Quantitative Analysis of Aspergillus nidulans Growth Rate using Live Microscopy and Open-Source Software

Summary

We present a label-free live imaging protocol using transmitted light microscopy techniques to capture images, analyze and quantify growth kinetics of the filamentous fungus A. nidulans in both submerged cultures and solid media. This protocol can be used in conjunction with fluorescence microscopy.

Abstract

It is well established that colony growth of filamentous fungi, mostly dependent on changes in hyphae/mycelia apical growth rate, is macroscopically estimated on solidified media by comparing colony size. However, to quantitatively measure the growth rate of genetically different fungal strains or strains under different environmental/growth conditions (pH, temperature, carbon and nitrogen sources, antibiotics, etc.) is challenging. Thus, the pursuit of complementary approaches to quantify growth kinetics becomes mandatory in order to better understand fungal cell growth. Furthermore, it is well-known that filamentous fungi, including Aspergillus spp., have distinct modes of growth and differentiation under sub-aerial conditions on solid media or submerged cultures. Here, we detail a quantitative microscopic method for analyzing growth kinetics of the model fungus Aspergillus nidulans, using live imaging in both submerged cultures and solid media. We capture images, analyze, and quantify growth rates of different fungal strains in a reproducible and reliable manner using an open source, free software for bio-images (e.g., Fiji), in a way that does not require any prior image analysis expertise from the user.

Introduction

Filamentous fungi are of great socioeconomic and ecological importance, being both crucial as industrial/agricultural tools for enzyme and antibiotic production^1,2 and as pathogens of crop plants³, pest insects⁴ and humans³. Moreover, filamentous fungi such as Aspergillus nidulans are widely used as model organisms for fundamental research, such as studies in genetics, cell and evolutionary biology as well as for the study of hyphal extension⁵. Filamentous fungi are highly polarized organisms ....

Protocol

1. Inoculum preparation

NOTE: All steps should be performed under a laminar flow cabinet.

1. Streak out fungal strain of interest, from a glycerol stock (-80 °C) using a sterile inoculation loop, onto plates of minimal media (MM) supplemented with the appropriate nutritional requirements relevant to the strain examined [MM: 10.0 g/L glucose, 20 mL/L salt solution (salt solution: 26 g/L KCl, 26 g/L MgSO₄·7H₂O, 76 g/L KH₂PO₄, 2.0 mL/L chloroform) and 1 mL/L trace elements (trace elements: 40 mg/L Na₂B₄O₇·H₂O, 400 mg/L CuSO_....

Results

Following this protocol, we captured and analyzed various images corresponding to different growth/developmental stages of the filamentous fungus A. nidulans. The data presented in this study were processed and analyzed using the Fiji software. Measurements were saved as csv files, statistically analyzed and prepared as graphs using commercial statistical software and/or Python programming language using software libraries like pandas, numpy, statsmodels, matplotlib and seaborn. More details can be found in the .......

Discussion

Monitoring fungal cell growth and phenotype by time-lapse microscopy is a powerful approach to assess cellular behavior in real-time and quantitatively and accurately determine whether a particular drug treatment and/or genetic intervention results in detectable cell growth or phenotypic differences over time.

In this study, a reliable live-cell imaging methodology was described to measure and quantitatively analyze fungal development, including the dynamics of germ tube and hyphal tip growth .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
µ-Slide 8 Well	Ibidi	80826	Imaging slides
4-Aminobenzoic acid	Merck	A9878
azhAΔ ngnAΔ			Genotype: zhAΔ::pyrGAf; ngnAΔ::pyrGAf; pyroA4 pantoB100 / References:Laboratory collection, Athanasopoulos et al., 2013
Bacto Casamino Acids	Gibco	223030
Biotin	Merck	B4639
Chloroform	Merck	67-66-3
Copper(II) sulfate pentahydrate	Merck	C8027
Glucose	Merck	G8270
GraphPad Prism 8.0	GraphPad Software		Statistical Software
ImageJ	NIH		Image processing and analysis software
Inoculating Loop	Merck	I8263-500EA
Iron(III) phosphate	Merck	1.03935
Leica Application Suite X	Leica Microsystems		Microscope software
Magnesium sulfate heptahydrate	Merck	63138
Manganese(II) sulfate monohydrate	Merck	M7899
Microscope Leica TCS SP8	Leica Microsystems
Nicotinamide (Niacinamide)	Supelco	47865-U
Peptone	Millipore	68971
Petri Dishes for Microbiology Culture	KISKER	G090
Potassium chloride	Merck	P4504
Potassium phosphate monobasic	Merck	P5655
Pyridoxine hydrochloride	Merck	P6280
Quali - Microcentrifuge Tubes, 1,7 mL, DNase-, RNase and pyrogen free, sterile	KISKER	G052-S
Quali - Microcentrifuge Tubes, 2.0 mL, sterile	KISKER	G053-S
Quali - Standard Tips, Bevelled, 100-1000 µL	KISKER	VL004G
Quali - Standard Tips, Bevelled, 1-200 µL	KISKER	VL700G
Quali Microvolume Tips, DNase-, RNase free, 0,1-10 µL/clear	KISKER	GC.TIPS.B
Riboflavin (B2)	Supelco	47861
Scalpel blades NO. 11	OdontoMed2011	S2771
Sodium chloride	Merck	S7653
Sodium hydroxide	Merck	S8045
Sodium tetraborate decahydrate	Merck	S9640
VS151 (PilA-GFP and H1-mRFP)			Genotype: pyrG89; pilA::sgfp::AfpyrG+ argB2 nkuAΔ::argB+  pyroA4 hhoA::mrfp::Afribo+ riboB2 / References:Laboratory collection, Biratsi et al., 2021
WT			Genotype: nkuAΔ::argB; pyrG89; pyroA4;pyrG89 / References: TN02A3 -FGSC A1149
Yeast Extract	Millipore	70161
ZnSO4