Increasing Acid Tolerance of an Engineered Lactic Acid Bacterium Pediococcus acidilactici for L-Lactic Acid Production

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.

Potential influence of dairy propionibacteria on the growth and acid metabolism of Streptococcus bovis and Megasphaera elsdenii

Ruminal acidosis is a prevalent disorder among dairy cows and feedlot cattle, which can significantly impair their health and productivity. This study, involving seven different strains of dairy propionibacteria, represents an in vitro investigation of the feasibility of using these organisms as direct-fed microbials to control lactic acid acumulation in the rumen. Interactions between the propionibacteria, Streptococcus bovis and Megasphaera elsdenii were evaluated in terms of effects on lactic, acetic and propionic acid metabolism, following co-incubation. Spot resistance tests showed slight but varying degrees of growth inhibition by S. bovis among the propionibacteria, while no inhibition was observed between M. elsdenii and the different strains of dairy propionibacteria. In the co-culture experiments comprising S. bovis in nutrient broth, significant differences in pH and the levels of production of lactic, acetic and propionic acid, were observed between treatments following inoculation with various propionibacteria and/or M. elsdenii. In general, lactic acid concentrations at the end of the incubation were significantly lower in the cultures containing propionibacteria compared with cultures comprising either S. bovis only or S. bovis + M. elsdenii, although efficacy of lactate metabolism varied between species and strains. Moreover,the accumulation of acetic and propionic acid in the combined cultures, but not in the solo S. bovis culture, indicated that these compounds were produced as a result of the metabolism of lactic acid by the propionibacteria and M. elsdenii.

Isolation and characterization of acid-sensitive Lactobacillus plantarum with application as starter culture for Nham production

The aim of this study was to derive new starter culture variants that are unable to grow below pH 4.6, the desirable pH of the Thai fermented pork sausage, Nham, specified by Thailand Food Standard, and apply them in Nham fermentation. Several acid-sensitive mutants of one of the commercial Nham starter cultures, Lactobacillus plantarum BCC 9546, were isolated as spontaneous neomycin-resistant mutants. The growth of three representative mutants was characterized in MRS broth, which revealed that their cell numbers and acid production were lower than that of the wild-type. The H(+)-ATPase activities of the three mutants were found significantly lower than that of the wild-type under either neutral or acidic conditions. Consequently, internal pH values of the mutants appeared to be lower, especially in acidic environment (pH 5). The most acid-sensitive mutant was applied in experimental Nham production and the pH of Nham fermented with the mutant had significantly higher pH at the end of fermentation (3 days) and after an additional 4 days of storage at 30 degrees C. These results indicate that the use of acid-sensitive L. plantarum as starter culture can reduce the severity of post-acidification and increase the shelf life of Nham at ambient temperature.

Engineering Propionibacterium acidipropionici for enhanced propionic acid tolerance and fermentation

Propionibacterium acidipropionici, a Gram-positive, anaerobic bacterium, has been the most used species for propionic acid production from sugars. In this study, the metabolically engineered mutant ACK-Tet, which has its acetate kinase gene knocked out from the chromosome, was immobilized and adapted in a fibrous bed bioreactor (FBB) to increase its acid tolerance and ability to produce propionic acid at a high final concentration in fed-batch fermentation. After about 3 months adaptation in the FBB, the propionic acid concentration in the fermentation broth reached approximately 100 g/L, which was much higher than the highest concentration of approximately 71 g/L previously attained with the wild-type in the FBB. To understand the mechanism and factors contributing to the enhanced acid tolerance, adapted mutant cells were harvested from the FBB and characterized for their morphology, growth inhibition by propionic acid, protein expression profiles as observed in SDS-PAGE, and H+-ATPase activity, which is related to the proton pumping and cell's ability to control its intracellular pH gradient. The adapted mutant obtained from the FBB showed significantly reduced growth sensitivity to propionic acid inhibition, increased H+-ATPase expression and activity, and significantly elongated rod morphology.

Yoghurt fermented by Lactobacillus delbrueckii subsp. bulgaricus H+-ATPase-defective mutants exhibits enhanced viability of Bifidobacterium breve during storage

Persistent acid production by Lactobacillus delbrueckii subsp. bulgaricus during refrigerated storage is a major cause of reduced viability of probiotic strains such as Bifidobacterium breve in yoghurt. It was established that H+ -ATPase-defective mutants of lactic acid bacteria have reduced growth and metabolism in low pH environments. Therefore, the aim of this study was to evaluate inhibition of post-acidification and maintenance of B. breve viability in yoghurt fermented by L. delbrueckii subsp. bulgaricus mutants with reduced membrane-bound H+ -ATPase activity during refrigerated storage. Spontaneous neomycin mutants of L. delbrueckii subsp. bulgaricus that had a significantly (P < or = 0.05) reduced H+ -ATPase activity were successfully isolated. Yoghurt fermented using L. delbrueckii subsp. bulgaricus SBT0164 No. 55-1 (mutant) starter culture had markedly reduced post-acidification and maintained viability (> or = 10(8) CFU/ml) of both Bifidobacteruim breve JCM 1192(T) and Bifidobacteruim breve JCM 7017 during storage at 10 degrees C for 21 days. These results clearly showed that yoghurt fermented by mutants of L. delbrueckii subsp. bulgaricus with reduced membrane-bound H+ -ATPase activity has reduced post-acidification that prolongs viability of B. breve in yoghurt during refrigerated storage.