Characterization of Aureobasidium pullulans isolated from airborne spores in Thailand

Role of melanin in the black yeast fungi Aureobasidium pullulans and Zalaria obscura in promoting tolerance to environmental stresses and to antimicrobial compounds

The role of melanin in Aureobasidium pullulans ATCC 15233 and Zalaria obscura LS31012019, under simulated osmotic, oxidative, and high temperature stress conditions, on the susceptibility to essential oils (EOs) or antifungals and on the resistance to UV-C radiation was investigated. 93.6% of melanized A. pullulans and 92% of Z. obscura survived to 40 °C for 1 h compared to 77% and 76% of the non-melanized ones, while both yeasts tolerated a high concentration of NaCl (up to 30%) and H₂O₂ (up to 400 mM) regardless of melanin production. Higher EOs antifungal efficacy was observed in non-melanized cells (growth inhibition zone >30 mm) compared to the melanized ones (25 mm). Similarly, the lowest Minimum Inhibitory Concentrations (MIC) and Minimum Fungicidal Concentration (MFC) values were evidenced for Fluconazole, Clotrimazole, Bifonazole and Amphotericin in the non-melanized fungi. Increasing UV-C intensity (up to 2004.5 J/m²) caused total death in the non-melanized strains compared to about 30% growth reduction in the melanized ones. The results of this investigation, the first focused on the biological role of melanin in "black-fungi", are novel and encourage a better understanding of the biochemical features of melanin in the environmental adaptive ability of the new species Z. obscura.

Production and characterization of thermostable acidophilic β-mannanase from Aureobasidium pullulans NRRL 58524 and its potential in mannooligosaccharide production from spent coffee ground galactomannan

The maximum yield of the crude mannanase from Aureobasidium pullulans NRRL 58524 was 8.42 ± 0.18 U mL-1 when cultured for 72 h in the optimized medium containing 3% (w v-1) defatted spent coffee grounds (SCG) and 0.67% (w v-1) ammonium sulphate. Two forms of mannanase were observed in the crude enzyme and the principal mannanase was enriched to apparent homogeneity via sequential filtration and anion exchange chromatography. The molecular mass of the enzyme was approximately 63 kDa as determined by SDS-PAGE. The enriched mannanase was active at high temperatures (45-75 °C) and a pH range from 3 to 6 with the maximum activity at 55 °C and pH 4.0. The enzyme was relatively thermostable with more than 75% of its initial activity remained after a 12 h incubation at 55 °C. The half-lives of the enriched mannanase were over 8 and 6 h at 60 and 65 °C, respectively. The enzyme was not adversely affected by chelator and most ions tested. This enzyme could hydrolyze both glucomannan and galactomannan and exhibited limited catalytic activity on beta-glucan. When the crude mannanase was used to hydrolyze galactomannan extracted from SCG, the maximum yield of reducing sugars mainly comprising of mannobiose (16.27 ± 0.84 mg 100 mg-1), and mannotriose (2.85 ± 0.20 mg 100 mg-1) was obtained at 58.22 ± 2.04 mg 100 mg-1 dry weight, under optimized condition (84.87 U g-1 mannanase, 41 h 34 min incubation at 55 °C and pH 4.0). These results suggested the prospect of the enzyme in mannan hydrolysis and mannooligosaccharide production at a larger scale.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03301-4.

Routes and rates of bacterial dispersal impact surface soil microbiome composition and functioning

Recent evidence suggests that, similar to larger organisms, dispersal is a key driver of microbiome assembly; however, our understanding of the rates and taxonomic composition of microbial dispersal in natural environments is limited. Here, we characterized the rate and composition of bacteria dispersing into surface soil via three dispersal routes (from the air above the vegetation, from nearby vegetation and leaf litter near the soil surface, and from the bulk soil and litter below the top layer). We then quantified the impact of those routes on microbial community composition and functioning in the topmost litter layer. The bacterial dispersal rate onto the surface layer was low (7900 cells/cm²/day) relative to the abundance of the resident community. While bacteria dispersed through all three routes at the same rate, only dispersal from above and near the soil surface impacted microbiome composition, suggesting that the composition, not rate, of dispersal influenced community assembly. Dispersal also impacted microbiome functioning. When exposed to dispersal, leaf litter decomposed faster than when dispersal was excluded, although neither decomposition rate nor litter chemistry differed by route. Overall, we conclude that the dispersal routes transport distinct bacterial communities that differentially influence the composition of the surface soil microbiome.

The signaling pathways involved in metabolic regulation and stress responses of the yeast-like fungi Aureobasidium spp

Aureobasidium spp. can use a wide range of substrates and are widely distributed in different environments, suggesting that they can sense and response to various extracellular signals and be adapted to different environments. It is true that their pullulan, lipid and liamocin biosynthesis and cell growth are regulated by the cAMP-PKA signaling pathway; Polymalate (PMA) and pullulan biosynthesis is controlled by the Ca²⁺ and TORC1 signaling pathways; the HOG1 signaling pathway determines high osmotic tolerance and high pullulan and liamocin biosynthesis; the Snf1/Mig1 pathway controls glucose repression on pullulan and liamocin biosynthesis; DHN-melanin biosynthesis and stress resistance are regulated by the CWI signaling pathway and TORC1 signaling pathway. In addition, the HSF1 pathway may control cell growth of some novel strains of A. melanogenum at 37 °C. However, the detailed molecular mechanisms of high temperature growth and thermotolerance of some novel strains of A. melanogenum and glucose derepression in A. melanogenum TN3-1 are still unclear.

Enzymatic hydrolysis of tropical weed xylans using xylanase from Aureobasidium melanogenum PBUAP46 for xylooligosaccharide production

The maximum yield of xylanase from Aureobasidium melanogenum PBUAP46 was 5.19 ± 0.08 U ml^-1 when cultured in a production medium containing 3.89% (w/v) rice straw and 0.75% (w/v) NaNO₃ as carbon and nitrogen sources, respectively, for 72 h. This enzyme catalyzed well and was relatively stable at pH 7.0 and room temperature (28 ± 2 °C). The produced xylanase was used to hydrolyze xylans from four tropical weeds, whereupon it was found that the highest amounts of reducing sugars in the xylan hydrolysates of cogon grass (Imperata cylindrical), Napier grass (Pennisetum purpureum), and vetiver grass (Vetiveria zizanioides) were at 20.44 ± 0.84, 17.50 ± 0.29, and 19.44 ± 0.40 mg 100 mg xylan^-1, respectively, but it was not detectable in water hyacinth (Eichhornia crassipes) hydrolysate. The highest combined amount of xylobiose and xylotriose was obtained from vetiver grass; thus, it was selected for further optimization. After optimization, xylanase digestion of vetiver grass xylan at 27.94 U g xylan^-1 for 92 h 19 min gave the highest amount of reducing sugars (23.65 ± 1.34 mg 100 mg xylan^-1), which were principally xylobiose and xylotriose. The enriched XOs exhibited a prebiotic property, significantly stimulating the growth of Lactobacillus brevis and L. casei by a factor of up to 3.5- and 6.5-fold, respectively, compared to glucose.

Protective role of trehalose during radiation and heavy metal stress in Aureobasidium subglaciale F134

An isolated black yeast-like strain was obtained from radiation-polluted soil collected from Xinjiang province in northwest China. On the basis of ITS and LSU rDNA sequence analysis, in combination with the colony morphology and phenotypic properties, the isolated strain was revealed to represent a novel variety of Aureobasidium subglaciale, designated as A. subglaciale F134. Compared to other yeasts and bacteria, this isolate displayed superior resistance to gamma irradiation, UV light, and heavy metal ions. It was discovered that the resistance of the isolate was correlated with the stress protector trehalose. Through the overexpression of the trehalose-6-phosphate synthase gene tps1 and the deletion of acid trehalase gene ath1, the APT∆A double mutant exhibited a survival rate of 1% under 20 kGy of gamma-radiation, 2% survival rate at a UV dosage of 250 J/m², and tolerance towards Pb²⁺ as high as 1500 mg/L, which was in agreement with the high accumulation of intracellular trehalose compared to the wild-type strain. Finally, the protective effects and the mechanism of trehalose accumulation in A. subglaciale F134 were investigated, revealing a significant activation of the expression of many of the stress tolerance genes, offering new perspectives on the adaptations of radioresistant microorganisms.

Fungal monitoring of the indoor air of the Museo de La Plata Herbarium, Argentina

BACKGROUND

Biological agents, such as fungal spores in the air in places where scientific collections are stored, can attack and deteriorate them.

AIMS

The aim of this study was to gather information on the indoor air quality of the Herbarium of Vascular Plants of the Museo de Ciencias Naturales de La Plata, Argentina, in relation to fungal propagules and inert particles.

METHODS

This study was made using a volumetric system and two complementary sampling methods: (1) a non-viable method for direct evaluation, and (2) a viable method by culture for viable fungal propagules.

RESULTS

The non-viable method led to ten spore morphotypes being found from related fungal sources. A total of 4401.88spores/m³ and 32135.18 inert suspended particles/m³ were recorded. The viable method led to the finding of nine fungal taxa as viable spores that mostly belonged to anamorphic forms of Ascomycota, although the pigmented yeast Rhodotorula F.C. Harrison (Basidiomycota) was also found. A total count of 40,500fungal CFU/m³ air was estimated for all the sites sampled.

CONCLUSIONS

Both the non-viable and viable sampling methods were necessary to monitor the bio-aerosol load in the La Plata Herbarium. The indoor air of this institution seems to be reasonably adequate for the conservation of vascular plants due to the low indoor/outdoor index, low concentrations of air spores, and/or lack of indicators of moisture problems.

Euphorbia plant latex is inhabited by diverse microbial communities

PREMISE OF THE STUDY

The antimicrobial properties and toxicity of Euphorbia plant latex should make it a hostile environment to microbes. However, when specimens from Euphorbia spp. were propagated in tissue culture, microbial growth was observed routinely, raising the question whether the latex of this diverse plant genus can be a niche for polymicrobial communities.

METHODS

Latex from a phylogenetically diverse set of Euphorbia species was collected and genomic microbial DNA extracted. Deep sequencing of bar-coded amplicons from taxonomically informative gene fragments was used to measure bacterial and fungal species richness, evenness, and composition.

KEY RESULTS

Euphorbia latex was found to contain unexpectedly complex bacterial (mean: 44.0 species per sample; 9 plants analyzed) and fungal (mean: 20.9 species per sample; 22 plants analyzed) communities using culture-independent methods. Many of the identified taxa are known plant endophytes, but have not been previously found in latex.

CONCLUSIONS

Our results suggest that Euphorbia plant latex, a putatively hostile antimicrobial environment, unexpectedly supports diverse bacterial and fungal communities. The ecological roles of these microorganisms and potential interactions with their host plants are unknown and warrant further research.

Effects of sugar and amino acid supplementation on Aureobasidium pullulans NRRL 58536 antifungal activity against four Aspergillus species

Cultured cell extracts from ten tropical strains of Aureobasidium pullulans were screened for antifungal activity against four pathogenic Aspergillus species (Aspergillus flavus, Aspergillus niger, Aspergillus fumigatus, and Aspergillus terreus) using the well diffusion and conidial germination inhibition assays. The crude cell extract from A. pullulans NRRL 58536 resulted in the greatest fungicidal activity against all four Aspergillus species and so was selected for further investigation into enhancing the production of antifungal activity through optimization of the culture medium, carbon source (sucrose and glucose) and amino acid (phenylalanine, proline, and leucine) supplementation. Sucrose did not support the production of any detectable antifungal activity, while glucose did with the greatest antifungal activity against all four Aspergillus species being produced in cells grown in medium containing 2.5 % (w/v) glucose. With respect to the amino acid supplements, variable trends between the different Aspergillus species and amino acid combinations were observed, with the greatest antifungal activities being obtained when grown with phenylalanine plus leucine supplementation for activity against A. flavus, proline plus leucine for A. terreus, and phenylalanine plus proline and leucine for A. niger and A. fumigatus. Thin layer chromatography, spectrophotometry, high-performance liquid chromatography, (1)H-nuclear magnetic resonance, and MALDI-TOF mass spectrometry analyses were all consistent with the main component of the A. pullulans NRRL 58536 extracts being aureobasidins.

Multilocus phylogenetic analyses, pullulan production and xylanase activity of tropical isolates of Aureobasidium pullulans

Aureobasidium pullulans is the source of the commercially valuable polysaccharide pullulan and the enzyme xylanase. Isolates are typically off-white to pale pink or black on solid media, while some tropical isolates have been described as 'color variants' with bright pigments of red, yellow or purple. We sequenced 5 loci (internal transcribed spacer, intergenic spacer 1, translation elongation factor-1 alpha, beta tubulin, and RNA polymerase II) from 45 new isolates from Thailand. Based on the phylogenetic analyses, isolates were classified into 12 clades. Each clade showed different colors on different culture media including two clades with 'color variants' and some clades exhibited high levels of pullulan production or xylanase activity. Colony characteristics do not correlate perfectly with DNA sequence phylogeny or the physiological characters, but DNA sequence differences rapidly identify isolates with genetic novelty.

Pullulan-hyperproducing color variant strain of Aureobasidium pullulans FB-1 newly isolated from phylloplane of Ficus sp

The studies were carried out for the isolation of efficient pullulan producing strains of Aureobasidium pullulans. Five strains were isolated from phylloplane of different plants. Amongst these, three were producing black pigment melanin, while the remaining two produced pink pigment. These two color variant isolates of A. pullulans were designated as FB-1 and FG-1, and obtained from phylloplane of Ficus benjamina and Ficus glometa, respectively. The parameters employed for the identification of the isolates included morphology, nutritional assimilation patterns and exopolysaccharide (EPS) production. Isolates were compared with standard cultures for EPS production. A. pullulans FB-1 was the best producer of pullulan giving up to 1.9, 1.4 and 1.7 times more pullulan than the control of A. pullulans NCIM 976, NCIM 1048 and NCIM 1049, respectively. The IR spectra of the isolates and standard strains revealed that the polysaccharide was pullulan, but not aubasidan. The study also supported the fact that A. pullulans is a ubiquitous organism and phylloplane being the important niche of the organism.

Redefinition of Aureobasidium pullulans and its varieties

Using media with low water activity, a large numbers of aureobasidium-like black yeasts were isolated from glacial and subglacial ice of three polythermal glaciers from the coastal Arctic environment of Kongsfjorden (Svalbard, Spitsbergen), as well as from adjacent sea water, sea ice and glacial meltwaters. To characterise the genetic variability of Aureobasidium pullulans strains originating from the Arctic and strains originating pan-globally, a multilocus molecular analysis was performed, through rDNA (internal transcribed spacers, partial 28 S rDNA), and partial introns and exons of genes encoding beta-tubulin (TUB), translation elongation factor (EF1alpha) and elongase (ELO). Two globally ubiquitous varieties were distinguished: var. pullulans, occurring particularly in slightly osmotic substrates and in the phyllosphere; and var. melanogenum, mainly isolated from watery habitats. Both varieties were commonly isolated from the sampled Arctic habitats. However, some aureobasidium-like strains from subglacial ice from three different glaciers in Kongsfjorden (Svalbard, Spitsbergen), appeared to represent a new variety of A. pullulans. A strain from dolomitic marble in Namibia was found to belong to yet another variety. No molecular support has as yet been found for the previously described var. aubasidani. A partial elongase-encoding gene was successfully used as a phylogenetic marker at the (infra-)specific level.

Pullulan production by tropical isolates of Aureobasidium pullulans

Tropical isolates of Aureobasidium pullulans previously isolated from distinct habitats in Thailand were characterized for their capacities to produce the valuable polysaccharide, pullulan. A. pullulans strain NRM2, the so-called "color variant" strain, was the best producer, yielding 25.1 g pullulan l(-1) after 7 days in sucrose medium with peptone as the nitrogen source. Pullulan from strain NRM2 was less pigmented than those from the other strains and was remarkably pure after a simple ethanol precipitation. The molecular weight of pullulan from all cultures dramatically decreased after 3 days growth, as analyzed by high performance size exclusion chromatography. Alpha-amylase with apparent activity against pullulan was expressed constitutively in sucrose-grown cultures and induced in starch-grown cultures. When the alpha-amylase inhibitor acarbose was added to the culture medium, pullulan of slightly higher molecular weight was obtained from late cultures, supporting the notion that alpha-amylase plays a role in the reduction of the molecular weight of pullulan during the production phase.

Aureobasidium pullulans keratitis

BACKGROUND

Corneal ulcer caused by Aureobasidium pullulans is considered to be a rare entity. So far very few reports have appeared in the world literature and the authors' hospital is the first to report from Nepal. Although A. pullulans is regarded as a contaminant, it should be considered as a pathogen if isolated from corneal ulcer specimen with clinical signs of infection and with growth of the organism on two or more culture media or growth in one medium with consistent direct microscopy findings or growth of the same organism on repeated corneal scrapings. In the present study, a series of proven cases of A. pullulans corneal ulcers at a tertiary eye care centre of Eastern Nepal is reported.

METHODS

A retrospective analysis of stored data of microbiological and clinical cases of corneal ulcer was carried out. All consecutive patients (447 patients) with presumed microbial keratitis from 1 August 1998 to 31 July 2001 were evaluated with regards to clinical details, microbiological examination and management.

RESULTS

Of 200 fungal organisms isolated from the cultures, 25 were identified as A. pullulans. These ulcers showed negligible improvement to topical natamycin and required either topical fluconazole or topical itraconazole in all along with systemic intravenous fluconazole in eight patients. Of 25 eyes, 22 responded well to antifungal therapy and 2 required therapeutic penetrating keratoplasty. One patient was lost to follow up for 3 months and revealed phthisis bulbi on subsequent examination.

CONCLUSIONS

Aureobasidium pullulans corneal infection should be considered as a cause of keratomycosis.

Identification and possible disease mechanisms of an under-recognized fungus, Aureobasidium pullulans

BACKGROUND

Investigations into the occurrence and health effects of yeast-like fungi in the outdoor air in the US have been limited. We sought to identify a respirable-sized fungus common in the Pasadena air, locate a major source for the emissions and investigate its relevance to allergic disease.

METHODS

Yeast-like fungi sampled from the environment were isolated, microscopically examined and sequenced. Pasadena allergy patients were skin tested with commercially available fungal extracts. Patient serum was immunoanalyzed for specific IgE reactivity. Nearby vegetation was analyzed in a controlled emission chamber to find a major source for the aerosols.

RESULTS

Hyaline unicellular conidia comprised up to 90% (41,250 m(-3) of air) of total fungal counts and generally peaked at night and during periods of rainfall and ensuing winds throughout the fall and winter. Flowers were determined to be a major source of the emissions. The cellular and colonial morphology of isolates were consistent with Aureobasidium species. The sequence of the D1/D2 region of the 26S ribosomal subunit of isolates from flowers showed identity to two strains of Aureobasidium pullulans (black yeast). Seventeen percent (16/94) of atopic individuals had positive skin testing with A. pullulans extract. Patient sera IgE identified several high molecular weight allergens in Aureobasidium extracts.

CONCLUSIONS

Respirable-sized conidia of A. pullulans are emitted from flowers and form high concentrations in the air. They are associated with immediate reactivity on skin tests, bind to patient sera IgE, and might be relevant in allergic upper and lower airway diseases.