The influence of dissolved CO(2) concentration on the death kinetics of Saccharomyces cerevisiae

Heat resistance of five spoilage microorganisms in a carbonated broth

Despite their acidic pH, carbonated beverages can be contaminated by spoilage microorganisms. Thermal treatments, before and/or after carbonation, are usually applied to prevent the growth of these microorganisms. However, the impact of CO2 on the heat resistance of spoilage microorganisms has never been studied. A better understanding of the combined impact of CO2 and pH on the heat resistance of spoilage microorganisms commonly found in carbonated beverages might allow to optimize thermal treatment. Five microorganisms were selected for this study: Alicyclobacillus acidoterrestris (spores), Aspergillus niger (spores), Byssochlamys fulva (spores), Saccharomyces cerevisiae (vegetative cells), and Zygosaccharomyces parabailii (vegetative cells). A method was developed to assess the impact of heat treatments in carbonated media on microbial resistance. The heat resistances of the five studied species are coherent with the literature, when data were available. However, neither the dissolved CO2 concentration (from 0 to 7 g/L), nor the pH (from 2.8 to 4.1) have an impact on the heat resistance of the selected microorganisms, except for As. niger, for which the presence of dissolved CO2 reduced the heat resistance. This study improved our knowledge about the heat resistance of some spoilage microorganisms in presence of CO2.

The association of air pollutants (CO2, MTBE) on Candida albicans and Candida glabrata drug resistance

Introduction

Therapeutic methods are very important in the prevalence of opportunistic fungal infections, which are an important cause of human diseases. In this study, air pollution agents that are in direct contact with microorganisms, and the effects of carbon sources using CO2 and MTBE on growth of fungi, and particularly the evaluation of changes in the expression of interfering genes in susceptibility and drug resistance in these fungi, were investigated.

Materials and Methods

Collecting samples and isolating Candida glabrata and Candida albicans with phenotypic methods were accomplished. We then evaluated the minimum inhibitory concentration (MIC) with the M27A4 protocol of CLSI. We adjusted 20 strains of C. albicans and 10 strains of C. glabrata whose sensitivity was evaluated in the MIC test with 5% CO2 and 5mg/ml methyl tert-butyl ether (MTBE) considered as air pollutants, and followed by re-evaluating MIC testing to separate azole-resistant strains. Interfering agents were also considered.

Results

Upregulation of some genes on the two mentioned yeasts had led to drug resistance in them; they were previously sensitive to both drugs. Correspondingly, 41% of C. glabrata samples in sputum showed sensitivity to these drugs. Upregulation of ERG11 (71%) and EPA1 (90%) were observed in resistant strains. Upregulation of genes associated with aspartate proteins and downregulation of SAP3 genes were recognized in C. glabrata in sputum and a 15% downregulation of bronchoalveolar lavage (BAL) isolate and 50% upregulation of SAP1 gene in C. albicans sensitive samples were observed and compared to fluconazole and itraconazole with the oral and joint sources. Remarkably, decreased SAP2 expression in oral sources and a 60% increase in resistant strains in C. albicans were observed. The downregulation of SAP3 expression showed in the joint samples. An increase in HWP1 expression (30%) was noted in isolated and drug-sensitive samples at the sputum and BAL source. CDR1 expression was increased in MTBE-affected species; however, it decreased in the vicinity of CT.

Conclusions

Air pollutants such as CO2 and MTBE eventually caused drug resistance in Candida, which can be one of the causes of drug resistance in candidiasis infections.

Effects of Killing Methods on Lipid Oxidation, Colour and Microbial Load of Black Soldier Fly (Hermetia illucens) Larvae

Black soldier fly (BSF) larvae represent a promising alternative ingredient for animal feed. Post-production processing can, however, affect their quality. This project aimed to optimize larval killing by comparing the effects on the nutritional and microbiological quality of 10 methods, i.e., blanching (B = 40 s), desiccation (D = 60 °C, 30 min), freezing (F20 = -20 °C, 1 h; F40 = -40 °C, 1 h; N = liquid nitrogen, 40 s), high hydrostatic pressure (HHP = 3 min, 600 MPa), grinding (G = 2 min) and asphyxiation (CO2 = 120 h; N2 = 144 h; vacuum conditioning, V = 120 h). Some methods affected the pH (B, asphyxiation), total moisture (B, asphyxiation and D) and ash contents (B, p < 0.001). The lipid content (asphyxiation) and their oxidation levels (B, asphyxiation and D) were also affected (p < 0.001). Killing methods altered the larvae colour during freeze-drying and in the final product. Blanching appears to be the most appropriate strategy since it minimizes lipid oxidation (primary = 4.6 ± 0.7 mg cumen hydroperoxide (CHP) equivalents/kg; secondary = 1.0 ± 0.1 mg malondialdehyde/kg), reduces microbial contamination and initiates dehydration (water content = 78.1 ± 1.0%). We propose herein, an optimized protocol to kill BSF that meet the Canadian regulatory requirements of the insect production and processing industry.

Morphological changes and induction of antifungal resistance in Aspergillus fumigatus due to different CO2 levels

Background and Purpose

Aspergillosis is one of the most common opportunistic fungal infections in immunocompromised and neutropenic patients. Aspergillus fumigatus (A. fumigatus) is the most common causative agent of this infection. Due to variable CO₂ concentrations that pathogens are exposed to during the infection process and to understand the role of CO₂, we examined the effects of various CO₂ concentrations as one of the environmental factors on morphological changes and induction of antifungal resistance in A. fumigatus.

Materials and Methods

A. fumigatus strains were cultured and incubated under 1%, 3%, 5%, and 12% CO₂ atmospheres, each time for one, two, and four weeks. The control culture was maintained for one week without CO₂ atmosphere. Morphological changes were investigated and antifungal susceptibility test was performed according to the recommendations of the Clinical and Laboratory Standards Institute (CLSI) M38-A2 document. The results of different CO₂ atmospheres were compared with that of the control sample.

Results

We found that 1%, 3%, 5%, and 12% CO₂ atmospheres were associated with morphological colony changes. Macroscopically, the colonies were shallow dark green, smooth, crisp to powdery with reduced growth; microscopic examination revealed the absence of conidiation. The induction of antifungal resistance in the susceptible strains to itraconazole, voriconazole, and amphotericin B increased after exposure to 12% CO₂ atmosphere and four weeks of incubation. The MIC values for itraconazole, voriconazole, and amphotericin B were 16 g/ml, 1 g/ml, and 16 g/ml, respectively. These values for the control group were 0.125 g/ml, 0.125 g/ml, and 2 g/ml, respectively.

Conclusion

Exposure to different CO₂ atmospheres induced morphological changes in A. fumigatus, it seems to increase the MIC values, as well. In parallel, resistance to both itraconazole and voriconazole was also observed.

Pseudohyphae formation in Candida glabrata due to CO2 exposure

BACKGROUND AND PURPOSE

Formation of pseudohyphae is considered a virulence factor in Candida species. Generally, Candida glabrata grows as budding yeast cells; however, reports illustrated that C. glabrata could form pseudohyphal cells in response to some stimuli. In this study, we provided insight into the ability of C. glabrata in forming pseudohyphal cells under different levels of carbon dioxide (CO₂).

MATERIALS AND METHODS

Candida glabrata reference strain (ATCC 90030) was used in this study. Yeast samples were cultured on Sabouraud dextrose broth (SDB) medium and incubated under 3%, 5%, and 10% CO₂ levels for 24, 48 and 72 h. Control cultures were prepared without CO₂ pressure for three days. The possibility of pseudohyphae and mycelium formation in C. glabrata was investigated.

RESULTS

The results of this study revealed that the most branching filament-like cells were obtained at high CO₂ pressure (10%) after 72 h. After three days of low CO₂ pressure (3%), only yeast and budding cells were observed without any pseudohyphae formation.

CONCLUSION

CO₂ could act as a stimulus and induced formation of pseudohyphae in Candida glabrata yeast cells.

Carbon Dioxide as a Potential Water Disinfestant for Phytophthora Disease Risk Mitigation

The spread of Phytophthora spp. through irrigation systems and natural waterways can have a significant impact on plant health and requires mitigation. Pressurized carbon dioxide (CO₂) can inactivate Phytophthora nicotianae zoospores but its effectiveness at low pressure and on other species was unknown. This study evaluated the effect of injected CO₂ at 63 to 4,000 ppm in irrigation water on zoospore survival of four Phytophthora spp. and infectivity of P. nicotianae zoospores. Zoospore survival of P. nicotianae, P. tropicalis, and P. pini was reduced by over 90% at 4,000 ppm and was reduced by 40% at 125 to 2,000 ppm after a 2-h exposure. Survival of P. megasperma was less affected by injected CO₂, with a reduction of 37.1% at ≤4,000 ppm. CO₂ treatments at 4,000 ppm for 30 or 120 min of water infested with P. nicotianae at zoospore concentrations of 1,000 and 5,000 ml^-1 reduced disease incidence of annual vinca (Catharanthus roseus) by 92 and 75%. Comparable efficacy was observed in the CO₂ treatment at 2,000 ppm. The CO₂ treatments at <2,000 ppm also significantly reduced disease caused by water infested at 1,000 zoospores ml^-1. These results indicate that CO₂ may have potential as a safe and effective water disinfestant for Phytophthora spp.

Supercritical Fluid Pasteurization and Food Safety

Supercritical fluid pasteurization has been studied for over 20 years and the state of science and technology is such that it is now a viable and economical alternative to thermal pasteurization for a number of food products. The manufacture and distribution of food faces increasingly strict demands in terms of both safety and quality. Traditional thermal pasteurization is both effective and well-accepted by the public for milk and other products. However, thermal treatment is less effective and sometimes infeasible for certain products, such as fruit juices, seafoods and fresh vegetables. This is particularly true when the food products are packaged and shipped long distances. Supercritical fluid technology, a non-thermal, low temperature process, has been shown to reduce the viability of a number of pathogenic organisms important to the food industry. In addition, supercritical fluids, particularly CO2, have promise in deactivating subcellular pathogens such as prions and viruses. Numerous basic science investigations reveal the mechanism of supercritical fluid pasteurization and how it differs from thermal methods. Several commercial companies have issued patents and built demonstration plants based on the technology. In addition, certain supercritical fluids may provide additional benefits for food processors. This chapter provides a comprehensive review of both science and technology of supercritical fluid technology as applied to foods.

Effects of hydrostatic pressure, agitation and CO2 stress on Phytophthora nicotianae zoospore survival

BACKGROUND

Phytophthora nicotianae Breda de Haan is a common pathogen of ornamental plants in recycled irrigation systems. In a previous study, annual vinca (Catharanthus roseus Don) inoculated with zoospore suspensions using a CO(2)-pressurized sprayer had less foliage blight than plants inoculated using a hand sprayer. Here, the impact of hydrostatic pressure, agitation and aeration with CO(2) on the survival of P. nicotianae zoospores was examined.

RESULTS

Exposure of zoospores to 840 kPa hydrostatic pressure for 8 min or agitation at a mixing intensity (G) of 6483 s(-1) for 4 min at 22-23 degrees C did not kill zoospores, but resulted in viable cysts. Motile and forcefully encysted zoospores of P. nicotianae were equally infectious on vinca or lupine (Lupinus polyphylus Lindl.). Bubbling CO(2) into zoospore-infested water at 110.4 mL (0.2 g) min(-1) for 5 min caused 81% reduction in the number of germinated zoospores. Pressure at 630 kPa (16.3 g CO(2)) or 70 kPa (3.85 g CO(2)) facilitated CO(2) injection and shortened the zoospore inactivation time to 30 s. When air was bubbled through the suspension, germination was similar to the control.

CONCLUSIONS

Exposure to CO(2) killed P. nicotianae zoospores in water. Neither pressure nor agitation had an effect on zoospore viability or infectivity. Based on results of this study, the authors designed a recycling CO(2) water treatment system that is currently under evaluation.

Modeling the inactivation of Salmonella typhimurium by dense phase carbon dioxide in carrot juice

The inactivation of Salmonella typhimurium inoculated into acidified carrot juice subjected to dense phase carbon dioxide (DPCD) was investigated. The pressures in the study were 10, 20 and 30 MPa, the temperatures were 32, 37 and 42 degrees C, and the treatment time was 5-90 min. The inactivation effect of DPCD was enhanced by increasing pressure and temperature. The sigmoid inactivation curves were characterized with the lag phase, exponential inactivation phase, and resistant phase. The inactivation curves were fitted to the modified Gompertz equation and the modified Logistic equation, the modified Gompertz equation was superior since its lowest residual sum of squares (RSS) was lower although there was no significant difference of goodness-of-fit between both models as indicated by F-test. The lambda (the duration of the lag phase) and t(4-D) (the time necessary to achieve 4-log cycles reduction) decreased with increasing pressure or temperature. The k(dm) (the maximum specific value of the inactivation rate, min(-1)) increased with increasing temperatures, and decreased with increasing pressures. The activation energy (Ea) and the activation volume (Va) necessary for inactivating S. typhimurium by DPCD were 19.06-29.39 kJ mol(-1) and 18.89-58.27 cm(3) mol(-1).

Microbial inactivation and shelf life of apple juice treated with high pressure carbon dioxide

Apple juice prepared from 'Annurca' apple puree was treated with a HPCD batch system. The pH, °Brix, color parameters and microbial load of the treated apple juice were compared with those of thermally processed juice. Thermal processes were carried out at 35, 50, 65, 85°C and treatment times ranging between 10 and 140 minutes. Microbial inactivation kinetics indicated that 5-log reduction of natural flora in apple juice was achieved at 85°C and 60 minutes of treatment time for conventional thermal process and at 16.0 MPa, 60°C and 40 minutes for HPCD process. Results suggested that temperature played a fundamental role on HPCD treatment efficiency, with inactivation significantly enhanced when it increased from 35 to 60°C. Less significant was the role of the pressure at the tested levels of 7.0, 13.0 and 16.0 MPa. Also, 5-log reduction of natural flora in apple juice was obtained at lower temperatures by cyclic treatments of six compression and decompression steps. There were no significant differences between treated and untreated samples in °Brix (α = 0.05). Significant differences were detected in pH values between the untreated and HPCD treated samples (α = 0.05). There was a significant decrease in 'L*' and 'b*' values and also differences were detected in 'a*' values between the untreated and the HPCD treated samples (α = 0.05). Statistical analysis for °Brix, pH and color data showed no differences between the untreated and HPCD treated samples in the first 2 weeks of storage at 4°C. These results emphasize the potential use of HPCD in industrial applications.

Inactivation of Escherichia coli inoculated into cloudy apple juice exposed to dense phase carbon dioxide

The inactivation of Escherichia coli in cloudy apple juice by dense phase carbon dioxide (DPCD) was investigated. With CO2 at 20 MPa and 37 degrees C or at 30 MPa and 42 degrees C, the inactivation of E. coli significantly increased (p<0.05) when increasing the exposure time, which conformed to a fast-to-slow two-stage kinetics. The two stages were well fitted to first-order reactions. Higher temperature or pressure significantly enhanced the bactericidal effect of DPCD (p<0.05), the maximum reduction was 7.66 log CFU at 45 MPa and 52 degrees C for 30 min. The survival curves against temperature or pressure were fitted using a linear equation with high regression coefficients (R2>0.94). The temperature inactivation rate (kT) and pressure inactivation rate (kP) were obtained. Higher kT or kP indicated higher susceptibility of E. coli to temperature or pressure. Moreover, there was good linear correlation of kT with pressure (R2=1.00). Also, kP increased with increasing temperature except for 37 degrees C. Greater inactivation of E. coli was obtained with 99.9% CO2 than with 99.5% CO2 or with the initial number of 10(5) CFU/mL than with that of 10(8) CFU/mL at 20 MPa and 37 degrees C.

Modeling the inactivation of Escherichia coli O157:H7 and generic Escherichia coli by supercritical carbon dioxide

In this study, supercritical carbon dioxide (SC-CO(2)) was applied in the inactivation of pathogenic Escherichia coli (E. coli) O157:H7 and generic E. coli. For both strains suspended in physiological saline (PS), colony forming units per ml were reduced by 8 log orders within 15-30 min, in a treatment range of 80-150 bar and 35-45 degrees C. Any significant differences between the two E. coli strains during the inactivation by SC-CO(2) were not noticed. The microbial inactivation curve, which was established by the modified Gompertz model describing the survival rate with treatment time, was divided into three distinct stages. When using cells in PS, k(dm), lambda and t(8) (the time for an 8-log reduction of cell counts) were 0 to 3 min(-1), 8 to 16 min and 11 to 29 min, respectively. The temperature-dependency of the microbial inactivation was verified via the correlation of the logarithm of k(dm) versus the inverse of temperature. We have observed lower inactivation rates in phosphate-buffered saline (PBS) than in PS, the lowered pH, and an increase of UV-absorbing substances in the cell suspension after SC-CO(2) treatment. Also, the deformation and collapse of the SC-CO(2)-treated cells were revealed by scanning and transmission electron microscopy, and the deactivation of cellular enzymes occurred. These all suggest that the inactivation of E. coli O157:H7 and generic E. coli was possibly caused in a concerted manner by acidification, damage to cell membranes and subsequent leakage of cellular materials, and the inactivation of cellular enzymes.

Modeling of the inactivation of Salmonella typhimurium by supercritical carbon dioxide in physiological saline and phosphate-buffered saline

In this study, we used supercritical carbon dioxide (SC-CO(2)) to inactivate Salmonella typhimurium suspended in physiological saline (PS) or phosphate-buffered saline (PBS). The colony forming activity of S. typhimurium was completely lost (i.e., 8-log reduction) under the following condition ranges: pressures of 80-150 bar, temperatures of 35-45 degrees C and 10-50 min treatment times. The microbial inactivation process had three distinct phases and was modeled by the modified Gompertz model. Generally, an increase in pressure at constant temperature, and an increase in temperature at a constant pressure, both enhanced S. typhimurium inactivation. When the cells were suspended in PBS rather than PS, the length of time for the complete inactivation significantly increased. We observed the surface and internal morphological changes of the cells by SEM and TEM, respectively. The extraction of proteinous substances, nucleic acids and outer membrane proteins into the suspension during SC-CO(2) treatment was also observed. Through SDS-PAGE analysis of the total proteins and major outer membrane proteins (OMPs) of SC-CO(2)-treated cells, we found that a substantial amount of the total soluble proteins had converted into insoluble protein.

High pressure carbon dioxide inactivation of microorganisms in foods: The past, the present and the future

Thermal pasteurization is a well known and old technique for reducing the microbial count of foods. Traditional thermal processing, however, can destroy heat-sensitive nutrients and food product qualities such as flavor, color and texture. For more than 2 decades now, the use of high-pressure carbon dioxide (HPCD) has been proposed as an alternative cold pasteurization technique for foods. This method presents some fundamental advantages related to the mild conditions employed, particularly because it allows processing at much lower temperature than the ones used in thermal pasteurization. In spite of intensified research efforts the last couple of years, the HPCD preservation technique has not yet been implemented on a large scale by the food industry until now. This review presents a survey of published knowledge concerning the HPCD technique for microbial inactivation, and addresses issues of the technology such as the mechanism of carbon dioxide bactericidal action, the potential for inactivating vegetative cells and bacterial spores, and the regulatory hurdles which need to be overcome. In addition, the review also reflects on the opportunities and especially the current drawbacks of the HPCD technique for the food industry.

Effective terminal sterilization using supercritical carbon dioxide

Gentle alternatives to existing sterilization methods are called for by rapid advances in biomedical technologies. Supercritical fluid technologies have found applications in a wide range of areas and have been explored for use in the inactivation of medical contaminants. In particular, supercritical CO(2) is appealing for sterilization due to the ease at which the supercritical state is attained, the non-reactive nature, and the ability to readily penetrate substrates. However, rapid inactivation of bacterial endospores has proven a barrier to the use of this technology for effective terminal sterilization. We report the development of a supercritical CO(2) based sterilization process capable of achieving rapid inactivation of bacterial endospores while in terminal packaging. Moreover, this process is gentle; as the morphology, ultrastructure, and protein profiles of inactivated microbes are maintained. These properties of the sterilization process suit it for possible use on a wide range of biomedical products including: materials derived from animal tissues, protein based therapies, and other sensitive medical products requiring gentle terminal sterilization.

A study on the inactivation of micro-organisms and enzymes by high pressure CO2

This study addresses some microbial inactivation phenomena induced by high pressure CO2 over micro-organisms and enzymes. The activity of four selected enzymes was measured before and after treatment with CO2 under pressure in both buffer solutions and natural cellular environment (E. coli cells and tomato paste). Results are reported for acid phosphatase, alkaline phosphatase, ATPase, and pectinase at different conditions of temperature, CO2 pressure, and treatment time (32-40 degrees C, 85-150 bar, 30-70 min). The results obtained show that the high pressure CO2 treatment induces an inactivation of cellular enzymatic activity higher than the one caused on the same enzymes in solution. However, the measured activity difference is not caused by a damage at the enzymes molecular level but is a consequence of the permeabilization of the cellular envelopes which leads to a release of unmodified enzymes from the cells with simultaneous drop of enzymatic cellular activity. The reported data suggest that the bacterial cell death is probably due not to a selective effect of high pressure CO2 treatment but to simultaneous detrimental action of CO2 on cellular membrane and cell wall.

Physiological and genome-wide transcriptional responses of Saccharomyces cerevisiae to high carbon dioxide concentrations

Physiological effects of carbon dioxide and impact on genome-wide transcript profiles were analysed in chemostat cultures of Saccharomyces cerevisiae. In anaerobic, glucose-limited chemostat cultures grown at atmospheric pressure, cultivation under CO(2)-saturated conditions had only a marginal (<10%) impact on the biomass yield. Conversely, a 25% decrease of the biomass yield was found in aerobic, glucose-limited chemostat cultures aerated with a mixture of 79% CO(2) and 21% O(2). This observation indicated that respiratory metabolism is more sensitive to CO(2) than fermentative metabolism. Consistent with the more pronounced physiological effects of CO(2) in respiratory cultures, the number of CO(2)-responsive transcripts was higher in aerobic cultures than in anaerobic cultures. Many genes involved in mitochondrial functions showed a transcriptional response to elevated CO(2) concentrations. This is consistent with an uncoupling effect of CO(2) and/or intracellular bicarbonate on the mitochondrial inner membrane. Other transcripts that showed a significant transcriptional response to elevated CO(2) included NCE103 (probably encoding carbonic anhydrase), PCK1 (encoding PEP carboxykinase) and members of the IMD gene family (encoding isozymes of inosine monophosphate dehydrogenase).

Effect of hyperbaric stress on yeast morphology: study by automated image analysis

The effects of hyperbaric stress on the morphology of Saccharomyces cerevisiae were studied in batch cultures under pressures between 0.1 MPa and 0.6 MPa and different gas compositions (air, oxygen, nitrogen or carbon dioxide), covering aerobic and anaerobic conditions. A method using automatic image analysis for classification of S. cerevisiae cells based on their morphology was developed and applied to experimental data. Information on cell size distribution and bud formation throughout the cell cycle is reported. The results show that the effect of pressure on cell activity strongly depends on the nature of the gas used for pressurization. While nitrogen and air to a maximum of 0.6 MPa of pressure were innocuous to yeast, oxygen and carbon dioxide pressure caused cell inactivation, which was confirmed by the reduction of bud cells with time. Moreover, a decrease in the average cell size was found for cells exposed for 7.5 h to 0.6 MPa CO2.