title: Metabolic and transcriptional analysis of acid stress in Lactococcus lactis, with a focus on the kinetics of lactic acid pools.
abstract: The effect of pH on the glucose metabolism of non-growing cells of L. lactis MG1363 was studied by in vivo NMR in the range 4.8 to 6.5. Immediate pH effects on glucose transporters and/or enzyme activities were distinguished from transcriptional/translational effects by using cells grown at the optimal pH of 6.5 or pre-adjusted to low pH by growth at 5.1. In cells grown at pH 5.1, glucose metabolism proceeds at a rate 35% higher than in non-adjusted cells at the same pH. Besides the upregulation of stress-related genes (such as dnaK and groEL), cells adjusted to low pH overexpressed H(+)-ATPase subunits as well as glycolytic genes. At sub-optimal pHs, the total intracellular pool of lactic acid reached approximately 500 mM in cells grown at optimal pH and about 700 mM in cells grown at pH 5.1. These high levels, together with good pH homeostasis (internal pH always above 6), imply intracellular accumulation of the ionized form of lactic acid (lactate anion), and the concomitant export of the equivalent protons. The average number, n, of protons exported with each lactate anion was determined directly from the kinetics of accumulation of intra- and extracellular lactic acid as monitored online by (13)C-NMR. In cells non-adjusted to low pH, n varies between 2 and 1 during glucose consumption, suggesting an inhibitory effect of intracellular lactate on proton export. We confirmed that extracellular lactate did not affect the lactate: proton stoichiometry. In adjusted cells, n was lower and varied less, indicating a different mix of lactic acid exporters less affected by the high level of intracellular lactate. A qualitative model for pH effects and acid stress adaptation is proposed on the basis of these results.
authors.affiliation: Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras, Portugal.
authors.forename: Ana Lúcia
authors.initials: AL
authors.lastname: Carvalho
authors.forename: David L
authors.initials: DL
authors.lastname: Turner
authors.forename: Luís L
authors.initials: LL
authors.lastname: Fonseca
authors.forename: Ana
authors.initials: A
authors.lastname: Solopova
authors.forename: Teresa
authors.initials: T
authors.lastname: Catarino
authors.forename: Oscar P
authors.initials: OP
authors.lastname: Kuipers
authors.forename: Eberhard O
authors.initials: EO
authors.lastname: Voit
authors.forename: Ana Rute
authors.initials: AR
authors.lastname: Neves
authors.forename: Helena
authors.initials: H
authors.lastname: Santos
mesh_terms: D001692:Biological Transport; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial; D005947:Glucose / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000737:chemistry / Q000378:metabolism*; D013294:Lactococcus lactis / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism*; D055442:Metabolome; D019906:Nuclear Magnetic Resonance, Biomolecular; D013312:Stress, Physiological
chemical_list: D019344:Lactic Acid; D005947:Glucose