Escherichia coli membrane fluidity as detected by excimerization of dipyrenylpropane: sensitivity to the bacterial fatty acid profile

Assessment of trans-cinnamaldehyde and eugenol assisted heat treatment against Salmonella Typhimurium in low moisture food components

Increased thermal resistance of Salmonella at low water activity (a_w) is a significant food safety concern in low-moisture foods (LMFs). We evaluated whether trans-cinnamaldehyde (CA, 1000 ppm) and eugenol (EG, 1000 ppm), which can accelerate thermal inactivation of Salmonella Typhimurium in water, can show similar effect in bacteria adapted to low a_w in different LMF components. Although CA and EG significantly accelerated thermal inactivation (55 °C) of S. Typhimurium in whey protein (WP), corn starch (CS) and peanut oil (PO) at 0.9 a_w, such effect was not observed in bacteria adapted to lower a_w (0.4). The matrix effect on bacterial thermal resistance was observed at 0.9 a_w, which was ranked as WP > PO > CS. The effect of heat treatment with CA or EG on bacterial metabolic activity was also partially dependent on the food matrix. Bacteria adapted to lower a_w had lower membrane fluidity and unsaturated to saturated fatty acids ratio, suggesting that bacteria at low a_w can change its membrane composition to increase its rigidity, thus increasing resistance against the combined treatments. This study demonstrates the effect of a_w and food components on the antimicrobials-assisted heat treatment in LMF and provides an insight into the resistance mechanism.

Toward Detecting Biosignatures of DNA, Lipids, and Metabolic Intermediates from Bacteria in Ice Grains Emitted by Enceladus and Europa

The reliable identification of biosignatures is key to the search for life elsewhere. On ocean worlds like Enceladus or Europa, this can be achieved by impact ionization mass spectrometers, such as the SUrface Dust Analyzer (SUDA) on board NASA's upcoming Europa Clipper mission. During spacecraft flybys, these instruments can sample ice grains formed from subsurface water and emitted by these moons. Previous laboratory analog experiments have demonstrated that SUDA-type instruments could identify amino acids, fatty acids, and peptides in ice grains and discriminate between their abiotic and biotic origins. Here, we report experiments simulating impact ionization mass spectra of ice grains containing DNA, lipids, and metabolic intermediates extracted from two bacterial cultures: Escherichia coli and Sphingopyxis alaskensis. Salty Enceladan or Europan ocean waters were simulated using matrices with different NaCl concentrations. Characteristic mass spectral signals, such as DNA nucleobases, are clearly identifiable at part-per-million-level concentrations. Mass spectra of all substances exhibit unambiguous biogenic patterns, which in some cases show significant differences between the two bacterial species. Sensitivity to the biosignatures decreases with increasing matrix salinity. The experimental parameters indicate that future impact ionization mass spectrometers will be most sensitive to the investigated biosignatures for ice grain encounter speeds of 4-6 km/s.

Enzymology of standalone elongating ketosynthases

The β-ketoacyl-acyl carrier protein synthase, or ketosynthase (KS), catalyses carbon-carbon bond formation in fatty acid and polyketide biosynthesis via a decarboxylative Claisen-like condensation. In prokaryotes, standalone elongating KSs interact with the acyl carrier protein (ACP) which shuttles substrates to each partner enzyme in the elongation cycle for catalysis. Despite ongoing research for more than 50 years since KS was first identified in E. coli, the complex mechanism of KSs continues to be unravelled, including recent understanding of gating motifs, KS-ACP interactions, substrate recognition and delivery, and roles in unsaturated fatty acid biosynthesis. In this review, we summarize the latest studies, primarily conducted through structural biology and molecular probe design, that shed light on the emerging enzymology of standalone elongating KSs.

Cell membrane fatty acid changes and desaturase expression of Saccharomyces bayanus exposed to high pressure homogenization in relation to the supplementation of exogenous unsaturated fatty acids

Aims: The aim of this work was to study the responses of Saccharomyces bayanus cells exposed to sub-lethal high-pressure homogenization (HPH) and determine whether the plasmatic membrane can sense HPH in the presence, or absence, of exogenous unsaturated fatty acids (UFAs) in the growth medium.

Methods and Results: High-pressure homogenization damaged and caused the collapse of cell walls and membranes of a portion of cells; however, HPH did not significantly affect S. bayanus cell viability (less than 0.3 Log CFU ml^-1). HPH strongly affected the membrane fatty acid (FA) composition by increasing the percentage of total UFA when compared with saturated fatty acids. The gene expression showed that the transcription of OLE1, ERG3, and ERG11 increased after HPH. The presence of exogenous UFA abolished HPH-induced effects on the OLE1 and ERG3 genes, increased the percentage of membrane lipids and decreased the expression of OLE1 and ERG3 within 30 min of treatment.

Conclusion: The results suggest a key role for UFA in the microbial cell response to sub-lethal stress. In addition, these data provide insight into the molecular basis of the response of S. bayanus to this innovative technology.

Significance and Impact of the Study: Elucidation of the mechanism of action for sub-lethal HPH will enable the utilization of this technology to modulate the starter performance at the industrial scale.

Site-specific acylation changes in the lipid A of Escherichia coli lpxL mutants grown at high temperatures

LPS is a major component of the outer membrane of Gram-negative bacteria. The lipid A region of LPS mediates stimulation of the immune system. In E. coli, the gene (formerly htrB) codes for a late lauroyltransferase (LpxL) in lipid A biosynthesis. E. coli lpxL mutants have been described previously with impaired growth above 33°C in rich media. However, we were able to grow lpxL mutants at 30°C, 37°C and 42°C, and investigate their lipid A moieties to gain insight into changes and regulatory effects in lipid A biosynthesis. Multiple-stage mass spectrometry was used to decipher unusual lipid A structures produced by lpxL mutant bacteria at high temperatures that rescue the temperature-sensitive phenotype. At 37°C and 42°C, E. coli lpxL mutants appear to activate different acyltransferases or biosynthetic pathways that generate atypical penta- and hexaacyl lipid A structures by incorporating longer fatty acids, such as a secondary palmitoleic acid (2'-O-position, distal) and a secondary palmitic acid (2-O-position, proximal) respectively. However, we observed no changes in these structures through various growth curve stages. This study indicates that E. coli (lpxL) lipid A biosynthesis, and specifically the 'late' acylation of lipid A, is temperature dependent, thus suggesting a highly regulated process.

Effects of microwave (2.45 GHz) irradiation on some biological characters of Salmonella typhimurium

The present study was carried out to evaluate the effects of sub-lethal doses of microwave radiation on some biological characteristics in Salmonella typhimurium. The aim was to show the relationship between this treatment and the development of radiotolerance in this pathogen because there is a need for more information on physiological responses of pathogens to sub-lethal doses of microwave radiation. So, the bacterial strain was treated with a dose of 3600J (40-s exposure with power P=90 W) to cause cellular damage. The results have shown that the exposure of bacteria to microwaves resulted in a significant inhibition of cellular growth. This treatment has notably increased the effectiveness of the most tested antibiotics by the amelioration or the appearance of sensitivity in exposed bacteria. Gas chromatography (GC) analysis was performed to demonstrate the modification of the fatty acids (FA) composition. Results obtained have shown that this treatment had a significant effect on the FA content with an increase of unsaturated FA percentage. The acquisition of sensitivity to the sodium deoxycholate and the significant increase in the amount of extracellular proteins in exposed bacteria has confirmed the weakening of the bacterial membrane by microwaves. This study represents one of the few demonstrating the modifications on the bacterial membrane as a cellular response to survive the non-ionising radiation stress.

Accumulation of exogenous amyloid-beta peptide in hippocampal mitochondria causes their dysfunction: a protective role for melatonin

Amyloid-beta (Aβ) pathology is related to mitochondrial dysfunction accompanied by energy reduction and an elevated production of reactive oxygen species (ROS). Monomers and oligomers of Aβ have been found inside mitochondria where they accumulate in a time-dependent manner as demonstrated in transgenic mice and in Alzheimer's disease (AD) brain. We hypothesize that the internalization of extracellular Aβ aggregates is the major cause of mitochondrial damage and here we report that following the injection of fibrillar Aβ into the hippocampus, there is severe axonal damage which is accompanied by the entrance of Aβ into the cell. Thereafter, Aβ appears in mitochondria where it is linked to alterations in the ionic gradient across the inner mitochondrial membrane. This effect is accompanied by disruption of subcellular structure, oxidative stress, and a significant reduction in both the respiratory control ratio and in the hydrolytic activity of ATPase. Orally administrated melatonin reduced oxidative stress, improved the mitochondrial respiratory control ratio, and ameliorated the energy imbalance.

Analysis of the membrane fluidity of erythrocyte ghosts in diabetic, spontaneously hypertensive rats

Diabetes and hypertension are closely related diseases associated with changes in membrane fluidity. Here, we measured the membrane fluidity of erythrocyte ghosts from spontaneously hypertensive rats (SHR), with or without streptozotocin (STZ)-induced diabetes, at the ages of 1, 3 and 6 months, by introducing the use of the intramolecular excimer forming dipyrenylpropane (DPyP) in this model. Type 2 diabetes mellitus (T2DM) was induced in 48-h-old, newborn male SHR by intraperitoneal injection of STZ. We found lower excimer to monomer (I (e)/I (m)) DPyP ratios in diabetic SHR than in control SHR at 3 and 6 months old, indicating a decrease in membrane fluidity. Simultaneously, the composition of fatty acids was determined and it was found that the unsaturated to saturated fatty acids ratio (U/S) was compatible with changes in membrane fluidity. These results suggest that the change in fatty acid composition of erythrocyte ghosts contributes significantly to the decreased membrane fluidity detected with DPyP in diabetic SHR.

Lateral organization of mixed, two-phosphatidylcholine liposomes as investigated by GPS, the slope of Laurdan generalized polarization spectra

The effect of the excitation or emission wavelengths on Laurdan generalized polarization (GP) can be evaluated by GPS, a quantitative, simplified determination of the GP spectrum slope, the thermotropic dependence of which allows the assessment of phospholipid lamellar membrane phase, as shown in a recent publication of our laboratory [J.B. Velázquez, M.S. Fernández, Arch. Biochem. Biophys. 455 (2006) 163-174]. In the present work, we applied Laurdan GPS to phase transition studies of mixed, two-phosphatidylcholine liposomes prepared from variable proportions of dimyristoyl- and dipalmitoylphosphatidylcholine (DMPC and DPPC, respectively). We have found that the GPS function reports a clear limit between the gel/liquid-crystalline phase coexistence region and the liquid-crystalline state, not only at a certain temperature T(c) for liposomes of constant composition submitted to temperature scans, but also at a defined mole fraction X(c), for two-component liposomes of variable composition at constant temperature. The T(c) or the X(c) values obtained from GPS vs. temperature or GPS vs. composition plots, respectively, allow the construction of a partial phase diagram for the DMPC-DPPC mixtures, showing the boundary between the two-phase coexisting region and the liquid-crystalline state. Likewise, at the onset of the transition region, i.e., the two-phase coexisting region as detected by GPS, it is possible to determine, although with less precision, a temperature T(o) or a mole fraction X(o) defining a boundary located below but near the limit between the gel and ripple phase, reported in the literature. These GPS results are consistent with the proposal by several authors that a fraction of L(alpha) phospholipids coexists with gel phospholipids in the rippled phase.

Cell envelope fluidity modification for an effective glutamate excretion in Corynebacterium glutamicum 2262

1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) was used to assess the cell envelope fluidity of Corynebacterium glutamicum 2262 during a temperature-triggered glutamate producing process. Because the fluorescence lifetime of TMA-DPH was shown to be constant all over the process, fluorescence anisotropy can be considered as a good index of cell envelope fluidity. When the temperature of the fed-batch culture was increased from 33 to 39 degrees C to induce glutamate excretion, the fluorescence anisotropy values decreased from 0.212 +/- 0.002 to 0.186 +/- 0.002 (corresponding to an increase in the cell fluidity), while the specific glutamate production rate reached its maximal value. The increase in fluidity of the C. glutamicum cell envelope was not due to a physical effect related to the temperature elevation, but rather to an alteration of the composition of the cell envelope. Using a mutant devoid of corynomycolates, significant differences in fluorescence anisotropy values were obtained compared to the wild-type strain, suggesting that TMA-DPH is mainly anchored into the corynomycomembrane. Differences in fluorescence anisotropy were also observed when the bacteria were cultivated at 33, 36, 38, and 39 degrees C in batch cultures, and a linear relationship was obtained between the maximum specific glutamate production rate and the measured fluidity. When using the glutamate non-producing variant of C. glutamicum 2262, the fluorescence anisotropy remained constant at 0.207 +/- 0.003 whatever the applied temperature shift. This suggests that the fluidity of the Corynebacteria mycomembrane plays an important role in glutamate excretion during the temperature-triggered process.

In vitro inhibition of Streptococcus mutans biofilm formation on hydroxyapatite by subinhibitory concentrations of anthraquinones

We report that certain anthraquinones (AQs) reduce Streptococcus mutans biofilm formation on hydroxyapatite at concentrations below the MIC. Although AQs are known to generate reactive oxygen species, the latter do not underlie the observed effect. Our results suggest that AQs inhibit S. mutans biofilm formation by causing membrane perturbation.

GPS, the slope of laurdan generalized polarization spectra, in the study of phospholipid lateral organization and Escherichia coli lipid phases

The dependence of Laurdan generalized polarization (GP) on the excitation or emission wavelengths has been employed, at a descriptive level, to estimate lipid membrane physical state, including the coexistence of phases. In this paper, we introduce GPS, a quantitative, simplified estimation of the GP spectrum slope, and present a novel approach to assessing phase states through a graphical representation of its temperature dependence. The thermotropic profile of GPS allows the detection of the main phase transition of liposomes from model phosphoglycerides and renders a clear identification of T(c), a temperature that is unique for each phospholipid studied, marking the apparent limit between coexistence of phases and liquid crystalline state. Since at this temperature GPS is equal to zero, the tenet that the absence of wavelength effect on generalized polarization always means pure gel phase, can be called into question. Interestingly, GPS allows the discrimination between the thermotropic behavior of vesicles of lipid extracts from Escherichia coli grown at 30, 37, 42 or 45 degrees C, consistent with the remodeling in phospholipid acyl chain composition induced by changes in culture temperature. Yet in all cases, GPS reports liquid crystalline state at a temperature equal to the growth temperature of the bacteria from which each extract was obtained.

Differential detection of phospholipid fluidity, order, and spacing by fluorescence spectroscopy of bis-pyrene, prodan, nystatin, and merocyanine 540

The properties of liquid-ordered, solid-ordered, and liquid-disordered phases were investigated by steady-state fluorescence spectroscopy in liposomes composed of mixtures of dipalmitoylphosphatidylcholine and cholesterol (0-40 mol %) as a function of temperature (24-51 degrees C). The fluorescent probes used (bis-pyrene, nystatin, prodan, and merocyanine) were chosen because they differ in the location they occupy in the membrane and in the types of properties they sense. Comparison of phase diagrams with contour plots of the fluorescence data suggested that bis-pyrene is sensitive primarily to lipid order. In contrast, nystatin fluorescence intensity responded to changes in lipid fluidity. The shape of the prodan emission spectrum detected both liquid-solid and order-disorder transitions in the phase diagram. Merocyanine's behavior was more complex. First, it was more sensitive than any of the other probes to the membrane pretransition that occurs in the absence of cholesterol. Second, regardless of whether emission intensity, anisotropy, or spectral shape was observed, the probe appeared to distinguish two types of liquid-ordered phases, one with tightly packed lipids and one in which the apparent spacing among lipids was increased. The prodan data supported these results by displaying modest versions of these two observations. Together, the results identify eight regions within the phase diagram of distinguishable combinations of these physical properties. As an example of how this combined analysis can be applied to biological membranes, human erythrocytes were treated similarly. Temperature variation at constant cholesterol content revealed three of the eight combinations identified in our analysis of liposomes.

Differences in membrane fluidity and fatty acid composition between phenotypic variants of Streptococcus pneumoniae

Phase variation in the colonial opacity of Streptococcus pneumoniae has been implicated as a factor in the pathogenesis of pneumococcal disease. This study examined the relationship between membrane characteristics and colony morphology in a few selected opaque-transparent couples of S. pneumoniae strains carrying different capsular types. Membrane fluidity was determined on the basis of intermolecular excimerization of pyrene and fluorescence polarization of 1,6-diphenyl 1,3,5-hexatriene (DPH). A significant decrease, 16 to 26% (P < or = 0.05), in the excimerization rate constant of the opaque variants compared with that of the transparent variants was observed, indicating higher microviscosity of the membrane of bacterial cells in the opaque variants. Liposomes prepared from phospholipids of the opaque phenotype showed an even greater decrease, 27 to 38% (P < or = 0.05), in the pyrene excimerization rate constant compared with that of liposomes prepared from phospholipids of bacteria with the transparent phenotype. These findings agree with the results obtained with DPH fluorescence anisotropy, which showed a 9 to 21% increase (P < or = 0.001) in the opaque variants compared with the transparent variants. Membrane fatty acid composition, determined by gas chromatography, revealed that the two variants carry the same types of fatty acids but in different proportions. The trend of modification points to the presence of a lower degree of unsaturated fatty acids in the opaque variants compared with their transparent counterparts. The data presented here show a distinct correlation between phase variation and membrane fluidity in S. pneumoniae. The changes in membrane fluidity most probably stem from the observed differences in fatty acid composition.

Instability of glutamate production by Corynebacterium glutamicum 2262 in continuous culture using the temperature-triggered process

Kinetics and physiology of Corynebacterium glutamicum 2262 cultured for extended periods in continuous mode were investigated at 33, 39 and 41 degrees C. At 33 degrees C no glutamate production occurred whatever the dilution rates tested (ranging between 0.05 and 0.5 h(-1)). When the continuous culture was performed at 39 degrees C and D=0.05 h(-1), the glutamate was actively produced, while the activities of 2-oxoglutarate dehydrogenase complex (ODHC) and pyruvate dehydrogenase (PDH) were, respectively completely inhibited and 35% decreased. Simultaneously, the intracellular glutamate was 62% reduced compared to the level found at 33 degrees C and the co-metabolites lactate and trehalose were excreted. The decrease in PDH activity during the glutamate production was suggested to be responsible for the accumulation of by-products and for limiting the carbon flux required for glutamate synthesis. When the culture was prolonged for more than 100 h, a cell selection occurred, in favor of growth and to the detriment of glutamate production. In fact, these selected cells presented high levels of ODHC and PDH activities even at 39 degrees C, resulting in a complete inhibition of the glutamate production after 150 h of culture. A further temperature increase till 41 degrees C restored the glutamate production and abolished the ODHC activity of these selected cells.

Role of membrane fluidity in pressure resistance of Escherichia coli NCTC 8164

The relationship among growth temperature, membrane fatty acid composition, and pressure resistance was examined in Escherichia coli NCTC 8164. The pressure resistance of exponential-phase cells was maximal in cells grown at 10 degrees C and decreased with increasing growth temperatures up to 45 degrees C. By contrast, the pressure resistance of stationary-phase cells was lowest in cells grown at 10 degrees C and increased with increasing growth temperature, reaching a maximum at 30 to 37 degrees C before decreasing at 45 degrees C. The proportion of unsaturated fatty acids in the membrane lipids decreased with increasing growth temperature in both exponential- and stationary-phase cells and correlated closely with the melting point of the phospholipids extracted from whole cells examined by differential scanning calorimetry. Therefore, in exponential-phase cells, pressure resistance increased with greater membrane fluidity, whereas in stationary-phase cells, there was apparently no simple relationship between membrane fluidity and pressure resistance. When exponential-phase or stationary-phase cells were pressure treated at different temperatures, resistance in both cell types increased with increasing temperatures of pressurization (between 10 and 30 degrees C). Based on the above observations, we propose that membrane fluidity affects the pressure resistance of exponential- and stationary-phase cells in a similar way, but it is the dominant factor in exponential-phase cells whereas in stationary-phase cells, its effects are superimposed on a separate but larger effect of the physiological stationary-phase response that is itself temperature dependent.

Effects of 2,2',5,5'-tetrachlorobiphenyl and biphenyl on cell membranes of Ralstonia eutropha H850

The effects of 2,2',5,5'-tetrachlorobiphenyl (TeCB), a PCB congener, and biphenyl on the cytoplasmic membranes of Ralstonia eutropha H850 were investigated by measuring fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene (DPH) as the probe, and determining the cellular fatty acid compositions. TeCB significantly affected the membrane of R. eutropha H850 cells grown on fructose by decreasing DPH fluorescence polarization. In contrast, the membrane of cells grown on biphenyl showed a considerably less significant effect of TeCB on membrane polarization than in fructose-grown cells. An increase in the ratio of total saturated to unsaturated fatty acids in cells grown on biphenyl suggested less of a fluidizing effect of TeCB on membranes in those cells. When biphenyl-grown cells were transferred back to a fructose medium, they required 25 generations for the membrane polarization and fatty acid compositions of these cells to revert back to those of the initial fructose-grown cells. The re-adaptation to a change in temperature required only five generations to return to normal. These results show that biphenyl affects cells in more ways than simply fluidizing the cytoplasmic membrane.