The continual plastic accumulation in the environment and the hazardous consequences determine the interest in thermophiles as possible effective plastic degraders, due to their unique metabolic mechanisms and change of plastic properties at elevated temperatures. PCL is one of major biodegradable plastics with promising application to replace existing non-biodegradable polymers. Metagenomic analysis of the phylogenetic diversity in plastic contaminated area of Marikostinovo hot spring, Bulgaria revealed a higher number taxonomic groups (11) in the sample enriched without plastic (Marikostinovo community, control sample, MKC-C) than in that enriched in the presence of poly-ε-caprolactone (PCL) (MKC-P), (7). A strong domination of the phylum Proteobacteria was observed for MKC-C, while the dominant phyla in MKC-P were Deinococcus-Thermus and Firmicutes. Among the strains isolated from MKC-P, the highest esterase activity was registered for Brevibacillus thermoruber strain 7 at 55 °C. Its co-cultivation with another isolate resulted in ~10% increase in enzyme activity. During a 28-day biodegradation process, a decrease in PCL molecular weight and weight loss were established resulting in 100% degradation by MKC-P and 63.6% by strain 7. PCL degradation intermediate profiles for MKC-P and pure strain were similar. Broken plastic pieces from PCL surface and formation of a biofilm by MKC-P were observed by SEM, while the pure strain caused significant deformation of PCL probes without biofilm formation.

Exopolysaccharides (EPSs) represent versatile biopolymers finding diverse applications in food, pharmaceuticals, and bioremediation industries. Extremophiles, thriving under extreme environmental conditions, have emerged as a promising source of novel EPSs with better stability and bioactivity. The present work reviews the complex influence of various abiotic factors and bioprocess parameters such as temperature, pH, carbon and nitrogen sources, C/N ratios, and oxygen transfer dynamics on the production of EPSs from extremophilic microorganisms. Results underline the important role of temperature for structural and functional properties of EPSs, from the synthesis of cryoprotective polymers in psychrophiles to the production of thermostable EPSs in thermophiles under cold stress. The pH has an extensive effect on enzymatic activities: optimal neutral to slightly acidic conditions exist for most strains. Carbon sources determine not only the yield of EPSs but also its structural features, while nitrogen sources and C/N ratios regulate the balance between biomass production and polymer biosynthesis. Besides that, oxygen transfer limitations—which may happen in particularly viscous or saline media—are overtaken by optimized bioreactor configuration and stirring strategies. These findings are highly relevant to the development of tailored cultivation conditions enabling the maximization of EPS yields and adaptation of its properties to comply with industrial requirements. This study provides a framework for enhancing EPS production by leveraging the adaptive traits of extremophiles. This approach supports the sustainable use of biopolymers, advances fermentation production processes, and helps uncover the underlying mechanisms involved.

Bulgaria stands out as a country rich in diverse extreme environments, boasting a remarkable abundance of mineral hot waters, which positions it as the second-largest source of such natural resources in Europe. Notably, several thermal and coastal solar salterns within its territory serve as thriving habitats for thermophilic and halophilic microorganisms, which offer promising bioactive compounds, including exopolysaccharides (EPSs). Multiple thermophilic EPS producers were isolated, along with a selection from several saltern environments, revealing an impressive taxonomic and bacterial diversity. Four isolates from three different thermophilic species, Geobacillus tepidamans V264, Aeribacillus pallidus 418, Brevibacillus thermoruber 423, and Brevibacillus thermoruber 438, along with the halophilic strain Chromohalobacter canadensis 28, emerged as promising candidates for further exploration. Optimization of cultivation media and conditions was conducted for each EPS producer. Additionally, investigations into the influence of aeration and stirring in laboratory bioreactors provided valuable insights into growth dynamics and polymer synthesis. The synthesized biopolymers showed excellent emulsifying properties, emulsion stability, and synergistic interaction with other hydrocolloids. Demonstrated biological activities and functional properties pave the way for potential future applications in diverse fields, with particular emphasis on cosmetics and medicine. The remarkable versatility and efficacy of biopolymers offer opportunities for innovation and development in different industrial sectors.

Several exopolymers with different chemical composition and correspondingly variety in their physico-chemical properties from halophilic microorganisms have still been described, however, with a low production yield. Chromohalobacter canadensis 28 isolated from Pomorie saltern synthesized an unusual exopolymer (EP) containing 72% γ-polyglutamic acid (PGA), an essential cosmeceutical additive. Current work suggests a novel approach for effective EP synthesis by C. canadensis 28 using continuous cultures. Highest production was observed at low dilution rates reaching a level of 2.1 mg/mL at D = 0.035, similar to those in batch cultures (2.34 mg/mL), however avoiding all disadvantages of discontinuous fermentation processes. At steady state, the total quantities of the synthesized EP after 48 h cultivation for the given equipment volume in D = 0.035 h⁻¹ and D = 0.075 h⁻¹ were 8.67 and 12 g, correspondingly, while it was 2.9 g for batch culture. Process parameters did not change after a ten-day run at D = 0.35 h⁻¹. A degree of purity of EP fraction received from continuous cultures was significantly increased up to 93–96%. A lack of cytotoxicity and high cell viability were observed for human dermal fibroblast cells after 24 h incubation with crude EP from C. canadensis 28 and purified PGA fraction that could suggest its high potential for cosmetic applications.

Nakaseomyces glabratus is an opportunistic fungal pathogen that primarily affects immunocompromised individuals. Unlike other Candida species, N. glabratus exhibits nondimorphic blastoconidial morphology and a haploid genome. It is a leading cause of both superficial (oral, esophageal, vaginal, or urinary) and systemic candidiasis. In this study, we evaluated 47 clinical isolates from Central Bulgaria (Plovdiv) and 1 wild strain isolated from the gut of the beetle Oxythyrea funesta (Coleoptera: Cetoniinae) collected in Sofia, Bulgaria. Growth was observed across a pH range of 3 to 9. The strains were assessed for the production of lipases, esterases, and proteases—enzymes associated with pathogenicity—and their relationship to virulence. Biofilm formation and exopolysaccharide production were also measured, with all strains showing similar profiles. No competitive inhibition of N. glabratus was observed against C. parapsilosis. All isolates exhibited resistance to micafungin, while resistance to both micafungin and anidulafungin was observed in 21 isolates (44%). These findings provide insight into the biochemical characteristics of N. glabratus populations from Southeast Europe, contributing to a better understanding of strain behavior under controlled laboratory conditions and addressing the gap in data on this species in the region.

Thermophilic microorganisms represent an untapped reservoir of thermostable biocatalysts and stress-resilient biomolecules for industrial biotechnology. Aeribacillus pallidus, a Gram-positive moderate thermophile, has attracted attention for its enzymatic versatility and environmental adaptability, yet its genomic potential remains underexplored. Here, we present a comparative genomic analysis of 13 A. pallidus strains to uncover conserved and strain-specific traits relevant to biotechnology. Genomes ranged from 3.24 to 4.98 Mb, with GC content largely conserved (~39%) except for GS3372 (57.4%), indicating possible horizontal gene transfer. All strains encoded complete central metabolic pathways, while carbohydrate-active enzyme profiling revealed abundant glycoside hydrolases and glycosyltransferases, with GS3372 and MHI3390 enriched for lignocellulose-degrading enzymes. Secondary metabolite mining identified diverse biosynthetic gene clusters, including terpenes, sactipeptides, and bacteriocins, with PI8, W-12, and 8m3 exhibiting the greatest biosynthetic diversity. A core set of heat shock and universal stress proteins underscored robust thermotolerance. Phylogenomic and pan-genome analyses revealed high intraspecific diversity and an open pan-genome structure. Collectively, these findings position A. pallidus as a promising thermophilic chassis organism for sustainable applications, including biomass conversion, biofuel production, bioremediation, and the synthesis of heat-stable antimicrobial agents.