Radoslav Abrashev

Katak, a traditional Bulgarian fermented dairy product distinguished by its unique sensory characteristics and extended shelf life, represents a valuable source of autochthonous microorganisms whose functional and genomic properties remain insufficiently explored. In the present study, strain L3, isolated from katak, was comprehensively characterized by a polyphasic approach integrating classical microbiological methods, molecular identification, and whole-genome sequencing. Functional assessment revealed a broad spectrum of antimicrobial activity against pathogenic bacteria and food-associated fungi, accompanied by pronounced radical-scavenging and antioxidant capacities. The strain also exhibited high transit tolerance in simulated gut passage and a favorable antibiotic susceptibility profile, indicating its potential as a probiotic candidate. Whole-genome sequencing confirmed the taxonomic affiliation of the isolate as Lactiplantibacillus plantarum and enabled the identification of genetic determinants associated with stress response, environmental adaptation, oxidative stress protection, biofilm formation, and antimicrobial activity. The genomic analysis complemented the in vitro findings by identifying genetic determinants consistent with the observed phenotypic traits, supporting the genetic potential of strain L3 for probiotic-associated functions. The integration of phenotypic and genomic evidence allowed a comprehensive evaluation of L. plantarum L3 and demonstrated that its probiotic-related properties are underpinned by a diverse repertoire of functional genes. These findings identify L3 as a promising candidate for further biotechnological/probiotic assessment.

Oxidative stress poses significant challenges for fungi inhabiting extreme environments. Elevated salinity frequently induces the excessive production of reactive oxygen species (ROS), which can damage cellular components and impair growth. In response, fungi—common inhabitants of extreme environments—activate coordinated adaptive mechanisms, including antioxidant defense systems and other stress-related pathways. In the present study, a newly isolated strain, Trichoderma afroharzianum B2.2, obtained from the poorly studied saline habitat of Atanasovsko Lake (Bulgaria), was investigated. The cellular response of this moderately halotolerant strain to increased salinity was characterized. Biomarkers of oxidative stress were evaluated, and the involvement of key enzymes from glycolysis and the pentose phosphate pathway in the strain’s adaptation to elevated salinity was examined. Understanding adaptations to salt environments is crucial not only for elucidating fungal survival mechanisms under extreme conditions but also for their potential applications in biotechnology, ecology, and food safety, particularly in the context of increasing ecosystem salinization and climate change.

The diverse health benefits of lactic acid bacteria (LAB) have made them a focal point of research in the fields of food and health sciences. Furthermore, probiotics and postbiotics have been demonstrated to directly or indirectly influence food quality and human health. A substantial body of research has been dedicated to the antimicrobial activity of pro- and postbiotics; however, their antioxidant properties remain relatively unexplored. Although LAB are facultative anaerobes, there are several species that have the potential to undergo aerobic respiration, thereby being exposed to the action of reactive oxygen species (ROS). The resultant oxidative stress has been shown to damage all intracellular molecules, thus requiring the presence of antioxidants in order to counteract this effect. The present review discusses the peculiarities of respiration, the role of ROS, the antioxidant potential of LAB, and the mechanisms underlying their activity. Furthermore, the study explores the antioxidant capacity of probiotics and postbiotics, as well as their role in controlling oxidative stress. The objective of the present review is to provide an overview of the current research on the oxidative stress tolerance and antioxidant capacity of LAB and its impact on food.

In recent years, sialidases (neuraminidases) derived from non-clinical sources have attracted considerable interest due to their potential applications in the food and pharmaceutical industries. A deeper understanding of the mechanisms regulating sialidase synthesis could lead to more efficient enzyme production. Induction is considered a key regulatory mechanism. However, there is a lack of data on the regulation of sialidase synthesis in filamentous fungi. This study examines how regulatory mechanisms influence the production of a sialidase enzyme exhibiting high activity at low temperatures in the Antarctic fungal strain Penicillium griseofulvum P29. The inclusion of high- and low-molecular-weight substances possessing terminal non-reducing N-acetylneuramyl groups in the tests led to a marked enhancement of sialidase activity. The strongest induction response was elicited by sialic acid, followed by glycomacropeptide, milk whey, N-acetylglucosamine, N-acetylmannosamine, and colominic acid. RT-qPCR experiments demonstrated that induction occurs at the transcriptional level of the sialidase gene. Biochemical analysis elucidates the function of inducers as triggers in the de novo synthesis of the enzyme protein. To our knowledge, this is the first study to highlight the importance of regulatory mechanism induction in the synthesis of cold-active sialidases.

Efficient decomposition of lignocellulosic biomass composed of cellulose, hemicellulose, and lignin is vital for sustainable life-support systems in long-duration space missions. Fungi offer a promising solution due to their adaptability, low maintenance, and enzymatic ability to break down complex plant materials. Their enzymes, such as cellulases and lignin-degradingenzymes, convert biomass into fermentable sugars and aromatic compounds, which can be used to produce biofuels, bioplastics, or serve as inputs in closed-loop systems. Fungal strains isolated from woody species, fungal isolates from sawdust and wood surface, and five fungal consortia were tested for cellulase and lignolytic enzyme production. Three fungal strains and the natural consortia were selected as the most promising for the biodegradation of lignocellulosic wastes generated during manned spaceflights.

Air pollution remains one of the most urgent global challenges, affecting both public health and environmental integrity, with its severity escalating in parallel with industrialization and urban expansion. Defined as the presence of harmful substances in the atmosphere, air pollution poses risks to human health and disrupts the development of plant and animal life. Urban areas, particularly large cities, frequently exhibit pollutant concentrations that exceed safety thresholds established by the World Health Organization (WHO). This study presents a comprehensive analysis of airborne fungal microbiota in two distinct districts of Sofia, Bulgaria: the highly urbanized city center (Orlov Most) and a less urbanized southwestern area (New Bulgarian University, Ovcha Kupel). Weekly fluctuations in mold spore abundance were monitored, revealing elevated contamination levels on Fridays, likely due to intensified vehicular traffic preceding weekends and public holidays. Taxonomic identification of dominant mold species was conducted using both classical and molecular genetic methods. The isolated fungal strains predominantly belonged to the phylum Ascomycota (80%), with Talaromyces and Alternaria emerging as the most prevalent genera. Additionally, antifungal susceptibility testing indicated that most isolates were sensitive to commonly used antifungal agents, although resistance was observed in two strains of Talaromyces wortmannii. These findings underscore the significance of fungal bioaerosols in urban air quality assessments and highlight the need for targeted monitoring and mitigation strategies.

Legumes contain variable amounts of bioactive substances, including protease inhibitors, which have a protective role against herbivorous insects and bacterial, fungal, and viral pathogens. However, their potential for application in agricultural and medicinal practices requires additional investigation. Bitter vetch (Vicia ervilia (L.) Willd.) is an ancient crop that is now underutilized, and its potential for various applications has recently been reevaluated. In this study, we report the purification, characterization, and bioactive properties of a protease inhibitor against trypsin/chymotrypsin-type proteases (vPI) from bitter vetch seeds. The inhibitor was purified by extraction under acidic conditions, ammonium sulfate fractionation, and size-exclusion chromatography. Its inhibitory specificity, thermostability, pH stability, and antioxidant and antimycotic activity against Alternaria alternata, Alternaria solani, Aspergillus fumigatus, Aspergillus niger, Candida albicans, Fusarium solani, Mucor michei, Penicillium griseofulvum, and Rhizopus oryzae were evaluated. Purified vPI presented superoxide anion scavenging power and antifungal activity in response to all tested strains except M. michei. It had the strongest effect on F.solani and A. solani, and a moderate effect on P. griseofulvum and C. albicans. The treatment of A. alternata, R. oryzae, A. fumigatus, and A. niger demonstrated high efficacy within the initial 24h but declined thereafter. The usefulness and limitations of the vPI application in practice are discussed.

Antimicrobial resistance represents one of the most critical public health challenges of the 21st century. The emergence of multidrug resistance (MDR) in bacterial and fungal pathogens to diverse chemical agents severely impedes the effective treatment of diseases such as cancer and systemic infections. The rapid escalation of microbial resistance underscores the urgent need for the discovery of novel antimicrobial agents and innovative approaches to drug development. In both clinical and industrial contexts, the identification of new antibiotics and antifungals remains pivotal for pathogen control. Current research efforts focus on the development of alternative formulations that offer high efficacy, reduced resistance potential, minimal side effects, and synergistic interactions, particularly those derived from natural sources. Filamentous fungi originating from extreme environments have evolved to thrive under harsh conditions, making them promising reservoirs of bioactive metabolites with unique structural and functional properties. These fungi exhibit potent antimicrobial activity through diverse mechanisms that disrupt essential cellular processes in pathogens. Despite their remarkable potential, the bioprospecting of extremophilic filamentous fungi for drug development remains underexplored. This highlights the necessity for expanded research into the efficacy and safety of their derived compounds. This review aims to emphasize the capacity of extremophilic fungi to produce antimicrobial agents, elucidate resistance mechanisms, characterize fungal bioactive extracts, and analyze their molecular actions in the context of their extreme ecological niches.

Sialidases/neuraminidases remove terminal sialic acid residues from glycoproteins, glycolipids, and oligosaccharides. Our previous research has revealed the distribution of sialidase in non-clinical fungal isolates from different ecological niches, including Antarctica. Fungi adapted to extremely low temperatures possess defense mechanisms necessary for their survival such as the response against oxidative stress. The relationship between oxidative stress and sialidase synthesis has been studied extremely sparsely. The aim of the present study was to investigate the involvement of sialidase in the cell response of the Antarctic strain P. griseofulvum P29 against oxidative stress induced by long- and short-term exposure to low temperatures. The changes in growth temperatures for 120 h (long-term stress) affected biomass accumulation, glucose consumption, sialidase synthesis, and the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). The short-term temperature downshift (6 h) caused oxidative stress, evidenced by changes in the levels of biomarkers, including lipid peroxidation, oxidatively damaged proteins, and the accumulation of reserve carbohydrates. Simultaneously, a sharp increase in SOD and CAT activity was found, which coincided with a significant increase in sialidase activity. This study marks the first demonstration of increased sialidase activity in filamentous fungi isolated from extreme cold environments as a response to oxidative stress.

Antarctica, with its harsh environmental conditions, is home to a wide variety of microorganisms, including filamentous fungi. The survival of Antarctic mycoflora has led to increasing interest in their adaptation. Extreme low temperatures, low water availability, frequent freeze–thaw cycles, strong winds, ultraviolet radiation, etc., are all prerequisites for accelerated production of reactive oxygen species (ROS) and the occurrence of oxidative stress (OS). Antarctic fungi have effective mechanisms to cope with the damaging effects of ROS. While the distribution, morphology, physiology, and biotechnological potential of these fungi are well documented, the role of oxidative stress in their adaptation is poorly understood. This has been one of the main research topics of our team for a long time, and a number of papers on our results have been published. This review summarizes the existing literature on the induction of oxidative stress in Antarctic psychrophilic, psychrotolerant, and mesophilic fungi by extreme conditions. The most recent and relevant studies reporting on the relationship between cold stress and OS biomarkers and the damaging effects of ROS on key intracellular biomolecules are presented. The contribution of both enzymatic and non-enzymatic antioxidant defenses to the fungal cell response is discussed.