To succeed, many cells must alternate between life-styles that permit rapid growth in the presence of abundant nutrients and ones that enhance survival in the absence of those nutrients. One such change in life-style, the “acetate switch,” occurs as cells deplete their environment of acetate-producing carbon sources and begin to rely on their ability to scavenge for acetate. This review explains why, when, and how cells excrete or dissimilate acetate. The central components of the “switch” (phosphotransacetylase [PTA], acetate kinase [ACK], and AMP-forming acetyl coenzyme A synthetase [AMP-ACS]) and the behavior of cells that lack these components are introduced. Acetyl phosphate (acetyl∼P), the high-energy intermediate of acetate dissimilation, is discussed, and conditions that influence its intracellular concentration are described. Evidence is provided that acetyl∼P influences cellular processes from organelle biogenesis to cell cycle regulation and from biofilm development to pathogenesis. The merits of each mechanism proposed to explain the interaction of acetyl∼P with two-component signal transduction pathways are addressed. A short list of enzymes that generate acetyl∼P by PTA-ACKA-independent mechanisms is introduced and discussed briefly. Attention is then directed to the mechanisms used by cells to “flip the switch,” the induction and activation of the acetate-scavenging AMP-ACS. First, evidence is presented that nucleoid proteins orchestrate a progression of distinct nucleoprotein complexes to ensure proper transcription of its gene. Next, the way in which cells regulate AMP-ACS activity through reversible acetylation is described. Finally, the “acetate switch” as it exists in selected eubacteria, archaea, and eukaryotes, including humans, is described.

The acetate switch.

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.

Lactic acid bacteria (LAB) and their probio-active cellular substances exert many beneficial effects in the gastrointestinal tract. LAB prevent adherence, establishment, and replication of several enteric mucosal pathogens through several antimicrobial mechanisms. LAB also release various enzymes into the intestinal lumen and exert potential synergistic effects on digestion and alleviate symptoms of intestinal malabsoption. Consumption of LAB fermented dairy products with LAB may elicit antitumor effects. These effects are attributed to the inhibition of mutagenic activity; decrease in several enzymes implicated in the generation of carcinogens, mutagens, or tumor-promoting agents; suppression of tumors; and the epidemiology correlating dietary regimes and cancer. Specific cellular components in LAB strains seem to induce strong adjuvant effects including modulation of cell-mediated immune responses, activation of reticuloendothelial system, augmentation of cytokine pathways and regulation of interleukins, and tumor necrosis factors. Oral administration of LAB is well tolerated and proven to be safe in 143 human clinical trials and no adverse effects were reported in any of the total 7,526 subjects studied during 1961-1998. In an effort to decrease the reliance on synthetic antimicrobials and control the emerging immunocompromised host population, the time has come to carefully explore the prophylactic and therapeutic applications of probiotic LAB.

http://www.nterminus.com/sublinks/publications/(43)%20Probiotic%20LAB%20reprint.pdf

Probiotic Spectra of Lactic Acid Bacteria (LAB)

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

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

/pdf/stress-physiology-of-lactic-acid-bacteria-37ja8l2pap.pdf

Stress Physiology of Lactic Acid Bacteria

Two strains of Lactobacillus plantarum accumulated H2O2 when grown aerobically in a complex glucose based medium. The H2O2 accumulation did not occur immediately on exposure of the culture to O2 but was delayed for a time which, in the case of one strain, was dependent on the amount of inoculum used to seed the culture. The accumulation was always preceded by an increase in the rate of O2 utilization by the cultures. The latter coincided approximately with an increase in specific activity of NADH oxidase, pyruvate oxidase and NADH peroxidase. H2O2 was not a product of NADH oxidase in vitro but was formed in substantial quantities from O2 during oxidation of pyruvate. The three enzymes were induced by O2 and H2O2; the induction of NADH oxidase responded to lower levels of O2 (but not of H2O2) than the pyruvate oxidase or the NADH peroxidase.

Correlation of oxygen utilization and hydrogen peroxide accumulation with oxygen induced enzymes in Lactobacillus plantarum cultures.

Lactobacillus plantarum P5 grew aerobically in rich media at the expense of lactate; no growth was observed in the absence of aeration. The oxygen-dependent growth was accompanied by the conversion of lactate to acetate which accumulated in the growth medium. Utilization of oxygen with lactate as substrate was observed in buffered suspensions of washed whole cells and in cell-free extracts. A pathway which accounts for the generation of adenosine triphosphate during aerobic metabolism of lactate to acetate via pyruvate and acetyl phosphate is proposed. Each of the enzyme activities involved, nicotinamide adenine dinucleotide independent lactic dehydrogenase, nicotinamide adenine dinucleotide dependent lactic dehydrogenase, pyruvate oxidase, acetate kinase and NADH oxidase were demonstrated in cell-free extracts. The production of pyruvate, acetyl phosphate and acetate was demonstrated using cell-free extracts and cofactors for the enzymes of the proposed pathway.

Oxygen dependent lactate utilization by Lactobacillus plantarum.

The growth rate of Lactobacillus plantarum in a complex medium with 55.6 mM glucose decreased during aerobic incubation (relative to anaerobic incubation). The decrease occurred much earlier than an increase in the rate of oxygen utilization by the culture which led to H2O2 accumulation. The concentration of H2O2 accumulated in the medium was easily tolerated by the culture and elimination of the H2O2 did not prevent the decrease in growth rate. Increased O2 utilization was accompanied by a switch in metabolism which resulted in acetate rather than lactate accumulation in aerobic cultures.

S. Condon

Papers

Correlation of oxygen utilization and hydrogen peroxide accumulation with oxygen induced enzymes in Lactobacillus plantarum cultures.

Oxygen dependent lactate utilization by Lactobacillus plantarum.

Comparison of aerobic and anaerobic growth of Lactobacillus plantarum in a glucose medium