Elm oyster- Hypsizygus ulmarius. Button mushroom - Agaricus bisporus. Almond mushroom - Agaricus blazei murril. Shiitake - Lentinula Edodes. Hen-of-the-Woods - Grifola Frondosa. Pioppino - Agrocybe aegerita. Winter mushroom Enokitake - Flammulina velutipes. Caterpillar fungus - Cordyceps sinensis. Reishi - Ganoderma lucidum. Umbrella Polypore - Polyporus Umbellatus. Turkey Tail - Coriolus versicolor. Shaggy Mane - Coprinus comatus. This suggests that we may be setting up potential fungi hazards.
Even ethylene glycol, which is a by-product of ethylene oxide fumigation, activates fungi spores. It is probably the presence of this spore activator in fumigated parchments which appears to make them more prone to support fungi growth.
This is indeed an exciting topic and a barren field waiting for future research. We have treated surface fungi growth much too simplistically in the past. We have looked at growth parameters of the mycelium but have not looked holistically at all stages, spore dormancy, activation and germination, and then mycelium growth. Obtaining a better understanding of spore superdormancy and its underlying mechanisms is crucial for the development of spore control strategies that are based on the germination-inactivation principle.
Therefore, several research groups are currently investigating the genotypic and phenotypic differences between SD spores and their dormant counterparts. Currently, the exact causes of spore superdormancy are unclear and there is no consistent conclusion on whether the superdormancy of isolated SD spores is stable Keynan et al.
Notably, although the germination capacity of nutrient-SD spores increased during cold storage, it did not reach the level of the initial dormant spore population Ghosh and Setlow, Nevertheless, this indicates that the superdormancy of isolated SD spores is not permanent and it decreases over time.
Based on their findings, Ghosh and Setlow proposed that there are probably at least two causative factors for spore nutrient superdormancy, one permanent and one transient Ghosh and Setlow, For the transient cause, Ghosh and Setlow suggested it might be related to the activation status of the spores, since heat activation, which is reversible, influences the frequency of nutrient-SD spores Ghosh and Setlow, For the permanent cause, research has revealed that it is not because of genetic changes, since re-sporulated nutrient-SD spores showed the same germination capacity as the initial dormant population Ghosh and Setlow, ; Chen et al.
It was suggested that the phenotypic heterogeneity in germination may correspond to the presence of lower GR levels in the nutrient-SD spores Ghosh and Setlow, , ; Wei et al.
Lower GR levels as a cause for spore nutrient superdormancy has been proposed in many studies Ghosh and Setlow, , ; Ghosh et al. For example, in the study of Ghosh and Setlow , the frequency of SD spores decreased dramatically when the level of GerB receptor increased. In their later research Ghosh et al. Moreover, Chen et al. Lower GR levels as a causative factor of spore nutrient superdormancy is also supported by other evidence.
First, the average amount of GRs per spore is low, thus, stochastic variation in the number could lead to the situation that a small proportion of spores have very few GRs and would probably germinate more slowly Paidhungat and Setlow, ; Cabrera-Martinez et al. Second, heat activation, which improved GR-mediated germination, can decrease the frequency of nutrient-SD spores Ghosh and Setlow, However, a lower number of GRs does not seem to explain the existence of other types of SD spores.
Their superdormancy could be due to lower levels of CwlJ, which is one of the cortex-lytic enzymes, and coat deficiency Perez-Valdespino et al. Moreover, previous research suggested that the cause of HP superdormancy is different from that of nutrient superdormancy, since nutrient-SD spores can germinate normally with HP treatment Wei et al.
Furthermore, Bacillus nutrient-SD spores showed a lower spore core water content than their dormant counterparts Ghosh et al. This finding is consistent with the observation that spores sporulated at a higher temperature, which leads to a lower water content of the spore core Melly et al. This might indicate that a lower spore core water content could also be a cause of spore nutrient superdormancy Cowan et al.
One of the factors that leads to a difference in spore core water content is the DPA content of the spore. So far, several types of SD spores have been characterized and mechanisms have been proposed for their superdormancy. However, the state of knowledge about some types of SD spores is still rudimentary and the exact mechanisms are not fully clear.
Therefore, further research is needed to better understand SD spores, which represent one of the biggest challenges to the application of germination-inactivation as a milder non-thermal spore control strategy. First, attention should be paid to HPSD spores in future research. To our knowledge, there have been no reports of the isolation and characterization of HPSD spores so far.
Moreover, HP triggers germination more homogeneously, while added nutrients or chemicals might have an inhomogeneous distribution, especially in solid foods, leading to inconsistent germination within the products. Additionally, previous research has suggested that the cause of spore HP superdormancy is different from spore nutrient superdormancy Wei et al. Therefore, it would be beneficial to isolate and characterize HPSD spores regarding the mechanisms of their superdormancy.
Such research would strongly support the implementation of milder HP-based spore control strategies. Since germination behavior varies among bacterial genera, further research is needed to clarify the properties of Clostridium SD spores and the underlying mechanisms of their superdormancy Rodriguez-Palacios and LeJeune, ; Xiao et al.
Third, improvement of enumeration and culturing methods would be beneficial. Classic plate count methods based on quantifying colony-forming units are widely used to assess the viability of microbes. However, the number of colony-forming units is a measure of the highest physiological fitness of microbes Bunthof, , which might not be the best indicator for SD spores, because the possibility that these spores would not germinate on culture plates might lead to a risk of underestimation their numbers Wells-Bennik et al.
Therefore, tools such as flow cytometry or phase-contrast microscopy should be used to facilitate the enumeration of SD spores in future research. On the other hand, the amounts of SD spores are largely dependent on the germination conditions. This is important for the accuracy of antimicrobial susceptibility tests, sterilization controls, and challenge tests Silvestri et al. Fourth, in order to successfully apply a germination-inactivation technology as a gentle safety control, several other aspects need to be considered besides spore germination.
For example, the timing to apply the inactivation step is crucial. On one hand, it should be applied after the majority of spores lost most of their resistance. This time can vary, depending on spore species, germination stimuli and intensities. Notably, not all spores would finalize all their germination steps under a certain trigger Wuytack et al.
On the other hand, the germination-inactivation approach focuses on the elimination of bacterial spores to ensure the microbiological safety of the products, but the absence of spores does not guarantee the absence of toxins.
Some pathogenic spore-forming bacteria can produce toxins, which could endanger consumers. Different situations need to be taken into account if the germination-inactivation approach is considered as a food safety control in this case. First, special focus needs to be put on spore species that can produce toxins during the growth phase after their germination. For example, B. It is essential to consider the germination velocity rates and control the time intervals between the germination and inactivation steps to ensure food safety for these cases.
Notably, in any case, an inactivation needs to be performed before germinated spores could sporulate again. The time needed to complete sporulation varies, and it takes approximately 8—10 h in B. Proper processing time windows need to be identified using predictive models and experimental validation tests to ensure that the inactivation step is performed in the specific time period where the majority of spores lost most of their resistances but did not start producing toxins or sporulation, yet.
Another situation is where toxins are already present in the product, either produced by vegetative cells in their late growth phases or during sporulation, e. In this case, the following inactivation step needs to be able to degrade the present toxins, e.
For heat stable toxins, e. In this case, other approaches to control the toxin levels are needed. Generally, it is important to control the quality of raw material inputs, ingredients and their storage conditions to prevent the toxin formation before germination-inactivation steps. Finally, knowledge obtained from SD spore research could be used to develop milder spore control strategies.
On one hand, germination-inactivation technologies by first triggering spore germination and followed by a gentle inactivation step to inactivate the sensitized spores could be further developed and improved. Spore germination could be maximized when we understand the mechanisms and the influencing factors for spore superdormancy. For example, germination percentages can be increased by combining various germination triggers or controlling the influencing factors. Important influencing factors include heat activation, germination stimulus type and intensity Wei et al.
Besides that, from the application point of view, it is important to understand the germination behavior of spores that are formed and present in the food products. This is especially relevant as the sporulation conditions, which influence the spore germination properties, are often unknown and not controlled in this case Wells-Bennik et al.
Moreover, spores germination behaviors might be completely different when spores are germinated in food matrices compared to buffer systems. For example, the germination of Bacillus spores by nutrient and HP were inhibited when they are present in foods with low water activity Al-Holy et al. Therefore, future research is needed to investigate the mechanisms of how different factors influence spore germination. Examples of these hurdles can be pH, temperature, or bacteriocins such as nisin Markland et al.
Research on SD spores will help reveal factors that contribute to their superdormancy and allow for the identification of the underlying mechanisms that lead to their extremely low germination capacity as compared to the whole population. It will also contribute to improved predictive models that take germination heterogeneity into account, which can provide a mechanistic understanding of spore germination processes. Additionally, it will provide a foundation for developing milder non-thermal spore control strategies based on the germination-inactivation principle.
This could help to ensure microbial safety and quality retention of food products, contributing significantly to providing fresher and more nutritional foods for consumers. Moreover, aside from the food sector, the medical, pharmaceutical, and bio chemical sectors, where spore eradication is needed, will also benefit from research on SD spores, especially for the sterilization of heat-sensitive products.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abee, T. Germination and outgrowth of spores of Bacillus cereus group members: diversity and role of germinant receptors. Food Microbiol.
Al-Holy, M. Doona, and F. Feeherry Ames: Blackwell , 41— Google Scholar. Baier, D. Fluorescence-based methods for the detection of pressure-induced spore germination and inactivation.
High Press. Banawas, S. The Clostridium perfringens germinant receptor protein GerKC is located in the spore inner membrane and is crucial for spore germination. Bolumar, T. Amsterdam: Elsevier, — Spore germination, as defined as those events that result in the loss of the spore-specific properties, is an essentially biophysical process.
It occurs without any need for new macromolecular synthesis, so the apparatus required is already present in the mature dormant spore.
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