Regulation of sugar transport and metabolism in lactic acid bacteria

Abstract The phosphoenolpyruvate (PEP)-dependent lactose: phosphotransferase system (PTS), P-β-galactosidase, and enzymes of the d -tagatose-6P pathway, are prerequisite for rapid homolactic fermentation of lactose by Group N ('starter') streptococci. Moreover, the reactions of transport and catabolism constitute an open cycle in which ATP and lactic acid are metabolic products. The efficient and controlled operation of this cycle requires 'fine-control' mechanisms to ensure: (i) tight coupling between transport and energy-yielding reactions, (ii) co-metabolism of both glucose and galactose moieties of the disaccharide, and (iii) coordination of the rate of sugar transport to the rate of sugar catabolism. The elucidation of these fine-control mechanisms in intact cells of Streptococcus lactis has required the isolation of glucokinase (GK) and mannose-PTS defective mutants, the synthesis of novel lactose analogs, and the use of high resolution [³¹P]NMR spectroscopy. It has been established that PEP provides the crucial link between transport and energy-yielding reactions of the PTS: glycolysis cycle, and that both ATP-dependent glucokinase and PEP-dependent mannose-PTS can participate in the phosphorylation of intracellular glucose. Finally, evidence has been obtained in vivo, for modulation of pyruvate kinase activity in response to fluctuation in, concentrations of positive (FDP), and negative (P_i) effectors of the allosteric enzyme. Fine-control of pyruvate kinase activity may in turn regulate: (i) the distribution of PEP to either the PTS or ATP synthesis, (ii) overall activity of the PTS: glycolysis cycle, and (iii) the formation of the endogenous PEP-potential in starved organisms. The accumulation of non-metabolizable PTS sugars (e.g., 2-deoxy- d -glucose) by growing cells can perturb these fine-control mechanisms and, by establishment of a PEP-dissipating futile cycle, may result in bacteriostasis.     

Metabolic engineering of sugar catabolism in lactic acid bacteria

Lactic acid bacteria are characterized by a relatively simple sugar fermentation pathway that, by definition, results in the formation of lactic acid. The extensive knowledge of traditional pathways and the accumulating genetic information on these and novel ones, allows for the rerouting of metabolic processes in lactic acid bacteria by physiological approaches, genetic methods, or a combination of these two. This review will discuss past and present examples and future possibilities of metabolic engineering of lactic acid bacteria for the production of important compounds, including lactic and other acids, flavor compounds, and exopolysaccharides.     

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.