Cell membranes and multilamellar vesicles: influence of pH on solvent induced damage

Listeria monocytogenes - How This Pathogen Survives in Food-Production Environments?

The foodborne pathogen Listeria monocytogenes is the causative agent of human listeriosis, a severe disease, especially dangerous for the elderly, pregnant women, and newborns. Although this infection is comparatively rare, it is often associated with a significant mortality rate of 20-30% worldwide. Therefore, this microorganism has an important impact on food safety. L. monocytogenes can adapt, survive and even grow over a wide range of food production environmental stress conditions such as temperatures, low and high pH, high salt concentration, ultraviolet lights, presence of biocides and heavy metals. Furthermore, this bacterium is also able to form biofilm structures on a variety of surfaces in food production environments which makes it difficult to remove and allows it to persist for a long time. This increases the risk of contamination of food production facilities and finally foods. The present review focuses on the key issues related to the molecular mechanisms of the pathogen survival and adaptation to adverse environmental conditions. Knowledge and understanding of the L. monocytogenes adaptation approaches to environmental stress factors will have a significant influence on the development of new, efficient, and cost-effective methods of the pathogen control in the food industry, which is critical to ensure food production safety.

Macrophages use a bet-hedging strategy for antimicrobial activity in phagolysosomal acidification

Microbial ingestion by a macrophage results in the formation of an acidic phagolysosome but the host cell has no information on the pH susceptibility of the ingested organism. This poses a problem for the macrophage and raises the fundamental question of how the phagocytic cell optimizes the acidification process to prevail. We analyzed the dynamical distribution of phagolysosomal pH in murine and human macrophages that had ingested live or dead Cryptococcus neoformans cells, or inert beads. Phagolysosomal acidification produced a range of pH values that approximated normal distributions, but these differed from normality depending on ingested particle type. Analysis of the increments of pH reduction revealed no forbidden ordinal patterns, implying that the phagosomal acidification process was a stochastic dynamical system. Using simulation modeling, we determined that by stochastically acidifying a phagolysosome to a pH within the observed distribution, macrophages sacrificed a small amount of overall fitness to gain the benefit of reduced variation in fitness. Hence, chance in the final phagosomal pH introduces unpredictability to the outcome of the macrophage-microbe, which implies a bet-hedging strategy that benefits the macrophage. While bet hedging is common in biological systems at the organism level, our results show its use at the organelle and cellular level.

Application of Chlorine Dioxide in Cell Surface Modification to Enhance Its Mechanical Stability and Metal Ion Adsorption

There has been a trend toward the use of microorganisms as the biomaterial for removing dyes and metals from wastewater. However, native microorganism cells have low mechanical stability, which limit their further application in industries. In this study, chlorine dioxide (ClO₂), a high-efficiency, low-toxicity, and environmentally benign disinfectant, was used for microorganism surface modification to enhance the mechanical stability and metal ion adsorption of the cell. ClO₂ can either modify cell walls to improve their metal adsorption capacity or modify cell membranes to improve their mechanical stability. Fourier-transform infrared spectroscopy analysis indicated that several cell surface groups were involved in the cell wall modification of Bacillus sp. Microscopic observation indicated that ClO₂ treatment could deter cell membranes from forming vesicles in sodium hydroxide (NaOH) aqueous solution, and freeze-etching showed that ClO₂ treatment could alter the erythrocyte membrane proteins which might also contribute to improving the cell stability. The experimental results on Bacillus sp., Pseudomonas aeruginosa, and Mucor rouxii show that ClO₂ treatment may increase, or at least not reduce, the ability of microbial cells to adsorb heavy metals, but it can significantly improve the resistance of these cells to NaOH cleavage. It seems ClO₂ is a promising auxiliary for biosorption of heavy-metal ions.

Inactivation mechanisms of pathogenic bacteria in several matrixes during the composting process in a composting toilet

This study aimed to compare the inactivation rate and the mechanisms of pathogenic bacteria in three matrixes (sawdust, rice husk and charcoal) during the composting process. The inactivation rate was evaluated with Escherichia coli strain and the damaged parts and/or functions were evaluated with three different media. Normalized inactivation rate constant in three media and from three matrixes had no significant difference in each process (pure, 1 month and 2 months). The value in rice husk was relatively increased during 2 months but there was no significant difference. The inactivation rate constants of Tryptic Soy Agar (TSA) and Compact Dry E. coli/Coliform in pure sawdust and rice husk were relatively lower than that of Desoxycholate Agar, but increased in 2 months. This indicated that damaging part was changed from outer membrane to enzymes and metabolisms during the 2-month composting process. In the case of charcoal, only the TSA value in apure matrix was relatively lower than that of others, but it increased in 2 months. This indicated that damaging part was changed from outer membrane and enzyme to metabolisms during the composting process. Composting matrix and composting process did not significantly affect inactivation rate of pathogenic bacteria during the process but affected the damaging part of the bacteria.

Apolipophorin III interaction with model membranes composed of phosphatidylcholine and sphingomyelin using differential scanning calorimetry

Apolipophorin III (apoLp-III) from Locusta migratoria was employed as a model apolipoprotein to gain insight into binding interactions with lipid vesicles. Differential scanning calorimetry (DSC) was used to measure the binding interaction of apoLp-III with liposomes composed of mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and sphingomyelin (SM). Association of apoLp-III with multilamellar liposomes occurred over a temperature range around the liquid crystalline phase transition (L(alpha)). Qualitative and quantitative data were obtained from changes in the lipid phase transition upon addition of apoLp-III. Eleven ratios of DMPC and SM were tested from pure DMPC to pure SM. Broadness of the phase transition (T(1/2)), melting temperature of the phase transition (T(m)) and enthalpy were used to determine the relative binding affinity to the liposomes. Multilamellar vesicles composed of 40% DMPC and 60% SM showed the greatest interaction with apoLp-III, indicated by large T(1/2) values. Pure DMPC showed the weakest interaction and liposomes with lower percentage of DMPC retained domains of pure DMPC, even upon apoLp-III binding indicating demixing of liposome lipids. Addition of apoLp-III to rehydrated liposomes was compared to codissolved trials, in which lipids were rehydrated in the presence of protein, forcing the protein to interact with the lipid system. Similar trends between the codissolved and non-codissolved trials were observed, indicating a similar binding affinity except for pure DMPC. These results suggested that surface defects due to non-ideal packing that occur at the phase transition temperature of the lipid mixtures are responsible for apolipoprotein-lipid interaction in DMPC/SM liposomes.

Survival of Escherichia coli after isoelectric solubilization and precipitation of fish protein

Protein recovery for fish processing by-products utilizes extreme pH shifts for isoelectric solubilization and precipitation. The purpose of this study was to determine if Escherichia coli would survive exposure to the extreme pH shifts during the protein recovery process. Fresh rainbow trout were beheaded, gutted, and minced and then inoculated with approximately 10(9) CFU of E. coli ATCC 25922 per g, homogenized, and brought to the target pH of 2.0, 3.0, 11.5, or 12.5 by the addition of concentrated hydrochloric acid or sodium hydroxide to solubilize muscle proteins. The homogenate was blended and centrifuged to separate the lipid and insoluble components (bones, skin, insoluble protein, etc.) from the protein solution. The protein solution was subjected to a second pH shift (pH 5.5) resulting in protein precipitation that was recovered with centrifugation. Microbial analysis was conducted on each fraction (i.e., lipid, insoluble components, protein, and water) with selective and nonselective media. The sums of the surviving E. coli in these fractions were compared with the initial inoculum. The greatest total microbial reduction occurred when the pH was shifted to 12.5 (P < 0.05), i.e., a 4.4-log reduction of cells on nonselective media and a 6.0-log reduction of cells on selective media. The use of selective and nonselective media showed that there was significant (P < 0.05) injury sustained by cells exposed to alkaline treatment (pH 11.5 and 12.5) in all fractions except the insoluble fraction at pH 11.5. Increasing the exposure time or the pH may result in greater bacterial reductions in the recovered protein.

Destruction of gram-negative food-borne pathogens by high pH involves disruption of the cytoplasmic membrane

High pH has been shown to rapidly destroy gram-negative food-borne pathogens; however, the mechanism of destruction has not yet been elucidated. Escherichia coli O157:H7, Salmonella enteritidis ATCC 13706, and Listeria monocytogenes F5069 were suspended in NaHCO3-NaOH buffer solutions at pH 9, 10, 11, or 12 to give a final cell concentration of approximately 5.2 x 10(8) CFU/ml and then held at 37 or 45 degrees C. At 0, 5, 10, and 15 min the suspensions were sterilely filtered and each filtrate was analyzed for material with A260. Viability of the cell suspensions was evaluated by enumeration on nonselective and selective agars. Cell morphology was evaluated by scanning electron microscopy and transmission electron microscopy. A260 increased dramatically with pH and temperature for both E. coli and S. enteritidis; however, with L. monocytogenes material with A260 was not detected at any of the pHs tested. At pH 12, numbers of E. coli and S. enteritidis decreased at least 8 logs within 15 s, whereas L. monocytogenes decreased by only 1 log in 10 min. There was a very strong correlation between the initial rate of release of material with A260 and death rate of the gram-negative pathogens (r = 0.997). At pH 12, gram-negative test cells appeared collapsed and showed evidence of lysis while gram-positive L. monocytogenes did not, when observed by scanning and transmission electron microscopy. It was concluded that destruction of gram-negative food-borne pathogens by high pH involves disruption of the cytoplasmic membrane.

Cholesterol limits estrogen uptake by liposomes and erythrocyte membranes

Multilamellar vesicles (MLV) were prepared from phospholipids with and without cholesterol in equimolar amounts and [4-14C]estradiol. Unincorporated estrogen was removed by petroleum ether extraction or by aqueous buffer washes. In either case, cholesterol-containing vesicles incorporated one-half the estradiol as vesicles without sterol. Addition of estradiol to preformed vesicles followed by buffer washes showed that vesicles without cholesterol invariably retained more estradiol than those with the sterol. Reduction of the cholesterol content to one-half increased estradiol incorporation. The pattern of estradiol removal from MLV with successive buffer washes indicated that much of the steroid associated with cholesterol-containing vesicles was superficially bound to the membrane but vesicles without cholesterol incorporated the estrogen into the bilayer structure. To test the role of cholesterol in limiting the uptake of an estrogen by cells, right-side out resealed ghosts of ox erythrocytes were prepared. They were partially depleted of cholesterol by exposure to small unilamellar vesicles of dioleoylphosphatidyl choline. A decrease in cholesterol content correlates with an increase in estradiol uptake by red cell ghosts. The experiments described point to a central role of cholesterol in limiting the uptake of steroids. The loss of cholesterol of steroid producing cells caused by tropic hormones may be key to their mode of action in promoting secretion of steroid hormones. Likewise, the long-term genomic responses of steroid target cells may depend upon their cholesterol content and the ease by which the steroid can penetrate the membrane barrier.