Lactic acid bacteria (LAB) constitute a group of gram-positive bacteria united by a constellation of morphological, metabolic, and physiological characteristics The general description of the bacteria included in the group is gram-positive, nonsporing, nonrespiring cocci or rods, which produce lactic acid as the major end product during the fermentation of carbohydrates The LAB term is intimately associated with bacteria involved in food and feed fermentation, including related bacteria normally associated with the (healthy) mucosal surfaces of humans and animals The boundaries of the group have been subject to some controversy, but historically the genera Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus form the core of the group Taxonomic revisions of these genera and the description of new genera mean that LAB could, in their broad physiological definition, comprise around 20 genera However, from a practical, food-technology point of view, the following genera are considered the principal LAB: Aerococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus, Tetragenococcus, Vagococcus, and Weissella The genus Bifidobacterium, often considered in the same context as the genuine lactic acid bacteria and sharing some of their typical features, is phylogenetically unrelated and has a unique mode of sugar fermentation The classification of lactic acid bacteria into different genera is largely based on morphology, mode of glucose fermentation, growth at different temperatures, configuration of the lactic acid produced, ability to grow at high salt concentrations, and acid or alkaline tolerance Chemotaxonomic markers such as fatty acid composition and constituents of the cell wall are also used in classification In addition, the present taxonomy relies partly on true phylogenetic relationships,

Lactic acid bacteria: classification and physiology.

The degradation of lactic acid under anoxic conditions was studied in several strains of Lactobacillus buchneri and in close relatives such as Lactobacillus parabuchneri, Lactobacillus kefir, and Lactobacillus hilgardii. Of these lactobacilli, L. buchneri and L. parabuchneri were able to degrade lactic acid under anoxic conditions, without requiring an external electron acceptor. Each mole of lactic acid was converted into approximately 0.5 mol of acetic acid, 0.5 mol of 1,2-propanediol, and traces of ethanol. Based on stoichiometry studies and the high levels of NAD-linked 1, 2-propanediol-dependent oxidoreductase (530 to 790 nmol min(-1) mg of protein(-1)), a novel pathway for anaerobic lactic acid degradation is proposed. The anaerobic degradation of lactic acid by L. buchneri does not support cell growth and is pH dependent. Acidic conditions are needed to induce the lactic-acid-degrading capacity of the cells and to maintain the lactic-acid-degrading activity. At a pH above 5.8 hardly any lactic acid degradation was observed. The exact function of anaerobic lactic acid degradation by L. buchneri is not certain, but some results indicate that it plays a role in maintaining cell viability.

https://aem.asm.org/content/aem/67/1/125.full.pdf

Anaerobic Conversion of Lactic Acid to Acetic Acid and 1,2-Propanediol by Lactobacillus buchneri

1,3-Propanediol (1,3-PD) production by fermentation of glycerol was described in 1881 but little attention was paid to this microbial route for over a century. Glycerol conversion to 1,3-PD can be carried out by Clostridia as well as Enterobacteriaceae. The main intermediate of the oxidative pathway is pyruvate, the further utilization of which produces CO2, H2, acetate, butyrate, ethanol, butanol and 2,3-butanediol. In addition, lactate and succinate are generated. The yield of 1,3-PD per glycerol is determined by the availability of NADH2, which is mainly affected by the product distribution (of the oxidative pathway) and depends first of all on the microorganism used but also on the process conditions (type of fermentation, substrate excess, various inhibitions). In the past decade, research to produce 1,3-PD microbially was considerably expanded as the diol can be used for various polycondensates. In particular, polyesters with useful properties can be manufactured. A prerequisite for making a “green” polyester is a more cost-effective production of 1,3-PD, which, in practical terms, can only be achieved by using an alternative substrate, such as glucose instead of glycerol. Therefore, great efforts are now being made to combine the pathway from glucose to glycerol successfully with the bacterial route from glycerol to 1,3-PD. Thus, 1,3-PD may become the first bulk chemical produced by a genetically engineered microorganism.

Microbial production of 1,3-propanediol.

Microbial production of 1,3-propanediol: recent developments and emerging opportunities.

Malolactic fermentation (MLF) in wine is a secondary fermentation that usually occurs at the end of alcoholic fermentation by yeasts, although it sometimes occurs earlier. It is practically a biological process of wine deacidification in which the dicarboxylic L-malic acid (malate) is converted to the monocarboxylic L-lactic acid (lactate) and carbon dioxide (Davis et al. 1985). Deacidification is particularly desirable for high-acid wine produced in cool-climate regions, such as New Zealand and Canada. This process is normally carried out by lactic acid bacteria (LAB) isolated from wine, including Oenococcus oeni (formerly Leuconostoc oenos; Dicks et al. 1995), Lactobacillus spp. and Pediococcus spp. (Wibowo et al. 1985). Various technologies, such as bioreactors with high-density cells and immobilized cells or enzymes, have been developed to facilitate wine deacidification (Maicas 2001). Oenococcus oeni is the preferred species used to conduct MLF due to its acid tolerance and flavour profile produced. In addition to its occurrence in wine, MLF occurs in other fermented beverages, such as cider (Carr 1987; Jarvis et al. 1995). Numerous reviews on MLF have been published since the 1960s, covering the history of research on MLF, taxonomy, ecology, genetics and physiology of wine LAB and the oenological significance of MLF (Bartowsky and Henschke 1995; Kunkee 1967, 1974, 1991; Lafon-Lafourcade 1983; Davis et al. 1985; Wibowo et al. 1985; Henick-Kling 1988, 1993, 1995; Henschke 1993; van Vuuren and Dicks 1993; Lonvaud-Funel 1995, 1999; Versari et al. 1999). However, a review that is specifically focused on the metabolic capacity of wine LAB and its impact on wine quality is still lacking. Furthermore, new information on the metabolism of wine LAB has emerged since the publication of these reviews. It is now accepted that the role of MLF is more than just a deacidification process, although deacidification via MLF is still a primary objective in wine fermentation in cool-climate regions. The complexity and diversity of the metabolic activity of LAB suggest that MLF may affect wine quality both positively and negatively. Knowledge of the metabolism of wine LAB is necessary to assess the impact of MLF on wine quality. The primary focus of this review is to evaluate the current knowledge of the metabolism of wine LAB (predominantly oenococci) with a view to stimulating further research in this area. The author also aims to assess the potential impact of MLF on wine flavour, mouthfeel/body and human health in relation to the metabolism of wine LAB. References are made with respect to the metabolism of LAB from other fermented beverages and foods, such as cider and cheese.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2002.01589.x

Malolactic fermentation in wine – beyond deacidification

The simultaneous fermentation of glycerol and sugar by lactobacillus brevis B22 and Lactobacillus buchneri B190 increases both the growth rate and total growth. The reduction of glycerol to 1,3-propanediol by the lactobacilli was found to influence the metabolism of the sugar cofermented by channelling some of the intermediate metabolites (e.g., pyruvate) towards NADH-producing (rather than NADH-consuming) reactions. Ultimately, the absolute requirement for NADH to prevent the accumulation of 3-hydroxypropionaldehyde leads to a novel lactate-glycerol cofermentation. As a result, additional ATP can be made not only by (i) converting pyruvate to acetate via acetyl phosphate rather than to the ethanol usually found and (ii) oxidizing part of the intermediate pyruvate to acetate instead of the usual reduction to lactate but also by (iii) reoxidation of accumulated lactate to acetate via pyruvate. The conversion of lactate to pyruvate is probably catalyzed by NAD-independent lactate dehydrogenases that are found only in the cultures oxidizing lactate and producing 1,3-propanediol, suggesting a correlation between the expression of these enzymes and a raised intracellular NAD/NADH ratio. The enzymes metabolizing glycerol (glycerol dehydratase and 1,3-propanediol dehydrogenase) were expressed in concert without necessary induction by added glycerol, although their expression may also be influenced by the intracellular NAD/NADH ratio set by the different carbohydrates fermented.

M A Foster

Papers

Sugar-glycerol cofermentations in lactobacilli: the fate of lactate.