Molecular Characterization of a Fungicidal Endoglucanase from the Cyanobacterium Calothrix elenkinii

Natural compounds from freshwater mussels disrupt fungal virulence determinants and influence fluconazole susceptibility in the presence of macrophages in Cryptococcus neoformans

Cryptococcus neoformans is a human fungal pathogen responsible for fatal infections, especially in patients with a depressed immune system. Overexposure to antifungal drugs due to prolonged treatment regimens and structure-similar applications in agriculture have weakened the efficacy of current antifungals in the clinic. The rapid evolution of antifungal resistance urges the discovery of new compounds that inhibit fungal virulence determinants, rather than directly killing the pathogen, as alternative strategies to overcome disease and reduce selective pressure toward resistance. Here, we evaluated the efficacy of freshwater mussel extracts (crude and clarified) against the production of well-defined virulence determinants (i.e., thermotolerance, melanin, capsule, and biofilm) and fluconazole resistance in C. neoformans. We demonstrated the extracts' influence on fungal thermotolerance, capsule production, and biofilm formation, as well as susceptibility to fluconazole in the presence of macrophages. Additionally, we measured the inhibitory activity of extracts against commercial peptidases (family representatives of cryptococcal orthologs) related to fungal virulence determinants and fluconazole resistance, and integrated these phenotypic findings with quantitative proteomics profiling. Our approach defined distinct signatures of each treatment and validated a new mechanism of anti-virulence action toward the polysaccharide capsule from a selected extract following fractionation. By understanding the mechanisms driving the antifungal activity of mussels, we may develop innovative treatment options to overcome fungal infections and promote susceptibility to fluconazole in resistant strains.

IMPORTANCE

As the prevalence and severity of global fungal infections rise, along with an increasing incidence of antifungal resistance, new strategies to combat fungal pathogens and overcome resistance are urgently needed. Critically, our current methods to overcome fungal infections are limited and drive the evolution of resistance forward; however, an anti-virulence approach to disarm virulence factors of the pathogen and promote host cell clearance is promising. Here, we explore the efficacy of natural compounds derived from freshwater mussels against classical fungal virulence determinants, including thermotolerance, capsule production, stress response, and biofilm formation. We integrate our phenotypic discoveries with state-of-the-art mass spectrometry-based proteomics to identify mechanistic drivers of these antifungal properties and propose innovative avenues to reduce infection and support the treatment of resistant strains.

Metabolite profiling of plant growth promoting cyanobacteria-Anabaena laxa and Calothrix elenkinii, using untargeted metabolomics

The metabolite profiles of two plant growth promoting cyanobacteria-Anabaena laxa and Calothrix elenkinii, which serve as promising biofertilizers, and biocontrol agents were generated to investigate their agriculturally beneficial activities. Preliminary biochemical analyses, in terms of total chlorophyll, total proteins, and IAA were highest at 14 days after inoculation (DAI). In A. laxa 20-25% higher values of reducing sugars, than C. elenkinii at both 14 and 21 DAI were recorded. Carbon and nitrogen assimilating enzyme activities-phosphoenol pyruvate carboxylase (PEPC), carbonic anhydrase (CA), and glutamine synthetase (GS) were highest at 14 DAI, albeit, nitrate reductase (NR) activity was higher by 0.73-0.84-fold at 21 DAI. Untargeted GC-MS (Gas chromatography-Mass spectrometric) analysis of metabolite profiles of 21d-old cyanobacterial cultures and characterization using NIST mass spectral library illustrated that A. laxa recorded highest number of metabolite hits in three chemical classes namely amino acid and peptides, nucleotides, nucleosides and analogues, besides other organic compounds. Based on the pathway analysis of identified metabolites, both A. laxa, and C. elenkinii were enriched in metabolites involved in aminoacyl-tRNA biosynthesis, and amino acid metabolism pathways, particularly lactose and glutamic acid, which are important players in plant-microbe interactions. Correlation-based metabolite network illustrated distinct and significant differences in the metabolic machinery of A. laxa and C. elenkinii, highlighting their novel identity and enrichment in C-N rich metabolites, as factors underlying their plant growth and soil fertility enhancing attributes, which make them valuable as inoculants.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03902-7.

Cyanobacteria: A Promising Source of Antifungal Metabolites

Cyanobacteria are a rich source of secondary metabolites, and they have received a great deal of attention due to their applicability in different industrial sectors. Some of these substances are known for their notorious ability to inhibit fungal growth. Such metabolites are very chemically and biologically diverse. They can belong to different chemical classes, including peptides, fatty acids, alkaloids, polyketides, and macrolides. Moreover, they can also target different cell components. Filamentous cyanobacteria have been the main source of these compounds. This review aims to identify the key features of these antifungal agents, as well as the sources from which they are obtained, their major targets, and the environmental factors involved when they are being produced. For the preparation of this work, a total of 642 documents dating from 1980 to 2022 were consulted, including patents, original research, review articles, and theses.

Microalgae as next generation plant growth additives: Functions, applications, challenges and circular bioeconomy based solutions

Sustainable agriculture practices involve the application of environment-friendly plant growth promoters and additives that do not negatively impact the health of the ecosystem. Stringent regulatory frameworks restricting the use of synthetic agrochemicals and the increase in demand for organically grown crops have paved the way for the development of novel bio-based plant growth promoters. In this context, microalgae biomass and derived agrochemicals offer novel sources of plant growth promotors that enhance crop productivity and impart disease resistance. These beneficial effects could be attributed to the presence of wide range of biomolecules such as soluble amino acid (AA), micronutrients, polysaccharides, phytohormones and other signaling molecules in microalgae biomass. In addition, their phototrophic nature, high photosynthetic efficiency, and wide environmental adaptability make them an attractive source of biostimulants, biofertilizers and biopesticides. The present review aims to describe the various plant growth promoting metabolites produced by microalgae and their effects on plant growth and productivity. Further, the effects elicited by microalgae biostimulants with respect to different modes of applications such as seed treatments, foliar spray and soil/root drenching is reviewed in detail. In addition, the ability of microalgae metabolites to impart tolerance against various abiotic and biotic stressors along with the mechanism of action is discussed in this paper. Although the use of microalgae based biofertilizers and biostimulants is gaining popularity, the high nutrient and water requirements and energy intensive downstream processes makes microalgae based technology commercially unsustainable. Addressing this challenge, we propose a circular economy model of microalgae mediated bioremediation coupled with biorefinery approaches of generating high value metabolites along with biofertilizer applications. We discuss and review new trends in enhancing the sustainability of microalgae biomass production by co-cultivation of algae with hydroponics and utilization of agriculture effluents.

Comparative genomics of Bacillus cereus sensu lato spp. biocontrol strains in correlation to in-vitro phenotypes and plant pathogen antagonistic capacity

Bacillus cereus sensu lato (Bcsl) strains are widely explored due to their capacity to antagonize a broad range of plant pathogens. These include B. cereus sp. UW85, whose antagonistic capacity is attributed to the secondary metabolite Zwittermicin A (ZwA). We recently isolated four soil and root-associated Bcsl strains (MO2, S-10, S-25, LSTW-24) that displayed different growth profiles and in-vitro antagonistic effects against three soilborne plant pathogens models: Pythium aphanidermatum (oomycete) Rhizoctonia solani (basidiomycete), and Fusarium oxysporum (ascomycete). To identify genetic mechanisms potentially responsible for the differences in growth and antagonistic phenotypes of these Bcsl strains, we sequenced and compared their genomes, and that of strain UW85 using a hybrid sequencing pipeline. Despite similarities, specific Bcsl strains had unique secondary metabolite and chitinase-encoding genes that could potentially explain observed differences in in-vitro chitinolytic potential and anti-fungal activity. Strains UW85, S-10 and S-25 contained a (~500 Kbp) mega-plasmid that harbored the ZwA biosynthetic gene cluster. The UW85 mega-plasmid contained more ABC transporters than the other two strains, whereas the S-25 mega-plasmid carried a unique cluster containing cellulose and chitin degrading genes. Collectively, comparative genomics revealed several mechanisms that can potentially explain differences in in-vitro antagonism of Bcsl strains toward fungal plant pathogens.

Seed biopriming with cyanobacterial extracts as an eco-friendly strategy to control damping off caused by Pythium ultimum in seedbeds

This work highlights the ability of various cyanobacterial extracts from Anabaena spp., Tolypothrix spp., Nostoc or Trichormus, among others genera, to control the incidence of damping-off caused by Pythium ultimum in cucumber seedlings. Protocols applied aimed at the preliminary characterization of the cyanobacterial collection were very useful for predicting their phytotoxic, phytostimulating and biopesticidal capacity. First, the phytostimulatory or phytotoxic potential of a collection of 31 sonicated cyanobacterial extracts was analyzed by calculating the germination index in watercress seeds and the increase or loss of seedling weight. Likewise, the collection was characterized according to its ability to inhibit the growth of P. ultimum by dual culture bioassays and detached-leaf test. Finally, after selecting the most effective extracts, a preventive damping-off bioassay was performed based on cucumber seed biopriming. The strain SAB-M465 showed to be the most efficient strain against the in vitro growth of P. ultimum, while SAB-B912 was more discreet in this regard, but proved to be the most effective as a germination stimulator. Seed biopriming strategy with sonicated extracts of cyanobacteria revealed a remarkable promoter effect in the early stages of plant development, although only SAB-M465 was positioned as an effective control agent against damping-off caused by P. ultimum in cucumber seedbeds.

The Oomycete Pythium oligandrum Can Suppress and Kill the Causative Agents of Dermatophytoses

Pythium oligandrum (Oomycota) is known for its strong mycoparasitism against more than 50 fungal and oomycete species. However, the ability of this oomycete to suppress and kill the causal agents of dermatophytoses is yet to be studied. We provide a complex study of the interactions between P. oligandrum and dermatophytes representing all species dominating in the developed countries. We assessed its biocidal potential by performing growth tests, on both solid and liquid cultivation media and by conducting a pilot clinical study. In addition, we studied the molecular background of mycoparasitism using expression profiles of genes responsible for the attack on the side of P. oligandrum and the stress response on the side of Microsporum canis. We showed that dermatophytes are efficiently suppressed or killed by P. oligandrum in the artificial conditions of cultivations media between 48 and 72 h after first contact. Significant intra- and interspecies variability was noted. Of the 69 patients included in the acute regimen study, symptoms were completely eliminated in 79% of the patients suffering from foot odour, hyperhidrosis disappeared in 67% of cases, clinical signs of dermatomycoses could no longer be observed in 83% of patients, and 15% of persons were relieved of symptoms of onychomycosis. Our investigations provide clear evidence that the oomycete is able to recognize and kill dermatophytes using recognition mechanisms that resemble those described in oomycetes attacking fungi infecting plants, albeit with some notable differences.

Beneficial cyanobacteria and eubacteria synergistically enhance bioavailability of soil nutrients and yield of okra

Microorganisms in the rhizosphere mediate the cycling of nutrients, their enhanced mobilisation and facilitate their uptake, leading to increased root growth, biomass and yield of plants. We examined the promise of beneficial cyanobacteria and eubacteria as microbial inoculants, applied singly or in combination as consortia or biofilms, to improve growth and yields of okra. Interrelationships among the microbial activities and the micro/macro nutrient dynamics in soils and okra yield characteristics were assessed along with the changes in the soil microbiome. A significant effect of microbial inoculation on alkaline phosphatase activity was recorded both at the mid-crop and harvest stages. Microbial biomass carbon values were highest due to the Anabaena sp. - Providencia sp. (CR1 + PR3) application. The yield of okra ranged from 444.6-478.4 g(-1) plant and a positive correlation (0.69) recorded between yield and root weight. The application of Azotobacter led to the highest root weight and yield. The concentration of Zn at mid-crop stage was 60-70% higher in the Azotobacter sp. and Calothrix sp. inoculated soils, as compared to uninoculated control. Iron concentration in soil was more than 2-3 folds higher than control at the mid-crop stage, especially due to the application of Anabaena-Azotobacter biofilm and Azotobacter sp. Both at the mid-crop and harvest stages, the PCR-DGGE profiles of eubacterial communities were similar among the uninoculated control, the Anabaena sp. - Providencia sp. (CW1 + PW5) and the Anabaena-Azotobacter biofilm treatments. Although the profiles of the Azotobacter, Calothrix and CR1 + PR3 treatments were identical at these stages of growth, the profile of CR1 + PR3 was clearly distinguishable. The performance of the inoculants, particularly Calothrix (T6) and consortium of Anabaena and Providencia (CR1 + PR3; T5), in terms of microbiological and nutrient data, along with generation of distinct PCR-DGGE profiles suggested their superiority and emphasized the utility of combining microbiological and molecular tools in the selection of effective microbial inoculants.

Influence of cyanobacterial inoculation on the culturable microbiome and growth of rice

Rice plants are selective with their associations with bacteria that are beneficial for growth, nutrient uptake, exhibit induced resistance or antagonism towards pathogens. Cyanobacteria as bioinoculants are known to promote the growth and health of rice plants. The present investigation was aimed at understanding whether and how cyanobacterial (Calothrix elenkinii) inoculation influenced the rice plant growth and the culturable bacterial populations and identifying the dominant culturable "microbiome" members. The plant tissue extracts were used to enumerate populations of the culturable microbiome members using selected enrichment media with different nutrient levels. About 10-fold increases in population densities of culturable microbiome members in different media were recorded, with some isolates having metabolic potential for nitrogen fixation and phosphorus solubilization. Fatty acid methyl ester (FAME) analysis and 16S rRNA sequencing of selected microbial morphotypes suggested the predominance of the members of Bacillaceae. Significant increases in plant growth attributes, nitrogenase activity and indole acetic acid production, and activities of hydrolytic and defense enzymes were recorded in the Calothrix inoculated plants. The PCR-based analysis and scanning electron microscopic (SEM) observations confirmed the presence of inoculated cyanobacterium inside the plant tissues. This investigation illustrated that cyanobacterial inoculation can play significant roles in improving growth and metabolism of rice directly and interact with the beneficial members from the endophytic microbiome of rice seedlings synergistically.