Heat activation of Phycomyces blakesleeanus spores: theromdynamics and effect of alcohols, furfural, and high pressure

Evaluation of the vital viability and their application in fungal spores' disinfection with flow cytometry

More attention was focused on fungi contamination in drinking water. Most researches about the inactivation of fungal spores has been conducted on disinfection efficiency and the leakage of intracellular substances. However, the specific structural damage of fungal spores treated by different disinfectants is poorly studied. In this study, the viability assessment methods of esterase activities and intracellular reactive oxygen species (ROS) were optimized, and the effects of chlorine-based disinfectants on fungal spores were evaluated by flow cytometry (FCM) and plating. The optimal staining conditions for esterase activity detection were as follows: fungal spores (10⁶ cells/mL) were stained with 10 μM carboxyfluorescein diacetate and 50 mM ethylene diamine tetraacetic acid at 33 °C for 10 min (in dark). The optimal staining conditions for intracellular ROS detection were as follows: dihydroethidium (the final concentration of 2 μg/mL) was added into fungal suspensions (10⁶ cells/mL), and then samples were incubated at 35 °C for 20 min (in dark). The cell culturability, membrane integrity, esterase activities, and intracellular ROS were examined to reveal the structural damage of fungal spores and underlying inactivation mechanisms. Disinfectants would cause the loss of the cell viability via five main steps: altered the morphology of fungal spores; increased the intracellular ROS levels; decreased the culturability, esterase activities and membrane integrity, thus leading to the irreversible death. It is appropriate to assess the effects of disinfectants on fungal spores and investigate their inactivation mechanisms using FCM.

Effects of high-pressure processing on fungi spores: Factors affecting spore germination and inactivation and impact on ultrastructure

Food contamination with heat-resistant fungi (HRF), and their spores, is a major issue among fruit processors, being frequently found in fruit juices and concentrates, among other products, leading to considerable economic losses and food safety issues. Several strategies were developed to minimize the contamination with HRF, with improvements from harvesting to the final product, including sanitizers and new processing techniques. Considering consumers' demands for minimally processed, fresh-like food products, nonthermal food-processing technologies, such as high-pressure processing (HPP), among others, are emerging as alternatives to the conventional thermal processing techniques. As no heat is applied to foods, vitamins, proteins, aromas, and taste are better kept when compared to thermal processes. Nevertheless, HPP is only able to destroy pathogenic and spoilage vegetative microorganisms to levels of pertinence for food safety, while bacterial spores remain. Regarding HRF spores (both ascospores and conidiospores), these seem to be more pressure-sensible than bacterial spores, despite a few cases, such as the ascospores of Byssochlamys spp., Neosartorya spp., and Talaromyces spp. that are resistant to high pressures and high temperatures, requiring the combination of both variables to be inactivated. This review aims to cover the literature available concerning the effects of HPP at room-like temperatures, and its combination with high temperatures, and high-pressure cycling, to inactivate fungi spores, including the main factors affecting spores' resistance to high-pressure, such as pH, water activity, nutritional composition of the food matrix and ascospore age, as well as the changes in the spore ultrastructure, and the parameters to consider regarding their inactivation by HPP.

Dormancy, activation and viability of Rhizopus oligosporus sporangiospores

Interruption of dormancy to improve viability of Rhizopus oligosporus sporangiospores is crucial for the application of stored starter cultures for fungal (tempe) production. We aimed to assess the extent of dormancy and factors that could result in activation. Whereas heat treatments were unsuccessful, Malt Extract Broth (MEB) showed to be a good activation medium, with 80% of dormant spores being activated as measured by fluorescence microscopy using a fluorescent marker, compared with 11% with the control. Peptone and yeast extract but not glucose played an important role in activating dormant spores. Metabolically active (fluorescent) and swollen spores, followed by germ tubes were obtained after activation in MEB for 25 min., 2 and 4 h, respectively, at 37 degrees C. Simultaneously, some interesting transitions took place. Dormant spores represent 85-90% of the total spores at harvest and after drying. Their number decreased to 21-32% after activation with MEB with a concomitant increase of metabolically active spores. As a result of storage, some dormancy was lost, yielding an increase of active spores from 11.2% at harvest to 28.8% after 3 months storage. Levels of active spores were well correlated with their viability. By activation of dormant spores, their viability increased; levels of viable and active spores were maximum in 1 month old starter (61.7% and 75.9% of total spores, respectively) but gradually decreased with concomitant increase of the number of dead spores.

Dormant ascospores of Talaromyces macrosporus are activated to germinate after treatment with ultra high pressure

AIMS

Ascospores of Talaromyces macrosporus are constitutively dormant and germinate after a strong external shock, classically a heat treatment. This fungus is used as a model system to study heat resistance leading to food spoilage after pasteurization. This study evaluates the effect of high pressure on the germination behaviour of these spores.

METHODS AND RESULTS

Ascospore containing bags were subjected to ultra high pressure and spores were plated out on agar surfaces. Untreated suspensions showed invariably very low germination. Increased germination of ascospores occurred after short treatments at very high pressure (between 400 and 800 MPa). Activation is partial compared with heat activation and did not exceed 6.9% (65 times that of untreated suspensions) of the spore population. Maximum activation was attained shortly (10 s-3 min) after the pressure was applied and accompanied by cell wall deformations as judged by scanning electron microscopy. The spores observed in this study were harvested from cultures that were 39-58 days old. The maturity of spores at similar developmental stages was measured by assessing the heat resistance of ascospores. Between 20 and 40 days heat resistance increased 2.4-fold, but only an additional increase of 1.3-fold was observed at later stages (40-67 days).

CONCLUSIONS

Our investigations show that high pressure constitutes a second type of shock that can activate heat-resistant ascospores to germinate. Activation is maximal after very short treatments and accompanied with changes in the cell wall structure. High-pressure activation is not the result of immaturity of the ascospores.

SIGNIFICANCE AND IMPACT OF THE STUDY

These observations are relevant for the application of high pressure as a novel pasteurization method.

Spore activation by acetate, propionate and heat in Phycomyces mutants

Exposure to heat, acetate, or propionate activates the spores of the fungus Phycomyces blakesleeanus and allows them to germinate. Using counterselection with the antibiotic N-glycosyl-polyfungin, seven mutants were isolated on the basis of decreased spore activation by propionate. The nine mutants showed decreased activation by both chemicals and by heat, increased heat lethality, and altered patterns of trehalase activation. These and other observations indicate that spore activation by the three agents and spore death by heat are mediated by the same cellular component(s), which is probably involved in the regulation of cyclic AMP concentration.

Characterization of Phycomyces blakesleeanus mutants temperature-sensitive for heat-shock induced germination

Sixty-nine mutants of the fungus Phycomyces blakesleeanus, temperature-sensitive for heat-shock induced germination, have been characterized. All of them show a low viability at 26 degrees C and normal viability at 16 degrees C. Eleven mutants recover the wild type phenotype if yeast extract is added to the minimal medium; the mutant phenotype of eight of these mutants is also suppressed by the addition of putrescine or other polyamines. The majority of the mutants are affected very early in germination. Spontaneous, heat-shock and acetate induced germination are not equally impaired by some of the mutations, so specific and independent steps seem to be involved in part of the activation mechanism of germination.

Activation and injury of Clostridium perfringens spores by alcohols

The activation properties of Clostridium perfringens NCTC 8679 spores were demonstrated by increases in CFU after heating in water or aqueous alcohols. The temperature range for maximum activation, which was 70 to 80 degrees C in water, was lowered by the addition of alcohols. The response at a given temperature was dependent on the time of exposure and the alcohol concentration. The monohydric alcohols and some, but not all, of the polyhydric alcohols could activate spores at 37 degrees C. The concentration of a monohydric alcohol that produced optimal spore activation was inversely related to its lipophilic character. Spore injury, which was manifested as a dependence on lysozyme for germination and colony formation, occurred under some conditions of alcohol treatment that exceeded those for optimal spore activation. Treatment with aqueous solutions of monohydric alcohols effectively activated C. perfringens spores and suggests a hydrophobic site for spore activation.

Trehalase activity in extracts of Phycomyces blakesleeanus spores following the induction of germination by heat activation

The heat activation of trehalase in extracts of sporangiospores of Phycomyces blakesleeanus, following the induction of germination by heat activation and the gelatinization of potato starch granules were studied under different conditions in order to discriminate between several phenomena as possible triggers in the activation of trehalase. Short-chain alcohols (from methanol to pentanol) lower the activation temperature of trehalase while long-chain alcohols (from heptanol to nonanol) raise it. Short-chain alcohols also lower the gelatinization temperature of potato starch granules, while long-chain alcohols, hexanol and heptanol have hardly any influence on the gelatinization temperature. Octanol raises the gelatinization temperature. More polar phenols lower the activation temperature of trehalase, while more apolar phenols will raise it. The gelatinization temperature of starch granules is more lowered by the polar polyphenols than by the more apolar phenols. The effect of high pressure on starch gelatinization was investigated in order to compare data from such a model system with the data on trehalase activation. The gelatinization temperature of starch granules is shifted upwards with about 3-5K/1000 atm (1.013 X 10(5) kPa). Pressures higher than 1500 atm do not further increase the gelatinization temperature. However, no reversal of the effect, as occurs with protein conformational changes, is seen with pressure up to 2500 atm. Also for trehalase activation we find a continuous upward shift of the activation temperature with about 5-9K/1000 atm. These data are in agreement with a thermal transition in a polysaccharide matrix, being the trigger in the heat activation of trehalase.

Effect of high pressure on the heat activation in vivo of trehalase in the spores of Phycomyces blakesleeanus

The effect of pressure on the heat activation in vivo of trehalase in the spores of Phycomyces blakesleeanus has been investigated in order to obtain information about the molecular mechanism of the activation. For a protein conformational change directly induced in the enzyme by the heat treatment an upward shift with about 2-6 K/1000 atm (1.013 X 10(5) kPa) is to be expected in the moderate high-pressure region. On the other hand, for a phospholipid phase transition causing the activation, a continuous upward shift with about 20 K/1000 atm is to be expected. For trehalase activation we find a continuous upward shift of the activation temperature with about 5-9 K/1000 atm. The denaturation of trehalase, which occurs at slightly higher temperatures, is influenced by pressure completely as expected for a protein conformational change. The application of high pressure during spore heat activation makes it possibe to break the dormancy of the spores without concomitant activation of trehalase.