Regulation of beta-galactoside transport and accumulation in heterofermentative lactic acid bacteria.

Galactose-grown cells of the heterofermentative lactic acid bacteria Lactobacillus brevis and Lactobacillus buchneri transported methyl-beta-D-thiogalactopyranoside (TMG) by an active transport mechanism and accumulated intracellular free TMG when provided with an exogenous source of energy, such as arginine. The intracellular concentration of TMG resultant under these conditions was approximately 20-fold higher than that in the medium. In contrast, the provision of energy by metabolism of glucose, gluconate, or glucosamine promoted a rapid but transient uptake of TMG followed by efflux that established a low cellular concentration of the galactoside, i.e., only two- to fourfold higher than that in the medium. Furthermore, the addition of glucose to cells preloaded with TMG in the presence of arginine elicited a rapid efflux of the intracellular galactoside. The extent of cellular TMG displacement and the duration of the transient effect of glucose on TMG transport were related to the initial concentration of glucose in the medium. Exhaustion of glucose from the medium restored uptake and accumulation of TMG, providing arginine was available for ATP generation. The nonmetabolizable sugar 2-deoxyglucose elicited efflux of TMG from preloaded cells of L. buchneri but not from those of L. brevis. Phosphorylation of this glucose analog was catalyzed by cell extracts of L. buchneri but not by those of L. brevis. Iodoacetate, at a concentration that inhibits growth and ATP production from glucose, did not prevent efflux of cellular TMG elicited by glucose. The results suggested that a phosphorylated metabolite(s) at or above the level of glyceraldehyde-3-phosphate was required to evoke displacement of intracellular TMG from the cells. Counterflow experiments suggested that glucose converted the active uptake of TMG in L. brevis to a facilitated diffusion mechanism that allowed equilibrium of TMG between the extra- and intracellular milieux. The means by which glucose metabolites elicited this vectorial regulation is not known, but similarities to the inducer expulsion that has been described for homofermentative Streptococcus and Lactobacillus species suggested the involvement of HPr, a protein that functions as a phosphocarrier protein in the phosphotransferase system, as well as a presumptive regulator of sugar transport. Indeed, complementation assays wit extracts of Staphylococcus aureus ptsH mutant revealed the presence of HPr in L. brevis, although this lactobacillus lacked a functional phaosphoenolpyruvate-dependent phosphortransferase system for glucose, 2-deoxyglucose, or TMG.     

Variations in the energy metabolism of biotechnologically relevant heterofermentative lactic acid bacteria during growth on sugars and organic acids

Heterofermentative lactic acid bacteria (LAB) such as Leuconostoc, Oenococcus, and Lactobacillus strains ferment pentoses by the phosphoketolase pathway. The extra NAD(P)H, which is produced during growth on hexoses, is transferred to acetyl-CoA, yielding ethanol. Ethanol fermentation represents the limiting step in hexose fermentation, therefore, part of the extra NAD(P)H is used to produce erythritol and glycerol. Fructose, pyruvate, citrate, and O₂ can be used in addition as external electron acceptors for NAD(P)H reoxidation. Use of the external acceptors increases the growth rate of the bacteria. The bacteria are also able to ferment organic acids like malate, pyruvate, and citrate. Malolactic fermentation generates a proton potential by substrate transport. Pyruvate fermentation sustains growth by pyruvate disproportionation involving pyruvate dehydrogenase. Citrate is fermented in the presence of an additional electron donor to acetate and lactate. Thus, heterofermentative LAB are able to use a variety of unusual fermentation reactions in addition to classical heterofermentation. Most of the reactions are significant for food biotechnology/microbiology.     

通气对乳酸发酵过程的影响

对拟干酪乳杆菌发酵产乳酸的过程进行研究,通过改变不同的通气量(不通气、0.1vvm、0.2 vvm、0.5 vvm)确定0.1vvm的通气量最有利于产生乳酸;再通过优化通气策略,在发酵0～15 h不通空气,15～50 h通0.1 vvm空气使得乳酸的产量比全程通0.1 vvm空气又提高了11.7%,同时乳酸产率也提高了16.2%。最后通过对胞内NAD~+、NADH、乳酸脱氢酶和NADH氧化酶活性、以及发酵过程氧化还原电位(Oxidation-reduction potential,ORP)变化进行分析,阐述了通气影响乳酸发酵过程的机理。     

Cancer chemopreventive effects of lactic acid bacteria

Lactic acid bacteria (LAB) provide several potential health and nutritional benefits, including improving the nutritional value of food, controlling serum cholesterol levels, and controlling some types of cancer. Numerous in vitro, in vivo, human, and epidemiological studies have provided evidence of the chemopreventive effects of LAB on colon, bladder, liver, breast, and gastric cancers. These effects act via diverse mechanisms, including alteration of the gastrointestinal microflora, enhancement of the host's immune response, and antioxidative and antiproliferative activities. This review discusses the recent progresses on the chemopreventive effects of LAB on specific cancer types and the underlying molecular mechanisms.     

Nicotinamide adenine dinucleotide-dependent and nicotinamide adenine dinucleotide-independent lactate dehydrogenases in homofermentative and heterofermentative lactic acid bacteria

Three homofermentative (Lactobacillus plantarum B38, L. plantarum B33, Pediococcus pentosaceus B30) and three heterofermentative (Leuconostoc mesenteroides 39, L. oenos B70, Lactobacillus brevis) lactic acid bacteria were examined for the presence or absence of nicotinamide adenine dinucleotide (NAD)-dependent and NAD-independent d- and l-lactate dehydrogenases. Two of the six strains investigated, P. pentosaceus and L. oenos, did not exhibit an NAD-independent enzyme activity capable of reducing dichlorophenol indophenol. The pH optima of the lactic dehydrogenases were determined. The NAD-dependent enzymes from homofermentative strains exhibited optima at pH 7.8 to 8.8, whereas values from 9.0 to 10.0 were noted for these enzymes from heterofermentative organisms. The optima for the NAD-independent enzymes were between 5.8 and 6.6. The apparent Michaelis-Menten constants determined for both NAD and the substrates demonstrated the existence of a greater affinity for d- than l-lactic acid. A comparison of the specific NAD-dependent and NAD-independent lactate dehydrogenase activities revealed a direct correlation of the d/l ratios of these activities with the type of lactic acid produced during the growth of the organism.     

Studies on the effects of low temperatures on lactic acid bacteria

Studies were conducted on different strains of L. bulgaricus, L. casei, S. thermophilus, S. lactis, and S. cremoris isolated in Bulgaria and applied as pure cultures and in combinations as starters. All the strains under investigation were found to preserve, on “freezing-thawing” their characteristic morphological and biochemical properties, regardless of the temperature and rate of cooling, but the optimum freezing temperature of the strains studied is ?196 °C (in liquid nitrogen). High cooling rates provide higher viability and activity of lactic acid bacterial cells. Lactic acid streptococci, S. lactis and S. thermophilus, are considerably more resistant than lactic acid rods, L. casei and L. bulgaricus, at all the freezing regimens tested.     

A differential medium for the enumeration of homofermentative and heterofermentative lactic Acid bacteria

A medium was developed for the differential enumeration of homofermentative and heterofermentative lactic acid bacteria. Essential components of the medium included fructose (14 mM), KH(2)PO(4) (18 mM), bromcresol green (as a pH indicator), and other nutrients to support growth. In agar medium, homofermentative colonies were blue to green, while heterofermentative colonies remained white. A total of 21 Lactobacillus, Pediococcus, Leuconostoc, and Streptococcus species were correctly classified with the medium.     

The genomics of lactic acid bacteria

The lactic acid bacteria (LAB) are one of the most industrially important groups of bacteria. These organisms are used in a variety of ways, including food production, health improvement and production of macromolecules, enzymes and metabolites. The genome sequencing of 20 LAB provides an expanded view of their genetic and metabolic capacities and enables researchers to perform functional and comparative genomic studies. This review highlights some of the findings from these analyses in the context of the numerous roles the LAB play.     

Ability of some strains of lactic acid bacteria to degrade phytic acid

Twelve strains of lactic acid bacteria were examined for their ability to degrade phytate. In media in which phytic acid was the source of phosphate, phytate degradation was observed. Phytate disappearance may however not only be due to phytase, as phytic acid coprecipitated with protein as a consequence of a fall in pH during fermentation.     

Evaluation of lactic acid bacteria autolysate for the supplementation of lactic acid bacteria fermentation

At the end of culture in a carbon-limited medium, i.e. the best conditions for subsequent autolysis, lactic acid bacteria were harvested and autolysed at 50 °C for 24 h. The resulting supernatant was then successfully tested as a substitute for industrial yeast extract for the supplementation of whey permeate and its conversion into lactic acid: for almost equivalent total nitrogen amounts of both supplements, the same growth and production rates were recorded.