Physiological actions of preservative agents: prospective of use of modern microbiological techniques in assessing microbial behaviour in food preservation

Critical review on biofilm methods

Biofilms are widespread in nature and constitute an important strategy implemented by microorganisms to survive in sometimes harsh environmental conditions. They can be beneficial or have a negative impact particularly when formed in industrial settings or on medical devices. As such, research into the formation and elimination of biofilms is important for many disciplines. Several new methodologies have been recently developed for, or adapted to, biofilm studies that have contributed to deeper knowledge on biofilm physiology, structure and composition. In this review, traditional and cutting-edge methods to study biofilm biomass, viability, structure, composition and physiology are addressed. Moreover, as there is a lack of consensus among the diversity of techniques used to grow and study biofilms. This review intends to remedy this, by giving a critical perspective, highlighting the advantages and limitations of several methods. Accordingly, this review aims at helping scientists in finding the most appropriate and up-to-date methods to study their biofilms.

Solid-state NMR spectroscopic studies on the interaction of sorbic acid with phospholipid membranes at different pH levels

2H, 31P, and 1H-magic-angle-spinning (MAS) solid-state NMR spectroscopic methods were used to elucidate the interaction between sorbic acid, a widely used weak acid food preservative, and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers under both acidic and neutral pH conditions. The linewidth broadening observed in the 31P NMR powder pattern spectra and the changes in the 31P longitudinal relaxation time (T1) indicate interaction with the phospholipid headgroup upon titration of sorbic acid or decanoic acid into DMPC bilayers over the pH range from 3.0 to 7.4. The peak intensities of sorbic acid decrease upon addition of paramagnetic Mn2+ ions in DMPC bilayers as recorded in the 1H MAS NMR spectra, suggesting that sorbic acid molecules are in close proximity with the membrane/aqueous surface. No significant 2H quadrupolar splitting (DeltanuQ) changes are observed in the 2H NMR spectra of DMPC-d54 upon titration of sorbic acid, and the change of pH has a slight effect on DeltanuQ, indicating that sorbic acid has weak influence on the orientation order of the DMPC acyl chains in the fluid phase over the pH range from 3.0 to 7.4. This finding is in contrast to the results of the decanoic acid/DMPC-d54 systems, where DeltanuQ increases as the concentration of decanoic acid increases. Thus, in the membrane association process, sorbic acids are most likely interacting with the headgroups and shallowly embedded near the top of the phospholipid headgroups, rather than inserting deep into the acyl chains. Thus, antimicrobial mode of action for sorbic acid may be different from that of long-chain fatty acids.

Growth inhibition mode of action of selected benzoic acid derivatives against the yeast Pichia anomala

Three benzoic acid derivatives (zinc p-iodobenzoate, zinc p-hydroxybenzoate, and zinc p-aminobenzoate) were synthesized and tested chemically and microbiologically in order to explain their mode of action against the yeast Pichia anomala. The yeast strains were cultivated in batch culture of chemically defined minimal medium (control) and with the addition of the studied compound at concentrations of 0.103 to 0.166% (wt/vol). The growth of microorganisms, H+ concentration, and the concentrations of both dissociated and undissociated forms of the appropriate weak acid in the medium were monitored at 1-h intervals during 24 h of incubation. The inhibitory effect of each compound on the growth of microorganisms was calculated based on measurement of optical density at 600 nm turbidity. The K parameter, defined as the ratio of the concentration of undissociated weak acid to the number of microorganisms in the medium, was determined. The K value is related to the degree of growth inhibition and provides new insight into the mode of action of weak organic acids against the studied yeasts. The buffering capacity of the chemicals studied was also found to be an inhibition parameter associated with microbial growth. Greater buffer capacity of a given compound reduced changes in the pH value of the medium, resulting in changes to antimicrobial effectiveness.

Synthesis of the food flavoring methyl benzoate by genetically engineered Saccharomyces cerevisiae

Current means of production for plant-derived aroma compounds include chemical synthesis and extraction from plant material. Both methods are environmentally detrimental and relatively expensive: plant material is only seasonally available and only a small subset of the plant biomass produces the desired aroma compounds, while organic synthesis inevitably involves waste byproducts with a negative ecological impact. Benzenoids are a class of plant metabolites that includes a number of aroma compounds. This paper explores, for the first time, the feasibility of producing benzenoids in yeast. We present a method for the production of the phenylpropanoid methyl benzoate in Saccharomyces cerevisiae using benzoic acid as a substrate, by heterologous expression of Antirrhinum majus benzoic acid methyl transferase. Production was pH dependent with a maximal yield of approximately 50 microg of methyl benzoate per liter of culture per hour, and with linear kinetics over at least 24 h. In addition, we have analyzed two alternative expression vectors for the production of benzoic acid methyl transferase in S. cerevisiae: a constitutive triosephosphate isomerase promoter-based system was compared with a copper-inducible CUP1 promoter system. We find major differences in the amounts of methylbenzoate produced by these respective systems. Potential applications are discussed.

Activation of the protein kinase C1 pathway upon continuous heat stress in Saccharomyces cerevisiae is triggered by an intracellular increase in osmolarity due to trehalose accumulation

This paper reports on physiological and molecular responses of Saccharomyces cerevisiae to heat stress conditions. We observed that within a very narrow range of culture temperatures, a shift from exponential growth to growth arrest and ultimately to cell death occurred. A detailed analysis was carried out of the accumulation of trehalose and the activation of the protein kinase C1 (PKC1) (cell integrity) pathway in both glucose- and ethanol-grown cells upon temperature upshifts within this narrow range of growth temperatures. It was observed that the PKC1 pathway was hardly activated in a tps1 mutant that is unable to accumulate any trehalose. Furthermore, it was observed that an increase of the extracellular osmolarity during a continuous heat stress prevented the activation of the pathway. The results of these analyses support our hypothesis that under heat stress conditions the activation of the PKC1 pathway is triggered by an increase in intracellular osmolarity, due to the accumulation of trehalose, rather than by the increase in temperature as such.

Assessment of heat resistance of bacterial spores from food product isolates by fluorescence monitoring of dipicolinic acid release

This study is aimed at the development and application of a convenient and rapid optical assay to monitor the wet-heat resistance of bacterial endospores occurring in food samples. We tested the feasibility of measuring the release of the abundant spore component dipicolinic acid (DPA) as a probe for heat inactivation. Spores were isolated from the laboratory type strain Bacillus subtilis 168 and from two food product isolates, Bacillus subtilis A163 and Bacillus sporothermodurans IC4. Spores from the lab strain appeared much less heat resistant than those from the two food product isolates. The decimal reduction times (D values) for spores from strains 168, A163, and IC4 recovered on Trypticase soy agar were 1.4, 0.7, and 0.3 min at 105 degrees C, 120 degrees C, and 131 degrees C, respectively. The estimated Z values were 6.3 degrees C, 6.1 degrees C, and 9.7 degrees C, respectively. The extent of DPA release from the three spore crops was monitored as a function of incubation time and temperature. DPA concentrations were determined by measuring the emission at 545 nm of the fluorescent terbium-DPA complex in a microtiter plate fluorometer. We defined spore heat resistance as the critical DPA release temperature (Tc), the temperature at which half the DPA content has been released within a fixed incubation time. We found Tc values for spores from Bacillus strains 168, A163, and IC4 of 108 degrees C, 121 degrees C, and 131 degrees C, respectively. On the basis of these observations, we developed a quantitative model that describes the time and temperature dependence of the experimentally determined extent of DPA release and spore inactivation. The model predicts a DPA release rate profile for each inactivated spore. In addition, it uncovers remarkable differences in the values for the temperature dependence parameters for the rate of spore inactivation, DPA release duration, and DPA release delay.

Benzoic acid, a weak organic acid food preservative, exerts specific effects on intracellular membrane trafficking pathways in Saccharomyces cerevisiae

Microbial spoilage of food causes losses of up to 40% of all food grown for human consumption worldwide. Yeast growth is a major factor in the spoilage of foods and beverages that are characterized by a high sugar content, low pH, and low water activity, and it is a significant economic problem. While growth of spoilage yeasts such as Zygosaccharomyces bailii and Saccharomyces cerevisiae can usually be retarded by weak organic acid preservatives, the inhibition often requires levels of preservative that are near or greater than the legal limits. We identified a novel synergistic effect of the chemical preservative benzoic acid and nitrogen starvation: while exposure of S. cerevisiae to either benzoic acid or nitrogen starvation is cytostatic under our conditions, the combination of the two treatments is cytocidal and can therefore be used beneficially in food preservation. In yeast, as in all eukaryotic organisms, survival under nitrogen starvation conditions requires a cellular response called macroautophagy. During macroautophagy, cytosolic material is sequestered by intracellular membranes. This material is then targeted for lysosomal degradation and recycled into molecular building blocks, such as amino acids and nucleotides. Macroautophagy is thought to allow cellular physiology to continue in the absence of external resources. Our analyses of the effects of benzoic acid on intracellular membrane trafficking revealed that there was specific inhibition of macroautophagy. The data suggest that the synergism between nitrogen starvation and benzoic acid is the result of inhibition of macroautophagy by benzoic acid and that a mechanistic understanding of this inhibition should be beneficial in the development of novel food preservation technologies.