Fluorescence polarization studies on Escherichia coli membrane stability and its relation to the resistance of the cell to freeze-thawing. I. Membrane stability in cells of differing growth phase

Role of the lipid bilayer in outer membrane protein folding in Gram-negative bacteria

β-Barrel outer membrane proteins (OMPs) represent the major proteinaceous component of the outer membrane (OM) of Gram-negative bacteria. These proteins perform key roles in cell structure and morphology, nutrient acquisition, colonization and invasion, and protection against external toxic threats such as antibiotics. To become functional, OMPs must fold and insert into a crowded and asymmetric OM that lacks much freely accessible lipid. This feat is accomplished in the absence of an external energy source and is thought to be driven by the high thermodynamic stability of folded OMPs in the OM. With such a stable fold, the challenge that bacteria face in assembling OMPs into the OM is how to overcome the initial energy barrier of membrane insertion. In this review, we highlight the roles of the lipid environment and the OM in modulating the OMP-folding landscape and discuss the factors that guide folding in vitro and in vivo We particularly focus on the composition, architecture, and physical properties of the OM and how an understanding of the folding properties of OMPs in vitro can help explain the challenges they encounter during folding in vivo Current models of OMP biogenesis in the cellular environment are still in flux, but the stakes for improving the accuracy of these models are high. OMP folding is an essential process in all Gram-negative bacteria, and considering the looming crisis of widespread microbial drug resistance it is an attractive target. To bring down this vital OMP-supported barrier to antibiotics, we must first understand how bacterial cells build it.

Microbiology: Peeling Back the Layers of Bacterial Envelope Mechanics

The Gram-negative cell envelope has two important mechanical elements. Whereas the cell wall bears the brunt of the turgor pressure during normal growth, the outer membrane also provides necessary rigidity under physical stress.

Mechanisms of pressure-mediated cell death and injury in Escherichia coli: from fundamentals to food applications

High hydrostatic pressure is commercially applied to extend the shelf life of foods, and to improve food safety. Current applications operate at ambient temperature and 600 MPa or less. However, bacteria that may resist this pressure level include the pathogens Staphylococcus aureus and strains of Escherichia coli, including shiga-toxin producing E. coli. The resistance of E. coli to pressure is variable between strains and highly dependent on the food matrix. The targeted design of processes for the safe elimination of E. coli thus necessitates deeper insights into mechanisms of interaction and matrix-strain interactions. Cellular targets of high pressure treatment in E. coli include the barrier properties of the outer membrane, the integrity of the cytoplasmic membrane as well as the activity of membrane-bound enzymes, and the integrity of ribosomes. The pressure-induced denaturation of membrane bound enzymes results in generation of reactive oxygen species and subsequent cell death caused by oxidative stress. Remarkably, pressure resistance at the single cell level relates to the disposition of misfolded proteins in inclusion bodies. While the pressure resistance E. coli can be manipulated by over-expression or deletion of (stress) proteins, the mechanisms of pressure resistance in wild type strains is multi-factorial and not fully understood. This review aims to provide an overview on mechanisms of pressure-mediated cell death in E. coli, and the use of this information for optimization of high pressure processing of foods.

Complex spatial distribution and dynamics of an abundant Escherichia coli outer membrane protein, LamB

Advanced techniques for observing protein localization in live bacteria show that the distributions are dynamic. For technical reasons, most such techniques have not been applied to outer membrane proteins in Gram-negative bacteria. We have developed two novel live-cell imaging techniques to observe the surface distribution of LamB, an abundant integral outer membrane protein in Escherichia coli responsible for maltose uptake and for attachment of bacteriophage lambda. Using fluorescently labelled bacteriophage lambda tails, we quantitatively described the spatial distribution and dynamic movement of LamB in the outer membrane. LamB accumulated in spiral patterns. The distribution depended on cell length and changed rapidly. The majority of the protein diffused along spirals extending across the cell body. Tracking single particles, we found that there are two populations of LamB--one shows very restricted diffusion and the other shows greater mobility. The presence of two populations recalls the partitioning of eukaryotic membrane proteins between 'mobile' and 'immobile' populations. In this study, we have demonstrated that LamB moves along the bacterial surface and that these movements are restricted by an underlying dynamic spiral pattern.

Morphological and physiological changes induced by high hydrostatic pressure in exponential- and stationary-phase cells of Escherichia coli: relationship with cell death

The relationship between a loss of viability and several morphological and physiological changes was examined with Escherichia coli strain J1 subjected to high-pressure treatment. The pressure resistance of stationary-phase cells was much higher than that of exponential-phase cells, but in both types of cell, aggregation of cytoplasmic proteins and condensation of the nucleoid occurred after treatment at 200 MPa for 8 min. Although gross changes were detected in these cellular structures, they were not related to cell death, at least for stationary-phase cells. In addition to these events, exponential-phase cells showed changes in their cell envelopes that were not seen for stationary-phase cells, namely physical perturbations of the cell envelope structure, a loss of osmotic responsiveness, and a loss of protein and RNA to the extracellular medium. Based on these observations, we propose that exponential-phase cells are inactivated under high pressure by irreversible damage to the cell membrane. In contrast, stationary-phase cells have a cytoplasmic membrane that is robust enough to withstand pressurization up to very intense treatments. The retention of an intact membrane appears to allow the stationary-phase cell to repair gross changes in other cellular structures and to remain viable at pressures that are lethal to exponential-phase cells.

Changes in membrane fluidity and fatty acid composition of Pseudomonas putida CN-T19 in response to toluene

A bacterial isolate, Pseudomonas putida CN-T19, could grow in a two-phase medium with toluene up to 50% (v/v). Changes in fatty acid composition and membrane fluidity of the isolate were investigated to understand how this microorganism responds toluene. The changes in the ratios of unsaturated to saturated fatty acids were insignificant between cells grown with and without toluene. The changes in the ratio of cis- to trans-fatty acids of C16:1 and C18:1 was, however, significantly lower in cells grown with toluene than cells grown without toluene, giving approximately 1.3 and 9.7, respectively. Toluene had a fluidizing effect on the membrane of cells grown without toluene, resulting in decrease in membrane polarization ratio. Less fluidizing effect of toluene on the membrane of cells grown with toluene was observed, giving 11% of polarization percentage, which was significantly lower than 53% in cells grown without toluene. These results suggest that cis/trans isomeration of C16:1 and C18:1 makes cell membranes more rigid to respond toluene, and is an adaptive strategy allowing P. putida CN-T19 to grow in the presence of organic solvent.

The making of a gradient: IcsA (VirG) polarity in Shigella flexneri

The generation and maintenance of subcellular organization in bacteria is critical for many cell processes and properties, including growth, structural integrity and, in pathogens, virulence. Here, we investigate the mechanisms by which the virulence protein IcsA (VirG) is distributed on the bacterial surface to promote efficient transmission of the bacterium Shigella flexneri from one host cell to another. The outer membrane protein IcsA recruits host factors that result in actin filament nucleation and, when concentrated at one bacterial pole, promote unidirectional actin-based motility of the pathogen. We show here that the focused polar gradient of IcsA is generated by its delivery exclusively to one pole followed by lateral diffusion through the outer membrane. The resulting gradient can be modified by altering the composition of the outer membrane either genetically or pharmacologically. The gradient can be reshaped further by the action of the protease IcsP (SopA), whose activity we show to be near uniform on the bacterial surface. Further, we report polar delivery of IcsA in Escherichia coli and Yersinia pseudotuberculosis, suggesting that the mechanism for polar delivery of some outer membrane proteins is conserved across species and that the virulence function of IcsA capitalizes on a more global mechanism for subcellular organization.

Lipid Content and Cryotolerance of Bakers' Yeast in Frozen Doughs

The relationship between lipid content and tolerance to freezing at −50°C was studied in Saccharomyces cerevisiae grown under batch or fed-batch mode and various aeration and temperature conditions. A higher free-sterol-to-phospholipid ratio as well as higher free sterol and phospholipid contents correlated with the superior cryoresistance in dough or in water of the fed-batch-grown compared with the batch-grown cells. For both growth modes, the presence of excess dissolved oxygen in the culture medium greatly improved yeast cryoresistance and trehalose content (P. Gélinas, G. Fiset, A. LeDuy, and J. Goulet, Appl. Environ. Microbiol. 26:2453-2459, 1989) without significantly changing the lipid profile. Under the batch or fed-batch modes, no correlation was found between the cryotolerance of bakers' yeast and the total cellular lipid content, the total sterol content, the phospholipid unsaturation index, the phosphate or crude protein content, or the yeast cell morphology (volume and roundness).

Changes in chemical structure and function in Escherichia coli cell membranes caused by freeze-thawing. I. Change of lipid state in bilayer vesicles and in the original membrane fragments depending on rate of freezing

The effect of different rates of freezing on the character of lipids in unilamellar lipid bilayer vesicles and in the original membrane fragments of Escherichia coli B cells was investigated by measuring the temperature-dependent fluorescence polarization ratio changes of cis- and trans-parinaric acids. In lipid bilayer vesicles, both slow and rapid freezing brought about significant alterations in fluorescence polarization ratios in the specimens derived from both logarithmic and stationary-phase cells. In the original membrane fragments derived from logarithmic-phase cells, slow freezing gave rise to a similar alteration in fluorescence polarization ratio change, but no such alteration was found in the case of rapid freezing. Logarithmic-phase cells suffered from a membrane permeability change during slow freezing, which subsequently resulted in low cell viability. The cells suffered only slight impairment in membrane function during rapid freezing, and maintained higher viability. These results suggest that the primary site of damage due to freezing of the cells is the cellular membranes, and this destruction is due to a lipid state change in the membranes brought about by freezing.

Changes in chemical structure and function in Escherichia coli cell membranes caused by freeze-thawing. II. Membrane lipid state and response of cells to dehydration

Glycerol and spermine prevented the freeze-induced alteration of lipid character in membrane fragments derived from Escherichia coli B logarithmic-phase cells, protecting against loss of membrane function and subsequently maintaining higher cell viability. The membrane specimens derived from the stationary-phase cells exhibited high resistance to freezing-induced alteration of the membrane lipid character, and to freezing injury. Freeze-drying of membrane fragment specimens from both logarithmic- and stationary-phase cells gave rise to alterations in lipid state similar to those shown in freeze-thawing of logarithmic-phase cell membrane specimens. Freeze-drying brought about reduction of cell viability in both growth phase specimens. It is suggested, therefore, that the fundamental cause of the injury induced by freezing of living materials is dehydration of lipid-rich systems such as cellular membranes.

The interaction of alpha-tocopherol and homologues with shorter hydrocarbon chains with phospholipid bilayer dispersions. A fluorescence probe study

The intrinsic fluorescence of tocopherol homologues with hydrocarbon chains ranging from 1 carbon (C1) to 16 carbons (alpha-tocopherol, C16) and their ability to quench the fluorescence of 9-anthroyloxy derivatives of fatty acids with the fluorophore located at different positions in the hydrophobic domain of phospholipid bilayers has been used to model the interaction of tocopherol with lipid bilayer membranes. All the tocopherol homologues used, C1, C6, C11 and alpha-tocopherol, showed a similar fluorescence emission intensity at 325 nm in cyclohexane but were almost completely self-quenched by aggregation in water. Fluorescence was restored when dispersions of dimyristoylglycerophosphocholine were added but the maximum intensity was lower with the longer-chain homologues. Full intensity was observed in all homologues on addition of the detergent Triton X-100. Studies using 9-anthracenecarboxylic acid and 9-anthracenecarboxymethyl ester, 6-(9-anthroyloxy)stearic acid and 16-(9-anthroyloxy)palmitelaidic acid showed that the tocopherol homologues partitioned into the hydrophobic domain of phospholipid dispersions composed of dimyristoylglycerophosphocholine at 40 degrees C and dioleoylglycerophosphocholine at 40 degrees C. The 9-anthroyloxy fatty acids and 1,6-diphenyl-1,3,5-hexatriene were quenched by all the homologues and Stern-Volmer plots of the concentration dependence of the quenching indicated that this was predominantly via dynamic processes. No fluorescence energy transfer was observed between diphenylhexatriene and tocopherols but efficient transfer was recorded to the 9-anthroyloxy fatty acid probes. The results are consistent with a model in which the chromanol nucleus of tocopherol is oriented towards the lipid-water interface of the phospholipid bilayer. As the phytol chain length increases there is an increasing tendency for the chromanol nucleus to reside in the hydrophobic interior of the structure. alpha-Tocopherol appears to form clusters within the phospholipid dispersion which are not fluorescent and do not quench the fluorescence of the different fluorescent probes nor transfer fluorescence energy to them. It is suggested that the monomeric form is responsible for the vitamin effects of tocopherol and the aggregated form acts as a reservoir that does not markedly perturb bilayer stability.

Fluorescence polarization studies on Escherichia coli membrane stability and its relation to the resistance of the cell to freeze-thawing. II. Stabilization of the membranes by polyamines

The effects of polyamines, spermine, spermidine and putrescine on the stabilization of the membrane organization of Escherichia coli cells were studied using measurements of fluorescence polarization change of extrinsic fluorescence probes in membrane specimens as a function of temperature. The effects of the polyamines on the restoration of the cell viability after freeze-thawing were also investigated. In logarithmic-phase membrane specimens, polyamines depressed the polarization ratio increase below the transition temperatures in a dose-dependent manner. The physiologically relevant concentration of polyamines repressed the ratios to the same levels as are obtained with the stationary-phase specimens. In the stationary-phase specimens, no effect of polyamines on repression of the polarization increase was observed. A preliminary exposure of logarithmic-phase cells to polyamines protected the cells from the reduction of viability in freeze-thawing. However, a considerably high concentration and a certain length of preincubation time were required in order to an effect to be exerted. These results indicate that the intracellular polyamines could stabilize the membrane organization of logarithmic-phase cells to the same extent as in the stationary-phase cell membranes. It is conjectured that the membrane stability which is mediated by the polyamines results in cellular resistance to freeze-thawing, as it is attained by increasing the growth phase of the cells.