The Role of Hydrophobicity and Surface Receptors at Hyphae of Lyophyllum sp. Strain Karsten in the Interaction with Burkholderia terrae BS001 - Implications for Interactions in Soil

Enhancement on migration and biodegradation of Diaphorobacter sp. LW2 mediated by Pythium ultimum in soil with different particle sizes

Introduction

The composition and structure of natural soil are very complex, leading to the difficult contact between hydrophobic organic compounds and degrading-bacteria in contaminated soil, making pollutants hard to be removed from the soil. Several researches have reported the bacterial migration in unsaturated soil mediated by fungal hyphae, but bacterial movement in soil of different particle sizes or in heterogeneous soil was unclear. The remediation of contaminated soil enhanced by hyphae still needs further research.

Methods

In this case, the migration and biodegradation of Diaphorobacter sp. LW2 in soil was investigated in presence of Pythium ultimum.

Results

Hyphae could promote the growth and migration of LW2 in culture medium. It was also confirmed that LW2 was able to migrate in the growth direction and against the growth direction along hyphae. Mediated by hyphae, motile strain LW2 translocated over 3 cm in soil with different particle size (CS1, 1.0-2.0 mm; CS2, 0.5-1.0mm; MS, 0.25-0.5 mm and FS, <0.25 mm), and it need shorter time in bigger particle soils. In inhomogeneous soil, hyphae participated in the distribution of introduced bacteria, and the total number of bacteria increased. Pythium ultimum enhanced the migration and survival of LW2 in soil, improving the bioremediation of polluted soil.

Discussion

The results of this study indicate that the mobilization of degrading bacteria mediated by Pythium ultimum in soil has great potential for application in bioremediation of contaminated soil.

Isolation of Diaphorobacter sp. LW2 capable of degrading Phenanthrene and its migration mediated by Pythium ultimum

Phenanthrene, one of the polycyclic aromatic hydrocarbons, is stubborn and persistent and exists widely in petroleum-contaminated soil. Filamentous fungi are good assistants to bacterial transport, by hyphae passing through soil pores and reaching further positions. An isolated bacterial strain, from the contaminated soil of the coking plant, was identified as Diaphorobacter and named LW2, which could use phenanthrene as the only carbon source and energy for its growth. LW2 could degrade phenanthrene in a wide range of pH, temperature and initial concentration. When pH was 6 and 10, the removal rate of phenanthrene was 38.59% and 76.44%, respectively, and the removal rate of phenanthrene was 68.25% at 15 ℃. And LW2 could degrade 86.64% phenanthrene when the initial concentration was 100 mg L-1. The detection of DI-N-octyl phthalate, phthalic acid and p-hydroxybenzoic acid revealed that the strain LW2 metabolised phenanthrene through the phthalic acid pathway. Meanwhile, swimming and swarming test results suggested that LW2 was motile. The auxiliary effect of Pythium ultimum on LW2 migration was assessed. In the presence of Pythium ultimum, LW2 could migrate within the range of centimters by its mycelium, which was also observed by fluorescence microscopy. Meanwhile, the degradation ability of LW2 after the migration was also explored. The results proved that the migration process had no significant effect on its degradation ability, and LW2 still showed good phenanthrene metabolism ability. This study provides more possibilities for the bioremediation of phenanthrene-contaminated soil by screening the degradation bacteria and testing the effect of fungi on its migration.

Hydrophobin Gene Cmhyd4 Negatively Regulates Fruiting Body Development in Edible Fungi Cordyceps militaris

A deep understanding of the mechanism of fruiting body development is important for mushroom breeding and cultivation. Hydrophobins, small proteins exclusively secreted by fungi, have been proven to regulate the fruiting body development in many macro fungi. In this study, the hydrophobin gene Cmhyd4 was revealed to negatively regulate the fruiting body development in Cordyceps militaris, a famous edible and medicinal mushroom. Neither the overexpression nor the deletion of Cmhyd4 affected the mycelial growth rate, the hydrophobicity of the mycelia and conidia, or the conidial virulence on silkworm pupae. There was also no difference between the micromorphology of the hyphae and conidia in WT and ΔCmhyd4 strains observed by SEM. However, the ΔCmhyd4 strain showed thicker aerial mycelia in darkness and quicker growth rates under abiotic stress than the WT strain. The deletion of Cmhyd4 could promote conidia production and increase the contents of carotenoid and adenosine. The biological efficiency of the fruiting body was remarkably increased in the ΔCmhyd4 strain compared with the WT strain by improving the fruiting body density, not the height. It was indicated that Cmhyd4 played a negative role in fruiting body development. These results revealed that the diverse negative roles and regulatory effects of Cmhyd4 were totally different from those of Cmhyd1 in C. militaris and provided insights into the developmental regulatory mechanism of C. militaris and candidate genes for C. militaris strain breeding.

The pleiotropic functions of intracellular hydrophobins in aerial hyphae and fungal spores

Higher fungi can rapidly produce large numbers of spores suitable for aerial dispersal. The efficiency of the dispersal and spore resilience to abiotic stresses correlate with their hydrophobicity provided by the unique amphiphilic and superior surface-active proteins-hydrophobins (HFBs)-that self-assemble at hydrophobic/hydrophilic interfaces and thus modulate surface properties. Using the HFB-enriched mold Trichoderma (Hypocreales, Ascomycota) and the HFB-free yeast Pichia pastoris (Saccharomycetales, Ascomycota), we revealed that the rapid release of HFBs by aerial hyphae shortly prior to conidiation is associated with their intracellular accumulation in vacuoles and/or lipid-enriched organelles. The occasional internalization of the latter organelles in vacuoles can provide the hydrophobic/hydrophilic interface for the assembly of HFB layers and thus result in the formation of HFB-enriched vesicles and vacuolar multicisternal structures (VMSs) putatively lined up by HFBs. These HFB-enriched vesicles and VMSs can become fused in large tonoplast-like organelles or move to the periplasm for secretion. The tonoplast-like structures can contribute to the maintenance of turgor pressure in aerial hyphae supporting the erection of sporogenic structures (e.g., conidiophores) and provide intracellular force to squeeze out HFB-enriched vesicles and VMSs from the periplasm through the cell wall. We also show that the secretion of HFBs occurs prior to the conidiation and reveal that the even spore coating of HFBs deposited in the extracellular matrix requires microscopic water droplets that can be either guttated by the hyphae or obtained from the environment. Furthermore, we demonstrate that at least one HFB, HFB4 in T. guizhouense, is produced and secreted by wetted spores. We show that this protein possibly controls spore dormancy and contributes to the water sensing mechanism required for the detection of germination conditions. Thus, intracellular HFBs have a range of pleiotropic functions in aerial hyphae and spores and are essential for fungal development and fitness.

Soil indigenous microorganisms weaken the synergy of Massilia sp. WF1 and Phanerochaete chrysosporium in phenanthrene biodegradation

Biodegradation is a promising way to reduce phenanthrene (PHE) in environment. PHE biodegradation by bioaugmentation of axenic and mixed cultures of Massilia sp. WF1 (a highly efficient PHE-degrading bacteria) and Phanerochaete chrysosporium (P. chrysosporium, an extensively researched model fungus in organic pollutant bioremediation) was investigated in aqueous and autoclaved/un-autoclaved soil cultures. In the liquid cultures, the strain WF1 could use PHE (ca. 10 mg L^-1) as the sole carbon source, and the presence of d-fructose (500 mg L^-1) had no obvious effect on its PHE degradation; while the opposite was observed for P. chrysosporium. The bioaugmentation of strain WF1 and P. chrysosporium co-culture showed the highest PHE-degradation efficiency, especially in the aqueous and the autoclaved soil (PHE, ca. 50 mg kg^-1) cultures, indicating a synergistic interaction of the co-culture during PHE dissipation. It was further observed that the indigenous microorganisms (mainly the Gram-positive bacteria) played a dominant role during PHE biodegradation and showed an antagonistic action against the strain WF1-P. chrysosporium co-culture, which weakened the synergistic action of the co-culture in the un-autoclaved soil. Besides, the abundances of PAH-RHDα GP and nidA genes were negatively correlated with residual PHE in the soil. Our findings provide the scientific support for bioremediation of PAHs in environment.

Potential use of fungal-bacterial co-cultures for the removal of organic pollutants

Fungi and bacteria coexist in a wide variety of natural and artificial environments which can lead to their association and interaction - ranging from antagonism to cooperation - that can affect the survival, colonization, spatial distribution and stress resistance of the interacting partners. The use of polymicrobial cultivation approaches has facilitated a more thorough understanding of microbial dynamics in mixed microbial communities, such as those composed of fungi and bacteria, and their influence on ecosystem functions. Mixed (multi-domain) microbial communities exhibit unique associations and interactions that could result in more efficient systems for the degradation and removal of organic pollutants. Several previous studies have reported enhanced biodegradation of certain pollutants when using combined fungal-bacterial treatments compared to pure cultures or communities of either fungi or bacteria (single domain systems). This article reviews: (i) the mechanisms of pollutant degradation that can occur in fungal-bacterial systems (e.g.: co-degradation, production of secondary metabolites, enhancement of degradative enzyme production, and transport of bacteria by fungal mycelia); (ii) case studies using fungal-bacterial co-cultures for the removal of various organic pollutants (synthetic dyes, polycyclic aromatic hydrocarbons, pesticides, and other trace or volatile organic compounds) in different environmental matrices (e.g. water, gas/vapors, soil); (iii) the key aspects of engineering artificial fungal-bacterial co-cultures, and (iv) the current challenges and future perspectives of using fungal-bacterial co-cultures for environmental remediation.

Intracellular Burkholderia Symbionts induce extracellular secondary infections; driving diverse host outcomes that vary by genotype and environment

Symbiotic associations impact and are impacted by their surrounding ecosystem. The association between Burkholderia bacteria and the soil amoeba Dictyostelium discoideum is a tractable model to unravel the biology underlying symbiont-endowed phenotypes and their impacts. Several Burkholderia species stably associate with D. discoideum and typically reduce host fitness in food-rich environments while increasing fitness in food-scarce environments. Burkholderia symbionts are themselves inedible to their hosts but induce co-infections with secondary bacteria that can serve as a food source. Thus, Burkholderia hosts are "farmers" that carry food bacteria to new environments, providing a benefit when food is scarce. We examined the ability of specific Burkholderia genotypes to induce secondary co-infections and assessed host fitness under a range of co-infection conditions and environmental contexts. Although all Burkholderia symbionts intracellularly infected Dictyostelium, we found that co-infections are predominantly extracellular, suggesting that farming benefits are derived from extracellular infection of host structures. Furthermore, levels of secondary infection are linked to conditional host fitness; B. agricolaris infected hosts have the highest level of co-infection and have the highest fitness in food-scarce environments. This study illuminates the phenomenon of co-infection induction across Dictyostelium associated Burkholderia species and exemplifies the contextual complexity of these associations.