Microbial Small Talk: Volatiles in Fungal-Bacterial Interactions

Abstract

There is increasing evidence that volatile organic compounds (VOCs) play an important role in the interactions between fungi and bacteria, two major groups of soil inhabiting microorganisms. Yet, most of the research has been focused on effects of bacterial volatiles on suppression of plant pathogenic fungi whereas little is known about the responses of bacteria to fungal volatiles. In the current study we performed a metabolomics analysis of volatiles emitted by several fungal and oomycetal soil strains under different nutrient conditions and growth stages. The metabolomics analysis of the tested fungal and oomycetal strains revealed different volatile profiles dependent on the age of the strains and nutrient conditions. Furthermore, we screened the phenotypic responses of soil bacterial strains to volatiles emitted by fungi. Two bacteria, Collimonas pratensis Ter291 and Serratia plymuthica PRI-2C, showed significant changes in their motility, in particular to volatiles emitted by Fusarium culmorum. This fungus produced a unique volatile blend, including several terpenes. Four of these terpenes were selected for further tests to investigate if they influence bacterial motility. Indeed, these terpenes induced or reduced swimming and swarming motility of S. plymuthica PRI-2C and swarming motility of C. pratensis Ter291, partly in a concentration-dependent manner. Overall the results of this work revealed that bacteria are able to sense and respond to fungal volatiles giving further evidence to the suggested importance of volatiles as signaling molecules in fungal-bacterial interactions.

Keywords: fungal–bacterial interactions; motility; signaling; soil microorganisms; terpenes; volatiles.

FIGURE 1

Principal Component Analysis (PCA) of fungal and oomycetal strains based on 106 volatiles that were significantly different (P < 0.05 with Bonferroni correction) in abundance between at least two strains when grown on water agar supplied with artificial root exudates at day 3 (early growth stage) and day 6 (late growth stage). MH, Mucor hiemalis; RS, Rhizoctonia solani; PU, Pythium ultimum; VD, Verticillium dahliae; FC, Fusarium culmorum; TH, Trichoderma harzianum.

FIGURE 2

Principal Component Analysis of fungal and oomycetal strains based on 173 volatiles that were significantly different (P < 0.05 with Bonferroni correction) in their abundance at least between two strains when grown on potato dextrose agar at day 3 (early growth stage) and day 6 (late growth stage). MH, Mucor hiemalis; RS, Rhizoctonia solani; PU, Pythium ultimum; VD, Verticillium dahliae; FC, Fusarium culmorum; TH, Trichoderma harzianum.

FIGURE 3

Effect of fungal and oomycetal volatiles on Collimonas pratensis Ter291 and Serratia plymuthica PRI-2C swarming motility (0.6% wt/vol agar) and swimming motility (0.3% wt/vol agar) on water agar supplied with artificial root exudates (A) and potato dextrose agar (B). Setup of experiment and effect of FC and RS volatiles on swimming motility of C. pratensis Ter291 and S. plymuthica PRI-2C (C). MH, M. hiemalis; RS, R. solani; PU, P. ultimum; VD, V. dahliae; FC, F. culmorum; TH, T. harzianum, Control: media. Five microliters of washed overnight cultures of C. pratensis Ter291 and S. plymuthica PRI-2C was spotted in the center of a soft agar partitioned Petri dish containing the fungal and oomycetal strains and incubated for 3 days at 20°C. As an indicator of motility the average swimming and swarming diameter (cm) was measured. The error bars represent standard errors of the mean of three independent biological replicates. The asterisks indicate statistically significant (P < 0.05) differences relative to the control.

FIGURE 4

Setup of experiment and effect of pure volatiles on Collimonas pratensis Ter291 and Serratia plymuthica PRI-2C swarming motility (0.6% wt/vol agar) (A,B) and swimming motility (0.3% wt/vol agar) (C,D) on water agar supplied with artificial root exudates. Control: media. Five microliters of washed overnight cultures of C. pratensis Ter291 and S. plymuthica PRI-2C was spotted in the center of a soft agar partitioned Petri dish containing the pure volatiles and incubated for 3 days at 20°C. As an indicator of motility the average swimming and swarming diameter (cm) was measured. The error bars represent standard errors of the mean of three independent biological replicates. The asterisks indicate statistically significant (P < 0.05) differences relative to the control.