Article

Significance of the physiological state of fungal spores

Laboratoire de Génie des Procédés Microbiologiques et Alimentaires, Université de Bourgogne, ENS.BANA, 21000 Dijon, France.
International journal of food microbiology (Impact Factor: 3.08). 03/2009; 134(1-2):16-20. DOI: 10.1016/j.ijfoodmicro.2009.02.005
Source: PubMed

ABSTRACT

In predictive mycology, most of the studies have been concerned with the influence of some environmental factors on fungal growth and production of mycotoxins, at steady-state. However, fluctuating conditions, interactions between organisms, and the physiological state of the organisms may also exert a profound influence on fungal responses in food and in the environment. In the laboratory, fungal spores are widely used as a biological material. They are produced under optimal conditions then, partially re-hydrated for obtaining standardized spore suspensions. In real conditions, spores are produced under suboptimal conditions and can be submitted to various stresses prior to their germination. It was illustrated how the sporulation/post-sporulation conditions, the re-hydration and the age of the spores affected greatly their physiological state and consequently their resistance to heat, inhibitors and their germinability. It was hypothesised that the observed responses to environmental factors during inactivation and germination could be correlated to the intracellular water activity of the spores.

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    • "Then it is important to take into account these fluctuations during the developing and validation of models, otherwise their applicability is compromised. Unfortunately very little information on the modelling of fungal germination and growth or mycotoxins production under fluctuating conditions is available (Dantigny and Nanguy, 2009; Gougouli and Koutsoumanis, 2012, 2010; Kalai et al., 2014; Peleg and Normand, 2013). On the other hand, prediction of bacterial growth under non-isothermal conditions has been studied during the past decade, where it has been demonstrated that the instantaneous specific growth rate adapts to the changing temperature practically immediately, except in extreme cases, when the temperature change is abrupt and close to the boundary of growth (Bovill et al., 2000). "
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    ABSTRACT: Human exposure to aflatoxins in foods is of great concern. The aim of this work was to use predictive mycology as a strategy to mitigate the aflatoxin burden in pistachio nuts postharvest. The probability of growth and aflatoxin B1 (AFB1) production of aflatoxigenic Aspergillus flavus, isolated from pistachio nuts, under static and non-isothermal conditions was studied. Four theoretical temperature scenarios, including temperature levels observed in pistachio nuts during shipping and storage, were used. Two types of inoculum were included: a cocktail of 25 A. flavus isolates and a single isolate inoculum. Initial water activity was adjusted to 0.87. Logistic models, with temperature and time as explanatory variables, were fitted to the probability of growth and AFB1 production under a constant temperature. Subsequently, they were used to predict probabilities under non-isothermal scenarios, with levels of concordance from 90 to 100% in most of the cases. Furthermore, the presence of AFB1 in pistachio nuts could be correctly predicted in 70-81 % of the cases from a growth model developed in pistachio nuts, and in 67-81% of the cases from an AFB1 model developed in pistachio agar. The information obtained in the present work could be used by producers and processors to predict the time for AFB1 production by A. flavus on pistachio nuts during transport and storage. Copyright © 2015 Elsevier Ltd. All rights reserved.
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    • "Depending on the experimental conditions, shelf-lives of 26 and 32 days; 38 and 20 days were described for the spot (hydrated conidia) and the air cabinet technique (non-hydrated conidia), respectively. The physiological state had a great effect on the inactivation and the germination of fungal conidia (Blaszyk et al., 1998; Dao and Dantigny, 2009; Dantigny and Nanguy, 2009). In particular, the hydration of the spore, even for a short period of time, i.e., 20 min, had a significant effect of the germination time (Nanguy et al., 2010). "
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    ABSTRACT: To reproduce a fungal contamination of food products by airborne conidia, a method to inoculate a few number (in the range 1-9) of conidia on the surface of agar media was developed. This technique would allow to determine accurately the time to detection of fungal colonies, then the mould free shelf-life of food products by using dry conidia. The method was based on dry-harvesting the conidia in the lid by gently taping the bottom of the dishes where sporulating mycelium was grown, retaining the conidia on glass beads, and, aseptically transferring the beads to successive Petri dishes to "dilute" the samples. Among the eleven factors tested by means of an experimental design, the most important factors were the incubation time of the sporulating mycelium, the resting time to dislodge as many conidia as possible from the lid, the number of beads and the number of successive dishes. The other factors were method used to produce conidia, the number of taps to remove as many conidia as possible from the sporulating culture before harvesting, the drop height of the harvest device, the number of taps to detached the harvested conidia from the lid, and the mixing times to attach conidia from the lid to the beads, and to detach conidia from the beads to the agar media. Decimal "dilutions" were achieved by transferring 10 beads to the successive dishes with a mixing time of 10 s. It was shown for Penicillium chrysogenum that an average of 3 colonies per dish were counted on the fourth of the successive dishes, for 3 days incubation time at 25 °C, 24 h resting time, and 10 beads.
    Full-text · Article · Oct 2015 · Food Microbiology
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    • "Unfortunately, very little information on modeling these biological responses under fluctuating conditions is available. The lack of experimental devices allowing automatic monitoring of growth and germination, in addition to the use of solid media, may explain this shortage of experiments carried out under transient conditions (Dantigny and Nanguy, 2009). Steady-state is a very poor assumption in the environment where non constant conditions prevail. "
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    ABSTRACT: In the environment, fungal conidia are subject to transient conditions. In particular, temperature is varying according to day/night periods. All predictive models for germination assume that fungal spores can adapt instantaneously to changes of temperature. The only study that supports this assumption (Gougouli and Koutsoumanis, 2012, Modelling germination of fungal spores at constant and fluctuating temperature conditions. International Journal of Food Microbiology, 152: 153-161) was carried out on Penicillium expansum and Aspergillus niger conidia that, in most cases, already produced germ tubes. In contrast, the present study focuses on temperature shifts applied during the first stages of germination (i.e., before the apparition of the germ tubes). Firstly, germination times were determined in steady state conditions at 10, 15, 20 and 25 °C. Secondly, temperature shifts (e.g., up-shifts and down-shifts) were applied at 1/4, 1/2, and 3/4 of germination times, with 5, 10 and 15 °C magnitudes. Experiments were carried out in triplicate on Penicillium chrysogenum conidia on Potato Dextrose Agar medium according to a full factorial design. Statistical analysis of the results clearly demonstrated that the assumption of instantaneous adaptation of the conidia should be rejected. Temperature shifts during germination led to an induced lag time or an extended germination time as compared to the experiments conducted ay steady state. The induced lag time was maximized when the amplitude of the shift was equal to 10 °C. Interaction between the instant and the direction of the shift was highlighted. A negative lag time was observed for a 15 °C down-shift applied at 1/4 of the germination time. This result suggested that at optimal temperature the rate of germination decreased with time, and that the variation of this rate with time depended on temperature.
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