Kinetics, stereospecificity, and expression of the malolactic enzyme

Expression of the malolactic enzyme gene (mle) from Lactobacillus plantarum under winemaking conditions

Malolactic fermentation (MLF) plays an important role in the production of wine, especially red wines, resulting in microbial stability, deacidification, as well as contributing to the aroma profile. MLF can be influenced by a number of factors. In this study, the influence of pH and ethanol on expression of the structural malolactic enzyme gene (mle) from Lactobacillus plantarum was investigated in a synthetic wine media, as well as in wine using quantitative PCR. Expression of mle was shown to be inducible by the presence of malic acid, with increased expression in the middle of MLF. Expression of mle was also shown to be increased at low pH values and decreased in the presence of ethanol. This indicates the role of MLF in acid tolerance and the negative impact of ethanol on the completion of MLF. The results therefore provide further evidence that L. plantarum should be applied as co-inoculation for MLF where alcohol will initially not have a negative impact on the malic acid degradation.

Degradation of 2,5-dimethylpyrazine by Rhodococcus erythropolis strain DP-45 isolated from a waste gas treatment plant of a fishmeal processing company

A bacterium, strain DP-45, capable of degrading 2,5-dimethylpyrazine (2,5-DMP) was isolated and identified as Rhodococcus erythropolis. The strain also grew on many other pyrazines found in the waste gases of food industries, like 2,3-dimethylpyrazine (2,3-DMP), 2,6-dimethylpyrazine (2,6-DMP), 2-ethyl-5(6)-dimethylpyrazine (EMP), 2-ethylpyrazine (EP), 2-methylpyrazine (MP), and 2,3,5-trimethylpyrazine (TMP). The strain utilized 2,5-DMP as sole source of carbon and nitrogen and grew optimally at 25 degrees C with a doubling time of 7.6 h. The degradation of 2,5-DMP was accompanied by the growth of the strain and by the accumulation of a first intermediate, identified as 2-hydroxy-3,6-dimethylpyrazine (HDMP). The disappearance of HDMP was accompanied by the release of ammonium into the medium. No other metabolite was detected. The degradation of 2,5-DMP and HDMP by strain DP-45 required molecular oxygen. The expression of the first enzyme in the pathway was induced by 2,5-DMP and HDMP whereas the second enzyme was constitutively expressed. The activity of the first enzyme was inhibited by diphenyliodonium (DPI), a flavoprotein inhibitor, methimazole, a competitive inhibitor of flavin-containing monooxygenases, and by cytochrome P450 inhibitors, 1-aminobenzotriazole (ABT) and phenylhydrazine (PHZ). The activity of the second enzyme was inhibited by DPI, ABT, and PHZ. Sodium tungstate, a specific antagonist of molybdate, had no influence on growth and consumption of 2,5-DMP by strain DP-45. These results led us to propose that a flavin-dependent monooxygenase or a cytochrome P450-dependent monooxygenase rather than a molybdenum hydroxylase catalyzed the initial hydroxylation step and that a cytochrome P450 enzyme is responsible for the transformation of HDMP in the second step.

The Bacillus subtilis ywkA gene encodes a malic enzyme and its transcription is activated by the YufL/YufM two-component system in response to malate

A transcriptome comparison of a wild-type Bacillus subtilis strain growing under glycolytic or gluconeogenic conditions was performed. In particular, it revealed that the ywkA gene, one of the four paralogues putatively encoding a malic enzyme, was more transcribed during gluconeogenesis. Using a lacZ reporter fusion to the ywkA promoter, it was shown that ywkA was specifically induced by external malate and not subject to glucose catabolite repression. Northern analysis confirmed this expression pattern and demonstrated that ywkA is cotranscribed with the downstream ywkB gene. The ywkA gene product was purified and biochemical studies demonstrated its malic enzyme activity, which was 10-fold higher with NAD than with NADP (kcat/Km 102 and 10 s(-1) mM(-1), respectively). However, physiological tests with single and multiple mutant strains affected in ywkA and/or in ywkA paralogues showed that ywkA does not contribute to efficient utilization of malate for growth. Transposon mutagenesis allowed the identification of the uncharacterized YufL/YufM two-component system as being responsible for the control of ywkA expression. Genetic analysis and in vitro studies with purified YufM protein showed that YufM binds just upstream of ywkA promoter and activates ywkA transcription in response to the presence of malate in the extracellular medium, transmitted by YufL. ywkA and yufL/yufM could thus be renamed maeA for malic enzyme and malK/malR for malate kinase sensor/malate response regulator, respectively.

Direct interface of chemistry to microbiological systems: membrane inlet mass spectrometry

Direct measurement of dissolved gases and low molecular weight volatiles through permeable membranes (e.g. 50-microm-thick silicone rubber), provides an invaluable tool for the investigation of the activities of microorganisms in the laboratory and in their natural environments. Multiple molecular species are monitored at a single point. Fast response times (t(90%)<1 min) and long-term stability, (<1% week(-1)); high specificity and high sensitivity (e.g. 0.2 microM for O(2), <0.5 mM for ethanol), provides a technique that can provide information on the kinetics of processes over many decades (10(0)-10(6)) of minutes. Spatial resolution of <1 mm enables 3D mapping of gases in complex ecosystems (sediments, peat, soils, biofilms, foodstuffs). Results with membrane inlet mass spectrometry (MIMS) when used in conjunction with confocal scanning laser microscopy, provides a powerful approach to the analysis of kinetic and spatial aspects of natural environments. Examples discussed are peat cores and cheese.

Membrane introduction mass spectrometry: trends and applications

Recent advances in membrane introduction mass spectrometry (MIMS) are reviewed. On-line monitoring is treated by focusing on critical variables, including the nature and dimensions of the membrane, and the analyte vapor pressure, diffusivity, and solubility in the membrane barrier. Sample introduction by MIMS is applied in (i) on-line monitoring of chemical and biological reactors, (ii) analysis of volatile organic compounds in environmental matrices, including air, water and soil, and (iii) in more fundamental studies, such as measurements of thermochemical properties, reaction mechanisms, and kinetics. New semipermeable membranes are discussed, including those consisting of thin polymers, low vapor pressure liquids, and zeolites. These membranes have been used to monitor polar compounds, selectively differentiate compounds through affinity-binding, and provide isomer differentiation based on molecular size. Measurements at high spatial resolution, for example, using silicone-capped hypodermic needle inlets, are also covered, as is electrically driven sampling through microporous membranes. Other variations on the basic MIMS experiment include analyte preconcentration through cryotrapping (CT-MIMS) or trapping in the membrane (trap-and-release), as well as differential thermal release methods and reverse phase (i.e., organic solvent) MIMS. Method limitations center on semivolatile compounds and complex mixture analysis, and novel solutions are discussed. Semivolatile compounds have been monitored with thermally assisted desorption, ultrathin membranes and derivatization techniques. Taking advantage of the differences in time of membrane permeation, mixtures of structurally similar compounds have been differentiated by using sample modulation techniques and by temperature-programmed desorption from a membrane interface. Selective ionization techniques that increase instrument sensitivity towards polar compounds are also described, and comparisons are made with other direct sampling (nonchromatographic) methods that are useful in mixture analysis.