with high energy density and low viscosity was prepared, using maize flour and barley malt. The porridge was fermented with four probiotic strains (grown separately): Lactobacillus reuteri, Lb. acidophilus (LA5 and 1748) and Lb. rhamnosus GG. These strains were inoculated at two levels; to obtain approx. 7 or 6 log cfu g(-1) in the porridge at 0 h. The porridge was fermented for 24 h at 37 degrees C, and analysed for viable cell count, pH, organic acids, volatile aromatic compounds and sugar content. The inoculated cell concentration was shown to be particularly important during the first hours of the fermentation period, showing a delayed production of most metabolites in porridge inoculated with approx. 6 log cfu g(-1). Most strains reached maximum cell count after 12-h fermentation (7.2-8.2 log cfu g(-1)), with a pH below 4.0. Depending on the strain, lactic acid was produced in amounts ranging from 1360 to 4000 mg kg(-1). Lb. reuteri metabolised succinate, while pyruvate and small amounts of diacetyl were detected in porridge inoculated with Lb. acidophilus LA5 and Lb. acidophilus 1748. High amounts of diacetyl (6 mg kg(-1)) and acetoin (27 mg kg(-1)) were detected in porridge inoculated with Lb. rhamnosus GG. Porridge inoculated with Lb. acidophilus LA5 and Lb. acidophilus 1748, contained acetaldehyde, while both Lb. reuteri and Lb. rhamnosus GG reduced the acetaldehyde to ethanol. Lb. reuteri utilised both maltose and glucose as carbohydrate sources, while Lb. acidophilus LA5, Lb. acidophilus 1748 and Lb. rhamnosus GG utilised only glucose.     

Growth and metabolism of selected strains of probiotic bacteria in milk

Growth and metabolism of five probiotic strains with well-documented health effects were studied in ultra-high temperature (UHT) treated milk, supplemented with 0.5% (w/v) tryptone or 0.75% (w/v) fructose. The probiotic strains were Lactobacillus acidophilus La5, Lb. acidophilus 1748, Lactobacillus rhamnosus GG, Lactobacillus reuteri SD 2112 and Bifidobacterium animalis BB12. Fermentation was followed for 72 h at 37 degrees C and the samples were analysed for pH, log cfu ml(-1), volatile compounds, organic acids and carbon dioxide. The strains reduced pH from 6.7 to between 3.9 and 4.4 after 24 h of incubation. All strains attained viable cell counts above 8.7-9.18 log cfu ml(-1) after 6-16 h of incubation. The two Lb. acidophilus strains showed a stable level of viable cells during 12-72 h of incubation but the three other strains showed a reduction of 0.4-1.1 log cfu ml(-1) from 24 to 72 h of incubation. However, all strains showed cell levels between 7.8 and 8.7 log cfu ml(-1) after 72 h of incubation. After 48 h of incubation, the amount of lactic acid produced varied according to strain from 6949 to 14,000 mg kg(-1) and acetic acid produced varied from 0 to 6901 mg kg(-1). Three of the strains metabolised citrate but only low amounts of diacetyl and acetoin were detected within strains, 0.2-0.8 and 6.5-10 mg kg(-1), respectively. Carbon dioxide produced varied from 221 to 3942 mg kg(-1) and was connected to the citrate-fermenting ability of the strain used and their carbohydrate fermentation pathway. Three of the strains produced detectable levels of acetaldehyde and the concentration varied from 9.4 to 12.6 mg kg(-1) after 24 h of incubation. All five probiotic strains showed very different profiles of metabolites during fermentation; however, the two Lb. acidophilus strains were the most alike. Our findings show the importance of controlling the fermentation time since the probiotic strains produced different amounts of metabolic products according to fermentation time.     

Functional characteristics of Lactobacillus spp. from traditional Maasai fermented milk products in Kenya

In this study functional characteristics of 23 representative Lactobacillus strains isolated from the Maasai traditional fermented milk 'Kule naoto' were determined. The Lb. acidophilus group strains showed resistance to gastric juice and bile. In addition, some Lb. acidophilus strains expressed bile salt hydrolase activity, and had ability to assimilate cholesterol in vitro. In-vitro adhesion to HT29 MTX cells of up to 70% was recorded. Lb. fermentum strains showed almost 100% survival under simulated stomach acidic conditions and physiological salt concentrations of bile salts, hydrophobicity values were over 80%. Most strains of the Lb. casei and Lb. acidophilus groups showed aggregation abilities of above 50%. Many strains expressed a protective effect against N-methyl-N'-nitro-N-nitrosoguanidine induced DNA damage according to the 'comet assay' and none was virulent. The antibiotic minimum inhibitory concentration of selected strains was established. According to these results, the Lactobacillus spp associated with 'Kule naoto', contain potentially probiotic (functional) strains.     

Evolution of carbohydrate fraction in carbonated fermented milks as affected by beta-galactosidase activity of starter strains

The influence of carbonation on the evolution of lactose, galactose and glucose in fermented milks with added probiotic bacteria (Lactobacillus casei, Lactobacillus acidophilus and/or Bifidobacterium bifidum) was evaluated and related to beta-galactosidase activity of starter strains. During incubation and first days of refrigeration, lactose hydrolysis resulting in the liberation of galactose and glucose occurred in CT (Streptococcus thermophilus/Lb. casei), AT (Str. thermophilus/Lb. acidophilus) and ABT fermented milks (Str. thermophilus/Lb. acidophilus/Bifid. bifidum). Levels of galactose were higher than those of glucose and could be related to the preferential consumption of glucose by actively growing bacteria. Through the incubation, lactose and monosaccharide levels were not affected by milk carbonation. However, during refrigerated storage the presence of this gas was associated with slightly lower content of lactose and higher levels of galactose and glucose in AT and ABT products but not in CT fermented milks. Through the refrigeration galactose was moderately utilised by Lb. acidophilus in AT products whereas the presence of Bifid. bifidum seems to prevent the consumption of this sugar in ABT fermented milks. Glucose remained constant, with minor variations in CT products but a continuous increase of this sugar occurred in carbonated AT and ABT fermented milks during storage. Beta-galactosidase activity displayed by Str. thermophilus strains was similar at pH 6.5 (initial pH of non-carbonated samples) and pH 6.3 (initial pH of carbonated samples) whereas Lb. acidophilus LaA3 showed greater beta-galactosidase activity at pH 6.3 than at higher pH values. Thus, the enhanced metabolic activity of Lb. acidophilus caused by the low initial pH of carbonated milk also promoted higher cellular beta-galactosidase activity that could have released greater amounts of galactose and glucose from lactose in AT and ABT fermented milks through the refrigerated period. In CT fermented milks, similar beta-galactosidase activity levels of Str. thermophilus at pH 6.5 and 6.3 together with the absence of beta-galactosidase activity in Lb. casei could explain the lack of differences on glucose and galactose content between carbonated and non-carbonated samples.     

Preparation and properties of probiotic cheese high in conjugated linoleic acid content

The development of probiotic Ras cheese rich in conjugated linoleic acid (CLA) was investigated using probiotic Lactobacillus casei and Lactobacillus acidophilus starters. The cheeses were assessed for composition, proteolysis, fatty acids and fat stability, and microbiology during 3 months of ripening. The cheese made with Lb. casei and Lb. acidophilus retained high counts of the probiotic strains (～log 8) throughout storage. Ripening changes followed the normal pattern of this type of cheese during ripening. Ras cheese made with Lb. casei and Lb. acidophilus contained the highest CLA content (0.84% after 3 months) as compared to control and cheese fat had acceptable oxidative stability.     

嗜酸乳杆菌和双歧杆菌发酵乳的流变特性研究

以两株嗜酸乳杆菌(KLDS AD1、KLDS AD2)和3株双歧杆菌(长双歧杆菌KLDS 2.0001、婴儿双歧杆菌KLDS 2.0002和KLDS 2.0604)分别发酵的酸乳为研究对象,测定其pH值、滴定酸度、质构及流变学特性。pH值和滴定酸度测定结果表明嗜酸乳杆菌产酸能力强于双歧杆菌。质构测定结果表明嗜酸乳杆菌发酵的酸奶质地较为结实。5种酸乳的剪切力升速和降速曲线都能形成触变环,触变环的面积大小为KLDS 2.0604>KLDS 2.0001>KLDS AD2>KLDS 2.0002>KLDS AD1,即婴儿双歧杆菌KLDS 2.0604在剪切力破坏下其组织状态的恢复力最差,很难恢复到起始状态。表观黏度曲线在下降时只有婴儿双歧杆菌KLDS 2.0002没有出现突增现象,其弹性最差。综合得出嗜酸乳杆菌KLDS AD2发酵乳的组织状态、黏弹性最好。     

Influence of controlled lactic fermentation on growth and sporulation of Bacillus cereus in milk

The growth and sporulation of Bacillus cereus NVH 45 in a fermentor with controlled pH or simulated pH conditions were investigated. The study was carried out in a fermentor to measure the influence of a rapid and a slow lactic acid production on the inhibition of B. cereus in a controlled environment during the initial part of fermentation and to observe if other factors than lactic acid influenced the inhibition. In the controlled pH experiments the pH was allowed to decrease to an end pH 5.0, 5.5 or 6.0 either by Lactobacillus casei 2756 (a fast acid producer) or Lactobacillus acidophilus NCFB 1748 (a slow acid producer). In co-cultures of Lb. casei 2756 and B. cereus NVH 45, low numbers (10-70 cfu/ml) of B. cereus NVH 45 were observed at end pH 5.5 (72 h) while at pH 5.0 no viable cells (<10 cfu/ml) were detected (48-72 h). B. cereus NVH 45 did not sporulate in co-culture with Lb. casei 2756. In co-culture with Lb. acidophilus NCFB 1748, B. cereus NVH 45 sporulated and survived as spores. In these co-cultures B. cereus NVH 45 grew to higher maximum counts (>10(7) cfu/ml) than with Lb. casei 2756 (<10(7) cfu/ml). Significantly different amounts of lactic acid were observed between the two co-cultures after 7 and 12 h. A rapid decrease of pH appears to prevent B. cereus from sporulating and it seems that it is enough to just reach pH 5.0 rapidly and keep that pH to achieve the desirable inhibition of B. cereus. In the simulated pH experiments B. cereus NVH 45 was inoculated in the fermentor and the different pH developments from different LAB strains were monitored by addition of lactic acid. These experiments showed the same tendency: a fast pH reduction during the initial hours of fermentation, simulating lactococci, resulted in complete inhibition of B. cereus NVH 45 (<10 cfu/ml). However, when simulating the pH development of the two different Lactobacillus strains, complete inhibition of B. cereus NVH 45 was not seen. In co-cultures competition for nutrients with consequences for cell density appears to be important. Based on these results it seems that B. cereus must reach a certain density to induce sporulation.     

富集钙乳酸菌的筛选及培养条件对富集效果的影响

为筛选具有较强钙转化富集能力的乳酸菌（后续用于鱼骨泥发酵）,本文研究了七种乳酸菌（植物乳杆菌CY1-1、植物乳杆菌Z7、戊糖片球菌DBY2-5-1、干酪乳杆菌D400、米酒乳杆菌DL10、嗜酸乳杆菌DL12、清酒乳杆菌YP4-5）及其复合菌株的生长能力、产酸能力和钙富集能力,研究了钙离子浓度、生长时间、pH、温度对初筛菌株钙富集能力的影响,并通过原子力显微镜观察菌体富集前后的表面结构差异,研究乳酸菌的形态变化及钙离子吸附机制。结果表明,单一菌株中钙富集量最高的是植物乳杆菌CY1-1,其次为植物乳杆菌Z7。钙富集能力较强的复合菌株为CY1-1+DL12、Z7+YP4-5,富集量分别较单一菌株提高了6.76和21.69 mg/g。试验菌株在稳定期时的钙富集能力优于对数期,最适富集的钙离子浓度为1.2 mg/mL,Z7菌株的最适pH为6,Z7+YP4-5、CY1-1、CY1-1+DL12菌株的最适pH为7.2,Z7+YP4-5复合菌株的最适温度为37 ℃,其余各组菌株的最适温度为40 ℃。原子力显微镜观察到乳酸菌与钙离子结合后表面结构凸起尖锐,与未结合菌株形态差异明显。上述结果表明,钙富集能力最强的单菌株是植物乳杆菌CY1-1,复合菌株是Z7+YP4-5;乳酸菌能有效吸附钙离子在菌体表面进行富集。     

Fatty acid composition and freeze-thaw resistance in lactobacilli

The fatty acid composition and freeze-thaw resistance of eight strains of thermophilic lactobacilli were studied. Seven of these contained the same polar and neutral lipids, the five major components making up 90% of the cellular fatty acid pool being 14:0, 16:0, 16:1, 18:1 and C19 cyclopropane (cyc19:0). Strain comparison by means of cluster analysis based on the fatty acid ratios using the overlap coefficient revealed two well defined clusters. One was formed by three strains of species Lactobacillus delbrueckii subsp. lactis and Lb. delbrueckii subsp. delbrueckii, the other included five strains of the species Lb. delbrueckii subsp. bulgaricus, Lb. acidophilus and Lb. helveticus. Resistance of strains with a high content of unsaturated fatty acids (66-70%) decreased with increasing cyc19:0 concentrations. In contrast, in strains with a low concentration of unsaturated fatty acids (42-49%), increasing cyc19:0 levels were associated with increased freeze-thaw resistance.     

Antimutagenicity of milk cultured with lactic acid bacteria against N-methyl-N'-nitro-N-nitrosoguanidine.

The antimutagenic effect of cultured milk using 71 strains of lactic acid bacteria belonging to the genus Lactobacillus, Streptococcus, Lactococcus, and Bifidobacterium on the mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine was investigated in vitro using Salmonella typhimurium TA 100 as an indicator bacterium. Each cultured milk sample displayed its characteristic antimutagenic effect on the mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine. The milk cultured with Lactobacillus acidophilus LA 106 (LA2) showed the highest inhibition of 77% against the mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine among the strains tested. Changes in the antimutagenic effect of the milk cultured by Lb. acidophilus LA 106 (LA2) during incubation were also examined using N-methyl-N'-nitro-N-nitrosoguanidine as a mutagen.