Characterization of the 16S-23S and 23S-5S rRNA intergenic spacer regions of dairy propionibacteria and their identification with species-specific primers by PCR.

In this study, the 16S-23S and 23S-5S rRNA intergenic spacer region sequences of Propionibacterium acidipropionici, P. freudenreichii ssp. freudenreichii and ssp. shermanii, P. jensenii and P. thoenii were determined. The sequences were shown to vary greatly between the species. Specific primer pairs were derived from the 16S-23S rRNA spacer sequences and used for the identification of the species by PCR.     

Capsular exopolysaccharide biosynthesis gene of Propionibacterium freudenreichii subsp. shermanii

In the dairy industry, exopolysaccharides (EPS) contribute to improving the texture and viscosity of cheese and yoghurt and also receive increasing attention because of their beneficial properties for health. For lactic acid bacteria, the production of EPS is well studied. However, for dairy propionibacteria the biosynthesis of EPS is poorly documented. A polysaccharide synthase-encoding gene was identified in the genome of Propionibacterium freudenreichii subsp. shermanii TL 34 (CIP 103027). This gene best aligns with Tts, the polysaccharide synthase gene of Streptococcus pneumoniae type 37 that is responsible for the production of a beta-glucan capsular polysaccharide. PCR amplification showed the presence of an internal fragment of this gene in twelve strains of P. freudenreichii subsp. shermanii with a ropy phenotype in YEL+ medium. The gene sequence is highly conserved, as less than 1% of nucleotides differed among the 10 strains containing the complete gtf gene. The same primers failed to detect the gene in Propionibacterium acidipropionici strain TL 47, which is known to excrete exopolysaccharides in milk. The presence of (1-->3, 1-->2)-beta-d-glucan capsule was demonstrated for 7 out of 12 strains by agglutination with a S. pneumoniae-type 37-specific antiserum. The presence of mRNA corresponding to the gene was detected by RT-PCR in three strains at both exponential and stationary growth phases. This work represents the first identification of a polysaccharide synthase gene of P. freudenreichii, and further studies will be undertaken to elucidate the role of capsular EPS.     

Research on some factors influencing acid and exopolysaccharide produced by dairy propionibacterium strains isolated from traditional homemade Turkish cheeses

In this study, a total of 32 isolated strains and 5 reference strains of dairy propionibacteria were analyzed for acid and exopolysaccharide (EPS) production in skim milk and yeast extract-lactate broth (YEL) media in order to investigate the physiological background and preservative role of acid and EPS. The effects of final culture pH and optical density on acid and EPS production were also determined. On average, all strains produced more acid and reached lower final pH values in skim milk than in YEL medium. While the correlations obtained between the acid produced by propionibacterium strains and their final culture pH in skim milk medium were significant (P < 0.01), no correlations were found between optical density, final pH, and produced acid in YEL medium. Sixteen isolated and five reference strains of propionibacteria were tested further for the ability to produce propionic and acetic acids. On average, Propionibacterium freudenreichii subsp. shermanii and P. freudenreichii subsp. freudenreichii strains produced higher amounts of propionic and acetic acids than did Propionibacterium jensenii in YEL medium. The acid produced by these strains may be used as a preservative in the food industry for replacement or reduction of the increasing use of chemical additives. The EPS production by propionibacterium strains during growth in YEL medium was 72 to 168 mg/liter, while in skim milk it was 94 to 359 mg/liter. The monomer compositions of the EPSs formed by the six selected dairy propionibacteria strains were analyzed. The EPSs may have applications as food grade additives and viscosity-stabilizing agents.     

Production of acetic and propionic acids from labneh whey by fermentation with propionibacteria

Whey produced during the manufacture of labneh was supplemented with yeast extract (10 g/1), and then fortified with lactose, treated with β-galactosidase or fermented with Lactobacillus helveticus, prior to inoculation with free living cells of Propionibacterium freudenreichii ssp shermanii or Propionibacterium acidipropionici or cells immobilized in aliginate beads. Under anaerobic batch conditions, fermentation of the whey with Lb helveticus followed by P acidipropionici (free cell system) for 2.5 days at 32°C gave a broth with 5.9 g/l of propionic acid and 2.4 gll of acetic acid, while immobilized cells of the same organisms gave a broth with 11.0 gll propionic acid and 3.2 g/l acetic acid over 4 days. These latter values were the maximum levels recorded with any of the treatments, and it is suggested that such yields might make recovery economically feasible in certain countries.     

瑞士奶酪生产工艺研究

以新鲜牛乳为主要原料,根据瑞士奶酪的生产工艺,采用L9(34)正交实验的方法,研究了不同切割尺寸、发酵剂添加比例、发酵剂添加量、发酵温度和切割pH对瑞士奶酪色泽、滋气味和组织状态的影响,确定了生产瑞士奶酪的最佳工艺条件为发酵剂添加量0.02%(质量分数),凝块切割尺寸0.6cm,切割pH6.60,发酵剂(唾液链球菌嗜热亚种∶瑞士乳杆菌∶谢氏丙酸杆菌)添加比例2∶2∶1,发酵温度37℃。     

Genetic manipulation system in propionibacteria

Members of the genus Propionibacterium are widely used in the production of vitamin B12, tetrapyrrole compounds, and propionic acid as well as in probiotic and cheese industries. Shuttle vectors were developed in propionibacteria using replicons from endogenous plasmids in Propionibacterium and Escherichia coli and an appropriate selection marker. The efficient transformation was achieved using the shuttle vector prepared from Propionibacterium freudenreichii to overcome the high restriction modification system in propionibacteria. Expression vectors with native promoters for use in propionibacteria were also developed. Using this system, cholesterol oxidase, which is used as a diagnostic enzyme, was produced in P. freudenreichii. Genes involved in 5-aminolevulinic acid (ALA) and vitamin B12 biosynthesis in propionibacteria were isolated. ALA in propionibacteria could be synthesized via both the C4 pathway (condensation of glycine and succinyl CoA) and the C5 pathway (from glutamate). The hemA gene encoding ALA synthase from Rhodobacter spheroides, was overexpressed and ALA accumulated in P. freudenreichii. Thus, the genetic manipulation systems in propionibacteria will facilitate genetic studies of probiotics and the vitamin B12 biosynthetic pathway.     

Aerobic culture of Propionibacterium freudenreichii ET-3 can increase production ratio of 1,4-dihydroxy-2-naphthoic acid to menaquinone

This is the first report on the production of both 1,4-dihydroxy-2-naphthoic acid (DHNA) and menaquinone by Propionibacterium freudenreichii ET-3. DHNA can be a stimulator of bifidogenic growth, and menaquinone has important roles in blood coagulation and bone metabolism. During anaerobic culture, DHNA and menaquinone concentrations reached 0.18 mM and 0.12 mM, respectively. The molar ratio between these products was approximately 3:2, which was not affected by culture pH and temperature over the ranges of 6.0-7.0 and 31-35 degrees C, respectively. As for organic acid, propionate and acetate accumulated at concentrations of 0.3 M and 0.15 M, respectively, and the propionate accumulation particularly inhibited further production of DHNA. To improve DHNA production, we switched from anaerobic condition to aerobic condition during the culture when lactose was depleted. DHNA concentration continued to increase even after lactose exhaustion, reaching 0.24 mM. In contrast to DHNA production, menaquinone production stopped after the switch to aerobic condition. The total molar production of DHNA and menaquinone was 0.3 mM irrespective of aerobic culture and anaerobic-aerobic switching culture. Therefore, the anaerobic-aerobic switching culture could increase the production ratio of DHNA to menaquinone. The DHNA concentration obtained from the anaerobic-aerobic switching culture was 1.3-fold higher than that in the anaerobic culture, because P. freudenreichii ET-3 utilized propionate accumulated in the medium via the reversed methylmalonyl CoA pathway under aerobic condition. The culture method proposed in this study could be applicable to industrial-scale fermentation using 1000 l of media, by which 0.23 mM DHNA was produced.     

Inhibition of Clostridium tyrobutyricum by bacteriocin-like substances produced by lactic acid bacteria

Lactic acid bacteria were selected for their inhibitory activity against Clostridium tyrobutyricum under conditions that eliminate the effects of lactic acid and hydrogen peroxide. Four strains were isolated belonging to the species Lactococcus lactis ssp. lactis. The sensitivity of the inhibitory substances to pronase and trypsine indicates that they are proteins or peptides different from nisin. Their resistance to phospholipase D indicates that they are also different from lactostrepcin. The inhibitory substances are produced during the exponential phase of growth. Their activity is bactericidal and directed toward some strains of Clostridium tyrobutyricum, Lactobacillus helveticus, and Streptococcus thermophilus, but strains used as dairy starters, Lactobacillus lactis, Streptococcus thermophilus, and Propionibacterium shermanii, are not all affected by the inhibition.     

Ability of Lactobacillus and Propionibacterium strains to remove aflatoxin B, from the chicken duodenum

The ability of Lactobacillus rhamnosus strains GG and LC-705 to remove AFB1 from the intestinal luminal liquid medium has been tested in vivo using a chicken intestinal loop technique. In this study, the GG strain of L. rhamnosus decreased AFB1 concentration by 54% in the soluble fraction of the luminal fluid within 1 min. This strain was more efficient in binding AFB1 compared with L. rhamnosus strain LC-705 (P < 0.05) that removed 44% of AFBl under similar conditions. Accumulation of AFB1 into the intestinal tissue was also determined. There was a 74% reduction in the uptake of AFB1 by the intestinal tissue, in the presence of L. rhamnosus strain GG compared with 63% and 37% in the case of Propionibacterium freudenreichii ssp. shermanii JS and L. rhamnosus strain LC-705, respectively. The complexes formed in vitro between either L. rhamnosus strain GG or L. rhamnosus strain LC-705 and AFB1 were stable under the luminal conditions for a period of 1 h.     

Production of extracellular bifidogenic growth stimulator (BGS) from Propionibacterium shermanii using a bioreactor system with a microfiltration module and an on-line controller for lactic acid concentration

Production of a bifidogenic growth stimulator (BGS) by Propionibacterium freudenreichii subsp. shermanii (Propionibacterium shermanii) using lactic acid as a carbon source was investigated using different cultivation methods. When a continuous bioreactor system with a filtration device was used at a dilution rate of 0.075 h(-1), the average BGS concentration was 2.4 mg/l, which corresponds to a BGS productivity per cultivation time of 1.8 x 10(-1) mg x l(-1) x h(-1). The BGS productivity per cultivation time in continuous cultivation with filtration was 1.9-fold that (9.4 x 10(-2) mg x l(-1).h(-1)) in a conventional batch cultivation. In fed-batch cultivation with feed-back control using an on-line lactic acid controller with a lactic acid biosensor, it was possible to prevent substrate inhibition by maintaining the lactic acid concentration in culture broth low at 3.3 g/l, and an enhanced BGS production (31 mg/l) was successfully attained. The BGS productivity per cultivation time (2.1x10(-1) mg x l(-1) x h(-1)) in the fed-batch cultivation with feed-back control was 2.2-fold that in the conventional batch cultivation. A new bioreactor system was developed by coupling a continuous bioreactor system with a filtration device to an on-line lactic acid controller. Using the new bioreactor system, we produced BGS continuously at a high level of 47 mg/l. The BGS productivities per cultivation time (3.5 mg.l(-1) x h(-1)) and the total volume of medium used (1.7 x 10(-1) mg x l(-1) x h(-1)) obtained in the new bioreactor system were 37-fold and 2.1-fold those in the conventional batch cultivation, respectively. These results described above clearly demonstrate the positive effects of both the continuous filtration for removal of metabolites (propionic and acetic acids) inhibitory to cell growth and feed-back control of lactic acid concentration in the culture broth on BGS production by P. shermanii. This paper is the first report on BGS production by the propionic acid bacterium using lactic acid as a carbon source.     

费氏丙酸杆菌费氏亚种在不同基质中转化生成共轭亚油酸的研究

研究了在MRS、乳酸钠和脱脂乳三种培养基质下温度、时间、底物浓等因素对P.freudenreichiissp.freudenreichii生成CLA的影响,结果表明P.freudenreichiissp.freudenreichii在MRS、乳酸钠和脱脂乳中均具有生成CLA的能力,三种培养基质中一定浓度的葵花籽油对P.freudenreichiissp.freudenreichii的生长都有明显的抑制作用,在MRS培养基中葵花籽油浓度为12mg/ml,菌液量10%(V/V),30℃培养24hCLA生成量最大为24.813μg/ml。     

Production of extracellular bifidogenic growth stimulator by anaerobic and aerobic cultivations of several propionibacterial strains

Production of a bifidogenic growth stimulator (BGS) by propionic acid bacteria was investigated under anaerobic and aerobic culture conditions. To measure the concentration of extracellular BGS produced by propionic acid bacteria, we evaluated the effects of bioassay conditions using Bifidobacterium longum as a test microorganism on the formation of a growth-stimulation zone. The diameter of the growth-stimulation zone was significantly affected by both the component concentrations and the pH of a bioassay medium. The optimum component concentrations and pH of a bioassay medium were one-half of the normal values and 8.5, respectively. Using the bioassay method, we can measure the concentration of BGS produced by propionic acid bacteria ranging in concentrations from 0.1 microg/l to 1 mg/l using 1,4-dihydroxy-2-naphthoic acid (DHNA) and 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) as standards. Of six dairy propionic acid bacterial strains tested, the four strains (Propionibacterium freudenreichii ET-3, P. shermanii PZ-3, P. acidipropionici JCM 6432, and P. jensenii JCM 6433) produced BGS at a concentration range of 4-23 mg/l under the anaerobic culture conditions. Analysis of high performance liquid chromatography (HPLC) showed that more than 70% of total BGS produced in supernatant samples was DHNA and no ACNQ was produced by the strains. The effect of oxygen supply on BGS production was investigated for the four BGS-producing strains. The aerobic conditions exerted in positive effects on BGS production by only P. acidipropionici JCM 6432. The concentration of BGS obtained in the aerobic cultivation of P. acidipropionici JCM 6432 was 1.3-fold than that in anaerobic cultivation. Different properties (BGS production as well as cell growth and glucose metabolism) occurring in response to the aerobic conditions were observed, depending on the propionic acid bacterial strain used. This paper is the first report on BGS production by propionibacterial strains except for P. freudenreichii.     

Antifungal effect of dairy propionibacteria--contribution of organic acids

Large amounts of food and feed are lost every year due to spoilage by moulds and yeasts. Biopreservation, i.e. the use of microorganisms as preservatives instead of chemicals, has gained increased interest. Lactic acid bacteria and propionibacteria might be particularly useful due to their important role in many food fermentations. Knowledge of the antifungal effects of the organic acids produced by these bacteria is necessary to understand their inhibitory activity. We evaluated the antifungal activity of the type strains of five dairy propionibacteria, Propionibacterium acidipropionici, P. jensenii, P. thoenii, P. freudenreichii subsp. freudenreichii and P. freudenreichii subsp. shermanii against eight food- and feedborne moulds and yeasts. A dual culture system assayed the inhibitory activity on three different agar media, sodium lactate (SL), de Man Rogosa Sharp (MRS) and MRS without acetate (MRS-ac). The amounts of organic acids produced during growth of propionibacteria in liquid SL, MRS and MRS-ac were also determined. The minimal inhibitory concentration (MIC) values of propionic, acetic and lactic acid were established for all fungi at pH 3, 5 and 7. Propionic acid, followed by acetic acid, was the most potent antifungal acid. Inhibition at pH 7 generally required concentrations above 500 mM for all three acids, at pH 5 the MIC values for propionic and acetic acids were 20-120 mM and above 500 mM for lactic acid. At pH 3, the MIC values were, with one exception, below 10 mM for both propionic and acetic acid and above 160 mM for lactic acid. The yeast Pichia anomala was the fungus most resistant to organic acids. The propionibacteria exhibited a pronounced species variation in antifungal activity on MRS (+/-acetate) agar, with P. thoenii being the most potent. Four of the five propionibacteria species produced more propionic and acetic acid in liquid SL medium than in MRS (+/-acetate) broth. However, when SL agar was used as the growth medium, none of the propionibacteria inhibited fungal growth.     

Removal of common Fusarium toxins in vitro by strains of Lactobacillus and Propionibacterium

This study was conducted to examine the ability of selected strains of Lactobacillus and Propionibacterium to remove common Fusarium toxins, trichothecenes, from liquid media. The trichothecenes studied were deoxynivalenol (DON), 3-acetyldeoxynivalenol (3-AcDON), nivalenol (NIV), fusarenon (FX), diacetoxyscirpenol (DAS), T-2 toxin (T-2) and HT-2 toxin (HT-2). The Lactobacillus rhamnosus strain GG (LGG), Lactobacillus rhamnosus strain LC-705 (LC-705) and Propionibacterium freudenreichii ssp. shermanii JS (PJS) were incubated in PBS buffer containing 20 µg toxin ml -1 for 1h at 37°C, and after centrifugation the concentration of the toxins was measured in the supernatant fraction. Both viable and heat-killed forms of LGG and PJS were more efficient than LC-705 in removing the toxins from the liquid media. LGG and PJS removed four of the seven tested toxins (the removal varying from 18 to 93%) and LC-705 two toxins (10-64%). Of the toxins, 3-AcDON was not removed by any of the bacteria; HT-2 was removed by the non-viable LGG and also slightly by non-viable LC-705; DAS was removed by all three bacteria tested. Binding is postulated as the possible mechanism of the removal, since no difference was observed between the ability of viable and heat-killed bacteria in removing the trichothecenes, and no degradation products of the toxins were detected by gas chromatography (GC)-mass spectrometry (MS) analysis. It is concluded that significant differences exist in the ability of the bacteria to bind trichothecenes in vitro.     

Autolysis of propionibacteria: detection of autolytic enzymes by renaturing SDS-PAGE and additional buffer studies

Five strains of propionibacteria with 70-90% autolysis in sodium lactate broth (SLB) were studied by renaturing sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Several lytic bands ranging in size between 25 and 143 kDa were detected by using propionibacteria cells or cell walls as substrate in the gel. Four Propionibacterium freudenreichii strains showed similar autolytic-enzyme profiles, consisting of two autolytic bands, one with molecular mass 162 kDa and one in the range 123-143 kDa. However, the Propionibacterium acidipropionici strain showed a completely different profile, consisting of 8 autolytic bands with molecular masses of 122, 97, 71, 55, 43, 39, 31, and 25 kDa. Lytic enzymes from P. freudenreichii INF-alpha, P. freudenreichii ISU P-59, P. freudenreichii ISU P-24, and P. freudenreichii ISU P-50 showed lytic activity against cells from all these four strains, but not against P. acidipropionici ATCC 4965. However, P. acidipropionici ATCC 4965 autolysed only its own cells. Effects of pH, temperature, and ions on autolytic activity were tested by renaturing SDS-PAGE and in buffer systems. Results from the SDS-PAGE electrophoresis showed optimal autolytic activity of P. acidipropionici ATCC 4965 at 37 degrees C and in the pH range 7 to 8.5 and of P. freudenreichii ISU P-59 at 20 degrees C and in the pH range 5 to 7. The autolytic activity of P. acidipropionici ATCC 4965 was extremely heat stable (100 degrees C, 2 h), in contrast to the lytic activity of P. freudenreichii ISU P-59, which was heat labile. The autolytic activities of P. acidipropionici ATCC 4965 were inhibited by divalent cations, however, the lytic activities of P. freudenreichii ISU P-59 were activated by Mn(2+), Ca(2+), and Co(2+). In buffer, optimum autolysis of P. acidipropionici ATCC 4965 was observed at pH 8.5 and at 40 degrees C. P. freudenreichii ISU P-59 showed optimum autolysis in buffer at pH 7.5 and at 30 degrees C.     

鲜牛奶中丙酸杆菌的筛选、鉴定及特性分析

从新鲜生牛奶中分离筛选产丙酸较高的菌株,经形态学特征、生理生化及糖发酵试验、16S rDNA基因序列同源性分析以及多位点序列分型(multilocus sequence typing)等实验鉴定该菌株,分析了其对不同碳源的利用率及产酸情况,并从溶血性试验及抗生素抗性试验方面来评估该菌株的安全性。结果表明,从4批次样品中共分离获得54株能产丙酸的菌株,其中一株菌的丙酸产量达到7.38 g/L,为费氏丙酸杆菌(Propionibacterium freudenreichii)。比较了葡萄糖和甘油对其生长的影响,发现其对葡萄糖的利用率大于对甘油的利用率,在含葡萄糖的培养基中于30 ℃厌氧培养120 h后丙酸产量达到7.38 g/L。该菌株无溶血性,对卡那霉素等氨基糖苷类抗生素有耐药性,对氨苄西林、万古霉素、氯霉素、克林霉素、四环素敏感,对红霉素中介。综上,费氏丙酸杆菌B1具有一定的潜在应用价值。     

Assessment of root uptake and systemic vine-transport of Salmonella enterica sv. Typhimurium by melon (Cucumis melo) during field production

Bifidobacterium adolescentis Int57 (Int57) and Propionibacterium freudenreichii subsp. shermanii ATCC 13673 (ATCC 13673) were grown either in coculture or as pure cultures in different media, such as cow's milk, soybean milk, and modified MRS medium. The viable cell counts of bacteria, changes in pH, concentrations of organic acids, and contents of various sugars were analyzed during incubation up to 7days. In soy milk, the survival of cocultured Int57 was six times higher than the monocultured cells, and ATCC 13673 cocultured with Int57 consumed 69.4% of lactic acid produced by Int57 at the end of fermentation. In cow's milk, coculture with ATCC 13673 increased the growth of Int57 from 24h until 120h by approximately tenfold and did not affect the survival of Int57 cells. After 96h of fermentation of modified MRS, the survival of ATCC 13673 cells cocultured with Int57 increased by 3.2- to 7.4-folds as compared with ATCC 13673 monoculture, whereas the growth of Int57 cells was unaffected. The growth and metabolic patterns of two strains during coculture showed noticeable differences between food grade media and laboratory media. The consumption of stachyose in soy milk during coculture of Int57 with ATCC 13673 was increased by more than twice compared with Int57 monoculture, and completed within 24h. The combinational use of Bifidobacterium and Propionibacterium could be applied to the development of fermented milk or soy milk products.     

丙酸对费氏丙酸杆菌生长及脱氧腺苷钴胺素合成的影响

通过添加控制丙酸浓度考查了费氏丙酸杆菌发酵生产VB12的过程中丙酸抑制细胞生长的浓度范围,在此基础上利用树脂对发酵过程中丙酸进行了选择性吸附后细胞的生长和脱氧腺苷钴胺素合成的变化。研究结果表明,丙酸的浓度在10.0g/L时,对菌体细胞生长产生了明显的抑制。在丙酸浓度积累到10.0g/L之前,利用树脂对其吸附分离出2.5g/L的丙酸后,生物量提高了37.5%,脱氧腺苷钴胺素产量提高了50.0%。该实验为实现丙酸的在线分离和丙酸/VB12高效联产的新型耦合发酵工艺提供了基础。     

Effect of Lactobacillus helveticus and Propionibacterium freudenrichii ssp. shermanii combinations on propensity for split defect in Swiss cheese

One of the least controlled defects in Swiss cheese is development of splits that appear during refrigerated storage after cheese is removed from the warm room. Such fissures, or cracks, in the body of the cheese can be as short as 1 cm, or long enough to span a 90-kg block. A 2 x 2 x 2 factorial experiment was used to determine the effect of different Lactobacillus helveticus/Propionibacterium freudenreichii ssp. shermanii starter culture combinations on the occurrence of split defect in Swiss cheese. Eights vats of cheese were made in summer and eight in winter. Each 90-kg block of cheese was cut into twenty-four 4-kg blocks and graded based on the presence of splits. Only small variations were found in the composition of cheeses made during the same season. There were no correlations between moisture, pH, fat, protein, calcium, lactose contents, D/L lactate ratio, or protein degradation that could be used to predict splits after 90 d of storage. However, cheese made in the summer had 2% higher moisture content and a greater prevalence of splits. There was a sixfold increase in amount of downgraded cheese between the best and worst culture combinations used during cheese manufacture. After 90-d storage, 14 to 90% of cheese had splits in the summer, and 1 to 6% in the winter. Split formation increased with time from 60 to 120 d of storage and extent of split formation was influenced by both the lactobacilli and propionibacteria cultures used.     

一株费氏丙酸杆菌生长特性及其代谢物抑菌作用分析

对一株费氏丙酸杆菌生长特性及其代谢物抑菌作用进行研究。结果表明：该菌株生长周期约为10 d,第1～3天为其对数生长期,第4～8天为菌体生长的稳定期,第9～10天为衰亡期。在生长过程中,菌体利用糖的速率很快,还原糖在发酵1 d内几乎被消耗殆尽,发酵液pH值在发酵的第1天下降到4.6左右,此后一直到第10天,pH值基本稳定在4.6左右。在同等条件下,该丙酸杆菌代谢物对大肠杆菌的抑制作用优于对金黄色葡萄球菌和酵母菌的抑制作用。对代谢产物的分析结果表明,发酵产物中抑菌物质除了丙酸之外,还存在其他物质,初步判定为细菌素。