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.

Factors affecting the fermentative lactic acid production from renewable resources1

http://sgpwe.izt.uam.mx/files/users/uami/acym/articulo_1.pdf

Food fermentations: Microorganisms with technological beneficial use

A small number of flavoprotein oxidase enzymes are responsible for the direct interaction of lactic acid bacteria (LAB) with oxygen; hydrogen peroxide or water are produced in these reactions. In some cultures exposed to oxygen, hydrogen peroxide accumulates to inhibitory levels. Through these oxidase enzymes and NADH peroxidase, O2 and H2O2 can accept electrons from sugar metabolism, and thus have a sparing effect on the use of metabolic intermediates, such as pyruvate or acetaldehyde, as electron acceptors. Consequently, sugar metabolism in aerated cultures of LAB can be substantially different from that in unaerated cultures. Energy and biomass yields, end-products of sugar metabolism and the range of substrates which can be metabolised are affected. Lactic acid bacteria exhibit an inducible oxidative stress response when exposed to sublethal levels of H2O2. This response protects them if they are subsequently exposed to lethal concentrations of H2O2. The effect appears to be related to other stress responses such as heat-shock and is similar, in some but not all respects, to that previously reported for enteric bacteria.

Responses of lactic acid bacteria to oxygen

During batch growth of Lactococcus lactis subsp. lactis NCDO 2118 on various sugars, the shift from homolactic to mixed-acid metabolism was directly dependent on the sugar consumption rate. This orientation of pyruvate metabolism was related to the flux-controlling activity of glyceraldehyde-3-phosphate dehydrogenase under conditions of high glycolytic flux on glucose due to the NADH/NAD+ ratio. The flux limitation at the level of glyceraldehyde-3-phosphate dehydrogenase led to an increase in the pool concentrations of both glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate and inhibition of pyruvate formate lyase activity. Under such conditions, metabolism was homolactic. Lactose and to a lesser extent galactose supported less rapid growth, with a diminished flux through glycolysis, and a lower NADH/NAD+ ratio. Under such conditions, the major pathway bottleneck was most probably at the level of sugar transport rather than glyceraldehyde-3-phosphate dehydrogenase. Consequently, the pool concentrations of phosphorylated glycolytic intermediates upstream of glyceraldehyde-3-phosphate dehydrogenase decreased. However, the intracellular concentration of fructose-1,6-bisphosphate remained sufficiently high to ensure full activation of lactate dehydrogenase and had no in vivo role in controlling pyruvate metabolism, contrary to the generally accepted opinion. Regulation of pyruvate formate lyase activity by triose phosphates was relaxed, and mixed-acid fermentation occurred (no significant production of lactate on lactose) due mostly to the strong inhibition of lactate dehydrogenase by the in vivo NADH/NAD+ ratio.

Control of the shift from homolactic acid to mixed-acid fermentation in Lactococcus lactis: predominant role of the NADH/NAD+ ratio.

All of the lactic streptococci examined except Streptococcus lactis ML8 fermented galactose to lactate, formate, acetate, and ethanol. The levels of pyruvate-formate lyase and lactate dehydrogenase were elevated and reduced, respectively, in galactose-grown cells compared with glucose- or lactose-grown cells. Reduced intracellular levels of both the lactate dehydrogenase activator (fructose, 1,6-diphosphate) and pyruvate-formate lyase inhibitors (triose phosphates) appeared to be the main factors involved in the diversion of lactate to the other products. S. lactis ML8 produced only lactate from galactose, apparently due to the maintenance of high intracellular levels of fructose 1,6-diphosphate and triose phosphates. The growth rates of all 10 Streptococcus cremoris strains examined decreased markedly with galactose concentrations below about 30 mM. This effect appeared to be correlated with uptake predominantly by the low-affinity galactose phosphotransferase system and initial metabolism via the D-tagatose 6-phosphate pathway. In contrast, with four of the five S. lactis strains examined, galactose uptake and initial metabolism involved more extensive use of the high-affinity galactose permease and Leloir pathway. With these strains the relative flux of galactose through the alternate pathways would depend on the exogenous galactose concentration.

K W Turner

Papers

Galactose fermentation by Streptococcus lactis and Streptococcus cremoris: pathways, products, and regulation.