Antimicrobial activity of Olea europaea Linné extracts and their applicability as natural food preservative agents.

The antimicrobial activity of phenolic compounds from Olea (O.) europaea Linné (L.) is part of the scientific discussion regarding the use of natural plant extracts as alternative food preservative agents. Although, the basic knowledge on the antimicrobial potential of certain molecules such as oleuropein, hydroxytyrosol or elenolic acid derivatives is given, there is still little information regarding their applicability for food preservation. This might be primarily due to the lack of information regarding the full antimicrobial spectrum of the compounds, their synergisms in natural or artificial combinations and their interaction with food ingredients. The present review accumulates available literature from the past 40 years, investigating the antimicrobial activity of O. europaea L. derived extracts and compounds in vitro and in food matrices, in order to evaluate their food applicability. In summary, defined extracts from olive fruit or leaves, containing the strongest antimicrobial compounds hydroxytyrosol, oleacein or oleacanthal in considerable concentrations, appear to be suitable for food preservation. Nonetheless there is still need for consequent research on the compounds activity in food matrices, their effect on the natural microbiota of certain foods and their influence on the sensorial properties of the targeted products.

Monitoring the live to dead transition of bacteria during thermal stress by a multi-method approach.

Rapid microbiological methods to assess cell physiological properties of microorganisms are gaining interest in the elucidation of the effect of antimicrobial agents or physical inactivation. Fluorescent probes combined with flow cytometry or microplate assays provide information about cellular targets of chemical or physical stressors and help to clarify the underlying mode of action. In this work we exemplarily monitored the bacterial response of Listeria innocua, Staphylococcus aureus, Salmonella enterica and Escherichia coli to a mild thermal treatment by applying various methods to illustrate bacterial vital functions like the redox activity, membrane potential, esterase activity, efflux activity, glucose uptake, membrane integrity and plate counts. It was observed that some cellular properties are affected earlier than others. Respiration, glucose-uptake and pump activity were the most sensitive parameters, followed by the loss of membrane potential and membrane integrity. Unspecific esterase was found to be relatively resistant to mild heat exposure. This study shows that such a multi-method approach is a suitable tool to monitor the impact of inactivation treatments on bacteria, providing information about the mode of action, the heterogeneity of populations, species-specific differences to stressors and valuable insight in vital functions beyond pure culturability.

Antimicrobial activity of hop extracts against foodborne pathogens for meat applications.

AIMS: The objective of this study was the fundamental investigation of the antimicrobial efficiency of various hop extracts against selected foodborne pathogens in vitro, as well as their activity against Listeria monocytogenes in a model meat marinade and on marinated pork tenderloins. METHODS AND RESULTS: In a first step, the minimum inhibitory concentrations (MIC) of three hop extracts containing either α- or ß-acids or xanthohumol were determined against test bacteria including L. monocytogenes, Staphylococcus aureus, Salmonella enterica and Escherichia coli by a colorimetric method based on the measurement of bacterial metabolic activity. Moreover, the influence of either lactic or citric acid on the antimicrobial activity of the hop extracts was evaluated. The efficiency of hop extracts as a natural food preservative was then tested in a model meat marinade at 2 and 8°C, respectively, and finally on marinated pork. The experiments showed that Gram-positive bacteria were strongly inhibited by hop extracts containing ß-acids and xanthohumol (MIC values of 6.3 and 12.5 ppm, respectively), whereas the antimicrobial activity of the investigated α-acid extract was significantly lower (MIC values of 200 ppm). Gram-negative bacteria were highly resistant against all tested hop extracts. Acidification of the test media led to a decrease of the MIC values. The inhibitory activity of the hop extracts against L. monocytogenes was strongly reduced in a fat-containing model meat marinade, but the efficiency of ß-acids in this matrix could be increased by lowering pH and storage temperatures. By applying 0.5 % ß-acids at pH = 5 in a model marinade, the total aerobic count of pork tenderloins was reduced up to 0.9 log10 compared with marinated pork without hop extract after 2 weeks of storage at 5°C. CONCLUSIONS: ß-acid containing hop extracts have proven to possess a high antimicrobial activity against Gram-positive bacteria in vitro and in a practice-related application for food preservation. SIGNIFICANCE AND IMPACT OF THE STUDY: Antimicrobial hop extracts could be used as natural preservatives in food applications to extend the shelf life and to increase the safety of fresh products.

Isolation, characterization and cDNA cloning of nicotianamine synthase from barley. A key enzyme for iron homeostasis in plants.

Basic cellular processes such as electron transport in photosynthesis and respiration require the precise control of iron homeostasis. To mobilize iron, plants have evolved at least two different strategies. The nonproteinogenous amino acid nicotianamine which is synthesized from three molecules of S-adenosyl-L-methionine, is an essential component of both pathways. This compound is missing in the tomato mutant chloronerva, which exhibits severe defects in the regulation of iron metabolism. We report the purification and partial characterization of the nicotianamine synthase from barley roots as well as the cloning of two corresponding gene sequences. The function of the gene sequence has been verified by overexpression in Escherichia coli. Further confirmation comes from reduction of the nicotianamine content and the exhibition of a chloronerva-like phenotype due to the expression of heterologous antisense constructs in transgenic tobacco plants. The native enzyme with an apparent Mr of approximately 105 000 probably represents a trimer of S-adenosyl-L-methionine-binding subunits. A comparison with the recently cloned chloronerva gene of tomato reveals striking sequence homology, providing support for the suggestion that the destruction of the nicotianamine synthase encoding gene is the molecular basis of the tomato mutation.

Two Polypeptides in the Inner Chloroplast Envelope of Dunaliella tertiolecta Induced by Low CO(2).

Unicellular green algae have a mechanism for concentrating dissolved inorganic carbon (DIC) only when grown in low CO(2). To find proposed transporter protein(s) for DIC, we isolated intact chloroplasts from Dunaliella tertiolecta cells, separated the chloroplast envelopes by isopyknic centrifugation, and separated their polypeptides by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two peptides of apparent molecular masses of 45 and 47 kD were constituents of the inner chloroplast envelope only if the cells had been adapted to low CO(2) in the light or grown in low CO(2). These two low CO(2)-induced peptides appear to be part of the algal DIC pump.

Antagonistic blue- and red-light regulation of cab-gene expression during photosynthetic adaptation in Scenedesmus obliquus.

During adaptation of the photosynthetic apparatus of the green alga Scenedesmus obliquus to various light qualities, the accumulation of chlorophylls and pigment-protein complexes (with specific consideration of chlorophyll a/b-binding (Cab) proteins) and cab-gene expression were determined. The fluence rate dependences for chlorophyll accumulation and cab-gene expression were very different. Very low fluence rates of violet (404 nm), blue (461 nm) and red (650 nm) light below the photosynthetic threshold, i.e. between 10(-3) and 10(-1) mumol m-2 s-1, inhibited all of these reactions in cells grown under heterotrophic conditions. At elevated fluence rates (above 1 mumol m-2 s-1), red light retained its negative regulation, whereas blue light stimulated pigment accumulation. Under autotrophic conditions the pattern was more complex, because chlorophyll accumulation was unaffected by light below the photosynthetic threshold. However, the expression of cab-genes was inhibited by red light but stimulated by blue light. Cells adapted to fluence rates, which ensured photosynthetic energy supply (above 1 mumol m-2 s-1), showed an increase in chlorophyll accumulation, blue light being more effective than red light. The results confirm and extend our previous discovery of two antagonistically acting photoreceptors in Scenedesmus which mediate and coordinate the complex functional and structural changes associated with photosynthetic adaptation. One of these receptor pigments is a blue-light receptor with positive action; the other is a violet-red-light receptor which can operate far below the photosynthetic threshold and exerts a negative regulation.

Action spectra for photosynthetic adaptation in Scenedesmus obliquus : I. Chlorophyll biosynthesis under autotrophic conditions.

Photosynthetic adaptation of the unicellular green alga Scenedemus obliquus to different light conditions was investigated with respect to chlorophyll synthesis. Cultures were grown under white light (20 W · m(-2)) from fluorescent lamps and were then transferred and subjected to the actual adaptation regime which consisted of a 24-h irradiation by different fluence rates and wavelengths. Fluence rate-response curves for chlorophyll synthesis were measured between 4 · 10(-2) and 1 · 10(2) W · m(-2). In white light from incandescent lamps, in blue and red light the fluence rate-response curves for chlorophyll (Chl) a and also for Chl b were bell-shaped. In red light the threshold was about the same as under blue light. The maximal amounts of Chl a and b were about twofold increased under blue light relative to the values obtained with red light. Action spectra for the stimulation of chlorophyll synthesis (Chl a + Chl b) as well as those for the separate chlorophylls showed two maxima near 450 and 500 nm. However, the action spectrum for Chl b synthesis demonstrated a considerably higher value in the 450-nm peak. Experiments with the photosynthesis inhibitor 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) indicated that photosynthetic energy supply supported the photostimulation of chlorophyll synthesis. The action spectra indicate the cooperation of two photoreceptors. The 460-nm peak is attributed to the "typical" blue-light receptor, being more active in Chl b formation. The peak at 500 nm may represent carotenoproteins acting as an accessory pigment system.

Action spectra for photosynthetic adaptation in Scenedesmus obliquus : II. Chlorophyll biosynthesis and cell growth under heterotrophic conditions.

The fluence-rate and wavelength dependence of chlorophyll synthesis and cell growth of Scenedesmus obliquus were measured under heterotrophic conditions. Cultures were first grown autotrophically under white light from fluorescent lamps (20 W · m(-2)) and were then transferred to a glucose-containing medium in which they were exposed to light of different fluence rates and wavelengths. Fluence rate-response curves, which were obtained with monochromatic blue (453 nm) and red (644 nm) light were complex and indicated the existence of a very low- and a low-irradiance photoreceptor-system with antagonistic modes of interaction. In the very low-irradiance region (10(-10)-10(-6) mol · m(-2) · s(-1)), blue light partially inhibited chlorophyll synthesis. In the low-irradiance region (> 10(-6) mol · m(-2) · s(-1)), chlorophyll synthesis and cell growth were stimulated. At these elevated fluence rates the curve was bell shaped. Red light inhibited chlorophyll synthesis in the very low- as well as in the low-irradiance region. The effects of blue and red light on chlorophyll synthesis and cell growth were unaffected by the photosynthesis inhibitor 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) showing that photosynthesis per se did not mediate these photoresponses. Based on detailed fluence rate-response curves and also on equal quantum-flux measurements, action spectra were obtained for chlorophyll synthesis and cell growth in the region below 10(-6) mol · m(-2) · s(-1). The action spectra for the very low-irradiance region (inhibition of chlorophyll synthesis and stimulation of cell growth) showed two sharp peaks near 408 and 645 nm. The action spectrum for the low-irradiance region showed stimulation of chlorophyll synthesis between 450 and 480-nm. Below and above this wavelength range an inhibitory action was found. We propose that two photoreceptor systems regulate chlorophyll synthesis, and that these can be distinguished by different thresholds, different spectral sensitivities and also by their antagonistic modes of action. The implications of these findings with respect to autotrophic conditions are discussed.

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus.

Scenedesmus cells grown on high CO(2), when adapted to air levels of CO(2) for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO(2) from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O(2) evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO(2) pump with K(0.5) for O(2) evolution of less than 5 micromolar CO(2) was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO(3) (-) concentrating process between pH 8 to 10 lowered the K(0.5) (DIC) from 7300 micromolar HCO(3) (-) in high CO(2) grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO(3) (-) pump was inhibited by vanadate (K(i) of 150 micromolar), as if it involved an ATPase linked HCO(3) (-) transporter. The CO(2) pump was formed on low CO(2) by high-CO(2) grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO(3) (-) pump was partially activated on low CO(2) within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO(3) (-) pump at pH 9 had requirements similar to the activation of the CO(2) pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO(2)-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO(2) pump and in some algae an additional vanadate-sensitive plasmalemma HCO(3) (-) transporter for a HCO(3) (-) pump.