A simple agar plate assay for screening siderophore producer yeasts

Molecular Basis of Yeasts Antimicrobial Activity-Developing Innovative Strategies for Biomedicine and Biocontrol

In the context of the growing concern regarding the appearance and spread of emerging pathogens with high resistance to chemically synthetized biocides, the development of new agents for crops and human protection has become an emergency. In this context, the yeasts present a huge potential as eco-friendly agents due to their widespread nature in various habitats and to their wide range of antagonistic mechanisms. The present review focuses on some of the major yeast antimicrobial mechanisms, their molecular basis and practical applications in biocontrol and biomedicine. The synthesis of killer toxins, encoded by dsRNA virus-like particles, dsDNA plasmids or chromosomal genes, is encountered in a wide range of yeast species from nature and industry and can affect the development of phytopathogenic fungi and other yeast strains, as well as human pathogenic bacteria. The group of the "red yeasts" is gaining more interest over the last years, not only as natural producers of carotenoids and rhodotorulic acid with active role in cell protection against the oxidative stress, but also due to their ability to inhibit the growth of pathogenic yeasts, fungi and bacteria using these compounds and the mechanism of competition for nutritive substrate. Finally, the biosurfactants produced by yeasts characterized by high stability, specificity and biodegrability have proven abilities to inhibit phytopathogenic fungi growth and mycelia formation and to act as efficient antibacterial and antibiofilm formation agents for biomedicine. In conclusion, the antimicrobial activity of yeasts represents a direction of research with numerous possibilities of bioeconomic valorization as innovative strategies to combat pathogenic microorganisms.

Detection and Characterization of Antibacterial Siderophores Secreted by Endophytic Fungi from Cymbidium aloifolium

Endophytic fungi from orchid plants are reported to secrete secondary metabolites which include bioactive antimicrobial siderophores. In this study endophytic fungi capable of secreting siderophores were isolated from Cymbidium aloifolium, a medicinal orchid plant. The isolated extracellular siderophores from orchidaceous fungi act as chelating agents forming soluble complexes with Fe³⁺. The 60% endophytic fungi of Cymbidium aloifolium produced hydroxamate siderophore on CAS agar. The highest siderophore percentage was 57% in Penicillium chrysogenum (CAL1), 49% in Aspergillus sydowii (CAR12), 46% in Aspergillus terreus (CAR14) by CAS liquid assay. The optimum culture parameters for siderophore production were 30 °C, pH 6.5, maltose and ammonium nitrate and the highest resulting siderophore content was 73% in P. chrysogenum. The total protein content of solvent-purified siderophore increased four-fold compared with crude filtrate. The percent Fe³⁺ scavenged was detected by atomic absorption spectra analysis and the highest scavenging value was 83% by P. chrysogenum. Thin layer chromatography of purified P. chrysogenum siderophore showed a wine-colored spot with R_f value of 0.54. HPLC peaks with R_ts of 10.5 and 12.5 min were obtained for iron-free and iron-bound P. chrysogenum siderophore, respectively. The iron-free P. chrysogenum siderophore revealed an exact mass-to-charge ratio (m/z) of 400.46 and iron-bound P. chrysogenum siderophore revealed a m/z of 453.35. The solvent-extracted siderophores inhibited the virulent plant pathogens Ralstonia solanacearum, that causes bacterial wilt in groundnut and Xanthomonas oryzae pv. oryzae which causes bacterial blight disease in rice. Thus, bioactive siderophore-producing endophytic P. chrysogenum can be exploited in the form of formulations for development of resistance against other phytopathogens in crop plants.

The role of Rhodotorula mucilaginosa in selected biological process of wild fish

Defense mechanisms of fish are investigated in many aspects. One of the most interesting systems is that based on non-specific immune factors whose mechanisms of biocontrol have evolved in complex processes of microbiological co-existence. The wild fish devoid of probiotic stimulation have developed their own system to control the biosynthesis of immunostimulating compounds based on commensal microflora. Results of this study demonstrated the gastrointestinal tract (GI) of wild fish (Abramis brama, Rutilus rutilus, Perca fluviatilis) was colonized by permanently residing strains of Rhodotorula mucilaginosa. The genetic profile of the tested strains (PCR-random amplification of polymorphic DNA) indicated their affinity only to the GI of the analyzed fish. The capability for biosynthesis of β-carotene, torulene, torularhodin, and exopolysaccharides (EPS) under conditions of fish gastrointestinal tract was found to be a strain-specific trait. Rhodotorula spp. interactions with fish should be considered as a mechanism of symbiotic relations based on the stimulation of non-specific mechanisms of fish immunoprotection and antioxidative properties of yeast.

Evaluation of Rhodosporidium fluviale as biocontrol agent against Botrytis cinerea on apple fruit

The aim of the present work was to evaluate the ability of the native yeast Rhodosporidium fluviale to control Botrytis cinerea on apple fruit and to study the possible mechanisms of action with the goal of improving the control of gray mold. For this, the influence of application time of the yeast was studied simulating preventive and curative effects. Also, the effect of nonviable cells of the yeast in the biocontrol was assessed. According to the results obtained, the following mechanisms of action of R. fluviale could be proposed: 1- competition for space, 2- direct interaction between antagonist and pathogen, 3- induction of β-1,3-glucanase in apple tissue, 4- Probable production of glucanase in the apple wounds and 5- antifungal action of cellular components, probably chitin, present in the wall of yeast cells that could be the explanation for the activity of nonviable cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

Significance and Impact of the Study: Botrytis cinerea Pers: Fr, which causes gray mold of fruits and vegetables around the world, is difficult to control successfully because it is genetically variable and rapidly develops resistance to the chemicals commonly used for its control. This study is a contribution to the biocontrol of this phytopathogen fungus. The evaluation of the native yeast Rhodosporidium fluviale as biocontrol agent and the elucidation of possible mechanisms of action, including the participation of nonviable cells of this yeast, have not been reported up to date.

Arsenic accumulating and transforming bacteria isolated from contaminated soil for potential use in bioremediation

Arsenic (As) is a metalloid and considered harmful due to its toxic and carcinogenic effects. Removal of arsenic is of great importance for human welfare. The main objective of this study was to isolate arsenic-resistant bacteria that are capable of removing arsenic from the environment. Soil samples were collected from an arsenic-affected area of West Bengal, India and 10 different bacterial strains were isolated. The minimum inhibitory concentration (MIC) values of the isolates varied widely in the range 50-125 mM (As) as arsenate and 10-100 mM (As) as arsenite. TEM and EDAX analysis were done to confirm intracellular accumulation of arsenic. The 16s RNA and phylogenetic analysis showed that seven isolates belonged to γ-proteobacterium, two isolates belonged to Firmicutes and one was identified as Kocuria genera. Some of these bacteria could oxidize arsenite to arsenate and all others could reduce arsenate to arsenite. The growth pattern of the bacterial strains in presence and absence of arsenic was also observed. All the 10 isolates exhibited multiple heavy metal (like Ni, Zn, Cu, Pb, Co, etc.) tolerances. Thus, these new bacterial strains could conveniently be used for bioremediation of soil and effluents and the enzymes produced by them may be used for commercial exploitation.