Name: measuring volatile and non-volatile antifungal activity of biocontrol products
Uploaded: 2020-12-05T14:00:00
Description: Watch this Scientific Journal Video about Measuring Volatile and Non-volatile Antifungal Activity of Biocontrol Products at JoVE.com

We are very grateful to Frank Yates for his precious advice. This work was supported by Sup&#39;Biotech.

1. Inocula preparation
<ol>
	<li>Prior to the experiment, lay 5 &#181;L of Trichoderma spp. SBT10-2018 spores stored at 4 &#176;C on potato dextrose agar medium (PDA) and incubate for 4 days at 30&#176;C with regular light exposure to promote conidia formation42 (Figure 1, panel A). 
	NOTE: Trichoderma spp. SBT10-2018 has been isolated from wood and is used as the model in this study for its rapid growth and ease of spore recovery. This strain...

The approach presented here allows for the evaluation of antifungal properties of minimally processed plant-derived products. In this protocol, homogenous distribution of spores on the agar surface is achieved using 2 mm glass beads. This step requires handling skills to properly distribute the beads and to obtain reproducible results, ultimately allowing the comparison of antifungal effects at different stages of fungal growth. We found that 5 mm glass beads or excessive rotation while homogenizing during spreading can ...

Global losses of fruits and vegetables can reach up to 50% of production1 and result mostly from food decay caused by fungi spoilage in field or during post-harvest storage2,3, despite the extensive employment of synthetic fungicides since the middle of the twentieth century4. The use of these substances is being reconsidered since it represents serious environmental and health hazards. As the harmful consequences of their use are showing up throughout ecosystems and evidence of potential health impacts has accumulated5,6, novel alternatives to old prophylactic strategies are being developed for pre- and post-harvest treatments7,8,9. Hence the challenge we face is two-fold. Novel fungicidal strategies must, firstly, maintain the levels of efficacy of food protection against phytopathogens and concomitantly, secondly, contribute to dramatically reducing the environmental footprint of agricultural practices. To fulfill this ambitious goal, strategies inspired by the natural defenses evolved in plants are being proposed as more than 1000 plants species have been highlighted for their antimicrobial properties8. For instance, plants which have developed natural fungicides to fight phytopathogens are a novel resource in exploring the development of new biocontrol products2. Essential oils are flagship molecules of this type. For example, Origanum essential oil protects tomato plants against gray mold in greenhouses 10 and Solidago canadensis L. and cassia essential oils have been shown to preserve post-harvested strawberries from gray mold damage11,12. These examples illustrate that biocontrol and notably plant-derived products represent a solution that combines biological efficacy and environmental sustainability.
Thus, plants are an important resource of molecules of potential interest for the crop-protection industry. However only a handful of plant products have been proposed to be used as biocontrol products even though they are generally recognized as safe, non-phytotoxic and eco-friendly2. Some difficulties in the transposition from the lab to the field have been observed, such as efficacy decreasing once applied in vivo2,9. Thus, it becomes important to improve the ability of lab tests to better predict field efficacy. In this context, antifungal testing methods for plant-derived products are necessary both to evaluate their antifungal efficacy and to define their optimal conditions for use. Specifically, biocontrol products are generally less efficient than chemical fungicides, so a better understanding of their mode of action is important for proposing suitable formulations, to identify the mode of application in fields, and to define which developmental stage of the pathogen is vulnerable to the candidate bioproduct.
Current approaches addressing antibacterial and antifungal activities include diffusion methods such as agar-disk diffusion, dilution, bioautography and flow cytometry13. Most of these techniques, and more specifically, the standard antifungal susceptibility testing - agar-disk diffusion and dilution assays - are well-adapted for evaluating the antimicrobial activity of soluble compounds on bacterial and fungal spores in liquid suspensions14. However, these methods are generally not suitable for testing solid compounds such as dried plant powder or to quantify antifungal activity during mycelium growth as they require spore dilution or spore spreading on agar plates and/or dilution of antifungal compounds13. In the food-poisoned method, agar plates containing the antifungal agent are inoculated with a 5-7 mm diameter disk sampled from a 7-day old fungi culture without considering the precise quantity of starting mycelium. After incubation, the antifungal activity is determined as a percent of radial-growth inhibition17,18,19. With this approach we can evaluate the antifungal activity on mycelial growth. By contrast, the agar-dilution method is performed to determine the antifungal activity on spores directly inoculated on the surface of the agar plate containing the antifungal compounds13,20,21. These two approaches give complementary results on antifungal activity. However these are two independent techniques used in parallel that do not provide accurate side-by-side comparison of the relative efficacy of antifungal compounds on spores and mycelium17,20,22 as the quantity of starting fungal material differs in the two approaches. Moreover, the antifungal activity of a plant-derived product often results from the combination of antifungal molecules synthesized by plants to face pathogens. These molecules encompass proteins, peptides23,24, and metabolites having wide chemical diversity and belonging to different classes of molecules such as polyphenols, terpenes, alcalo&#239;ds25, glucosinolates8, and organosulfur compounds26. Some of these molecules are volatile or become volatile during pathogen attack27. These agents are most often poorly water soluble and high vapor-pressure compounds that have to be recovered through water distillation as essential oils, some of whose antimicrobial activities have been well established28. Vapor-phase mediated susceptibility assays have been developed to measure the antimicrobial activity of volatile compounds following evaporation and migration via the vapor phase29. These methods are based on the introduction of antifungal compounds at a distance from the microbial culture29,30,31,32,33. In the commonly used vapor-phase agar assay, essential oils are deposited on a paper disk and placed in the center of the cover of the Petri dish at distance from the bacterial or fungal spore suspension, which is spread on agar medium. The diameter of the zone of growth inhibition is then measured in the same way as for the agar-disk diffusion method20,24. Other approaches have been developed to provide quantitative measurement of the vapor-phase antifungal susceptibility of essential oils, derived from the broth-dilution method from which an inhibitory vapor-phase mediated antimicrobial activity was calculated32, or derived from agar-disk diffusion assays31. These methods are generally specific to vapor-phase activity studies andnot appropriate to contact-inhibition assays. This precludes the determination of the relative contribution of volatile and non-volatile agents to the antifungal activity of a complex bioactive mixture.
The quantitative method we have developed aims to measure the antifungal effect of dried-plant powder on controlled quantities of spores and grown mycelium deposited on the surface of an agar medium to reproduce the aerial growth of phytopathogens during infection of plants15 as well as an interconnected mycelial network16. The approach is a modified experimental setup based on the agar-dilution and food-poisoned methods that also allows, in the same experimental setup, side-by-side quantification of the contribution of both volatile and non-volatile antifungal metabolites. In this study, the method has been benchmarked against the activity of three well-characterized antifungal preparations.

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		<tr>
			<td>Autoclave-vacuclav 24B+</td>
			<td>Melag</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Carbendazim</td>
			<td>Sigma&#160;</td>
			<td>378674-100G</td>
			<td></td>
		</tr>
		<tr>
			<td>Distilled water</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Eppendorf tubes</td>
			<td>Sarstedt</td>
			<td>72.706</td>
			<td>1.5 mL</td>
		</tr>
		<tr>
			<td>Falcons tubes</td>
			<td>Sarstedt</td>
			<td>547254</td>
			<td>50 mL</td>
		</tr>
		<tr>
			<td>Five millimeters diameter stainless steel tube</td>
			<td>retail store</td>
			<td></td>
			<td>/</td>
		</tr>
		<tr>
			<td>Food dehydrator</td>
			<td>Sancusto</td>
			<td></td>
			<td>six trays</td>
		</tr>
		<tr>
			<td>Garlic powder</td>
			<td>Organic shop</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glass beads</td>
			<td>CLOUP</td>
			<td>65020</td>
			<td><img alt="Equation 1" src="/files/ftp_upload/61798/61798eq1.png" /> 2 mm</td>
		</tr>
		<tr>
			<td>Hemocytometer counting cell</td>
			<td>Jeulin</td>
			<td>713442</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Incubator</td>
			<td>Memmert</td>
			<td>&#160;UM400</td>
			<td>30 &#176;C</td>
		</tr>
		<tr>
			<td>Knife mill</td>
			<td>Bosch</td>
			<td>TSM6A013B</td>
			<td></td>
		</tr>
		<tr>
			<td>Manual cell counter</td>
			<td>Labbox</td>
			<td>HCNT-001-001</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Measuring ruler</td>
			<td>retail store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Microbiological safety cabinets</td>
			<td>FASTER</td>
			<td></td>
			<td>FASTER BHA36, TYPE II, Cat 2</td>
		</tr>
		<tr>
			<td>Micropipette</td>
			<td>Mettler-Toledo</td>
			<td>17014407</td>
			<td>100 - 1000 &#181;L</td>
		</tr>
		<tr>
			<td>Micropipette</td>
			<td>Mettler-Toledo</td>
			<td>17014411</td>
			<td>20 - 200 &#181;L</td>
		</tr>
		<tr>
			<td>Micropipette</td>
			<td>Mettler-Toledo</td>
			<td>17014412</td>
			<td>2 - 20 &#181;L</td>
		</tr>
		<tr>
			<td>Petri dish</td>
			<td>Sarstedt</td>
			<td>82-1194500</td>
			<td><img alt="Equation 1" src="/files/ftp_upload/61798/61798eq1.png" /> 55 mm</td>
		</tr>
		<tr>
			<td>Petri dish</td>
			<td>Sarstedt</td>
			<td>82-1473&#160;</td>
			<td><img alt="Equation 1" src="/files/ftp_upload/61798/61798eq1.png" /> 90 mm</td>
		</tr>
		<tr>
			<td>Pipette Controllers-EASY 60</td>
			<td>Labbox</td>
			<td>EASY-P60-001</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Potato Dextrose Agar</td>
			<td>Sigma&#160;</td>
			<td>70139-500G</td>
			<td></td>
		</tr>
		<tr>
			<td>Precision scale-RADWAG</td>
			<td>Grosseron</td>
			<td>B126698</td>
			<td>AS220.R2-ML 220g/0.1mg&#160;</td>
		</tr>
		<tr>
			<td>Rake</td>
			<td>Sarstedt</td>
			<td>86-1569001</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Reverse microscope AE31E trinocular</td>
			<td>Grosseron</td>
			<td>M097917</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Sterile graduated pipette</td>
			<td>Sarstedt</td>
			<td>1254001</td>
			<td>10 mL</td>
		</tr>
		<tr>
			<td>Thymus essential oil</td>
			<td>Drugstore</td>
			<td></td>
			<td>Essential oil 100%</td>
		</tr>
		<tr>
			<td>Tips 1000 &#181;L&#160;</td>
			<td>Sarstedt</td>
			<td>70.762010</td>
			<td></td>
		</tr>
		<tr>
			<td>Tips 20 &#181;L&#160;</td>
			<td>Sarstedt</td>
			<td>70.760012</td>
			<td></td>
		</tr>
		<tr>
			<td>Tips 200 &#181;L</td>
			<td>Sarstedt</td>
			<td>70.760002</td>
			<td></td>
		</tr>
		<tr>
			<td>Tooth pick</td>
			<td>retail store</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Trichoderma spp strain</td>
			<td></td>
			<td></td>
			<td>Strain of LRPIA laboratory</td>
		</tr>
		<tr>
			<td>Tween-20&#160;</td>
			<td>Sigma&#160;</td>
			<td>P1379-250ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Tween-80</td>
			<td>Sigma&#160;</td>
			<td>P1754-1L</td>
			<td></td>
		</tr>
		<tr>
			<td>Tweezers</td>
			<td>Labbox</td>
			<td>FORS-001-002</td>
			<td>/</td>
		</tr>
	</tbody>
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measuring volatile and non-volatile antifungal activity of biocontrol products

The protocol described is based on a plug-transfer technique that allows accurate determination of microorganism quantities and their developmental stages. A specified number of spores are spread on an agar plate. This agar plate is incubated for a defined period to allow the fungi to reach the expected developmental stage, except for spores where incubation is not required. Agar plugs covered by spores, hyphae, or mycelium are next withdrawn and transferred onto agar media containing the antifungal compound to be tested either placed at a distance from the fungi or in contact. This method is applicable to test both liquid extracts and solid samples (powders). It is particularly well suited for quantifying the relative contributions of volatile and non-volatile agents in bioactive mixtures and for determining their effects, specifically on spores, early hyphae, and mycelium.
The method is highly relevant for the characterization of the antifungal activity of biocontrol products, notably plant-derived products. Indeed, for plant treatment, the results can be used to guide the choice of mode of application and to establish the trigger thresholds.

To evaluate the ability of the quantitative method to discriminate the mode of action of different types of antifungal compounds, we compared the efficacy of three well-known antifungal agents. Carbendazim is a non-volatile synthetic fungicide which has been widely used to control a broad range of fungal diseases in plants39,40. Thymus vulgaris essential oil has been largely described for its antibacterial and antifungal activity and is used as natural f...

Watch this Scientific Journal Video about Measuring Volatile and Non-volatile Antifungal Activity of Biocontrol Products at JoVE.com

Measuring Volatile and Non-volatile Antifungal Activity of Biocontrol Products

We describe a modified agar-based method designed to quantify the antifungal effects of plant-derived products. Both volatile and non-volatile contributions to the antifungal activity can be assessed through this protocol. In addition, efficacy against fungi can be measured at key developmental stages in a single experimental setup.

The protocol described is based on a plug-transfer technique that allows accurate determination of microorganism quantities and their developmental ...

This protocol provides a method for making accurate side-by-side comparisons of the relative efficacy of volatile and non-volatile antifungal compounds at different fungal growth stages. This method is well-suited for evaluating the antifungal activity of plant-derived products used in dried or liquid forms that contain a wide diversity of molecules. This method can provide valuable information about the mode of application of plant-derived products and is particularly well-suited in the field of biocontrol.Demonstrating the procedure will be Valentina Gligorijevic, an engineer assistant from the Sup'Biotech laboratory. For conidia collection, layer three milliliters of 0.05%Tween 20 on a trichoderma mycelium culture and use a rake to release the conidia from the conidiophores taking care not to press down on the mycelium to prevent the hyphae from being torn away. When all of the conidia have been released, use a micropipette to quickly recover and add the conidia solution to a 15 milliliter tube for counting.To prepare fungal plates, deposit 100 microliters of spores at a three times 10 to the six spores per milliliter concentration onto a nine centimeter Petri dish containing PDA medium, and use a sterile spatula to add 10 grams of two millimeter diameter glass beads to the plate. Then gently shake the plate forward and backward and spin and rotate the plate in 90 degree segments until it has been fully rotated to evenly distribute the spores across the surface of the agar. Then incubate the plate at 30 degrees Celsius until growth inhibition analysis.To perform a contact-inhibition assay with garlic powder, use a sterile spatula to weigh the desired garlic powder quantity into individual 50 milliliter tubes. to obtain concentrations generally ranging from 0.25 to 16 milligrams per milliliter. Add 10 milliliters of approximately 45 degrees Celsius PDA to each tube and carefully invert each tube several times to evenly distribute the powder throughout the agar.Quickly, pour the homogenized suspensions into five centimeter diameter Petri dishes and allow the agar to solidify at room temperature. To perform a contact-inhibition assay, use a five millimeter diameter sterile stainless steel tube to plot a circle in the center of the agar in the control and antifungal compound-treated dishes, and use a sterile toothpick to dispose of the agar cylinders. Next, use a new five millimeter diameter stainless steel tube to plot 15 to 20 circles randomly into the fungal plates, and use sterile toothpicks to carefully transfer the spore, hyphae, or mycelium-covered cylinders into the empty regions of the PDA plates.Then return the plates to the 30 degree Celsius incubator. To prepare a plates for a vapor-phase antifungal inhibition assay, pour 10 milliliters of PDA medium into the lid of a five centimeter diameter Petri dish containing 10 milliliters of PDA medium containing antifungal compounds or PDA medium alone, and allow the agar to solidify at room temperature. Using a 50 milliliter centrifical tube as a calibration tool, make a circle of PDA in the center of the agar and use a sterile spatula to remove the agar around the circle.Use a five millimeter diameter sterile stainless steel tube to plot a circle in the center of the medium in the lid and use a sterile toothpick to discard the auger cylinder. Then use a five millimeter diameter sterile stainless steel tube to randomly make plugs in the prepared fungal plates, and use a sterile toothpick to carefully transfer spore, hyphae, or mycelium-coated plugs into the lids of the assay plates. In this analysis, trichoderma species SBT 10-2018 growth inhibition triggered by three antifungal compounds was evaluated using contact and vapor-phase inhibition assays as demonstrated for each fungal stage.A higher spore sensitivity to carbendazim was observed compared to early hyphae and mycelium networks when trichoderma and antifungal compounds were in contact. In contrast, carbendozim had no antifungal effect on trichoderma when the fungus was placed at a distance from the fungicide in accordance with the low volatility of this substance. When thymus vulgaris essential oil was used as an antifungal compound, a higher sensitivity was observed for spores and early hyphae as compared to mycelium growth inhibition at 0.025%essential oil.As expected, thymus vulgaris essential oil presented identical antifungal activity irrespective of the distance between the fungus and the oil. When garlic powder was used, a higher efficacy was observed against spore germination and early hyphae elongation than for mycellium growth. Particular attention must be paid during the fungal plate preparation to make sure that the spores are evenly distributed on the surface of the agar plate to ensure comparable results.

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