Filamentous fungal biofilms: Conserved and unique aspects of extracellular matrix composition, mechanisms of drug resistance and regulatory networks in Aspergillus fumigatus

Unveiling the menace: a thorough review of potential pandemic fungal disease

Fungal diseases have emerged as a significant global health threat, with the potential to cause widespread outbreaks and significant morbidity and mortality. Anticipating future pandemic fungal diseases is essential for effective preparedness and response strategies. This comprehensive literature review aims to provide a comprehensive analysis of the existing research on this topic. Through an extensive examination of scholarly articles, this review identifies potential fungal pathogens that have the potential to become pandemics in the future. It explores the factors contributing to the emergence and spread of these fungal diseases, including climate change, globalization, and antimicrobial resistance. The review also discusses the challenges in diagnosing and treating these diseases, including limited access to diagnostic tools and antifungal therapies. Furthermore, it examines the strategies and interventions that can be employed to mitigate the impact of future pandemic fungal diseases, such as improved surveillance systems, public health education, and research advancements. The findings of this literature review contribute to our understanding of the potential risks posed by fungal diseases and provide valuable insights for public health professionals and policymakers in effectively preparing for and responding to future pandemic outbreaks. Overall, this review emphasizes the importance of proactive measures and collaborative efforts to anticipate and mitigate the impact of future pandemic fungal diseases.

Biofilm inhibition/eradication: exploring strategies and confronting challenges in combatting biofilm

Biofilms are complex communities of microorganisms enclosed in a self-produced extracellular matrix, posing a significant threat to different sectors, including healthcare and industry. This review provides an overview of the challenges faced due to biofilm formation and different novel strategies that can combat biofilm formation. Bacteria inside the biofilm exhibit increased resistance against different antimicrobial agents, including conventional antibiotics, which can lead to severe problems in livestock and animals, including humans. In addition, biofilm formation also imposes heavy economic pressure on industries. Hence it becomes necessary to explore newer alternatives to eradicate biofilms effectively without applying selection pressure on the bacteria. Excessive usage of antibiotics may also lead to an increase in the number of resistant strains as bacteria employ an advanced antimicrobial resistance mechanism. This review provides insight into multifaceted technologies like quorum sensing inhibition, enzymes, antimicrobial peptides, bacteriophage, phytocompounds, and nanotechnology to neutralize biofilms without developing antimicrobial resistance (AMR). Furthermore, it will pave the way for developing newer therapeutic agents to deal with biofilms more efficiently.

Antifungal Resistance in Pulmonary Aspergillosis

Aspergilli may cause various pulmonary diseases in humans, including allergic bronchopulmonary aspergillosis (ABPA), chronic pulmonary aspergillosis (CPA), and acute invasive pulmonary aspergillosis (IPA). In addition, chronic colonization may occur in cystic fibrosis (CF). Aspergillus fumigatus represents the main pathogen, which may employ different morphotypes, for example, conidia, hyphal growth, and asexual sporulation, in the various Aspergillus diseases. These morphotypes determine the ease by which A. fumigatus can adapt to stress by antifungal drug exposure, usually resulting in one or more resistance mutations. Key factors that enable the emergence of resistance include genetic variation and selection. The ability to create genetic variation depends on the reproduction mode, including, sexual, parasexual, and asexual, and the population size. These reproduction cycles may take place in the host and/or in the environment, usually when specific conditions are present. Environmental resistance is commonly characterized by tandem repeat (TR)-mediated mutations, while in-host resistance selection results in single-resistance mutations. Reported cases from the literature indicate that environmental resistance mutations are almost exclusively present in patients with IA indicating that the risk for in-host resistance selection is very low. In aspergilloma, single-point mutations are the dominant resistance genotype, while in other chronic Aspergillus diseases, for example, ABPA, CPA, and CF, both TR-mediated and single-resistance mutations are reported. Insights into the pathogenesis of resistance selection in various Aspergillus diseases may help to improve diagnostic and therapeutic strategies.

Transcription factor CreA is involved in the inverse regulation of biofilm formation and asexual development through distinct pathways in Aspergillus fumigatus

The exopolysaccharide galactosaminogalactan (GAG) contributes to biofilm formation and virulence in the pathogenic fungus Aspergillus fumigatus. Increasing evidence indicates that GAG production is inversely linked with asexual development. However, the mechanisms underlying this regulatory relationship are unclear. In this study, we found that the dysfunction of CreA, a conserved transcription factor involved in carbon catabolite repression in many fungal species, causes abnormal asexual development (conidiation) under liquid-submerged culture conditions specifically in the presence of glucose. The loss of creA decreased GAG production independent of carbon sources. Furthermore, CreA contributed to asexual development and GAG production via distinct pathways. CreA promoted A. fumigatus GAG production by positively regulating GAG biosynthetic genes (uge3 and agd3). CreA suppressed asexual development in glucose liquid-submerged culture conditions via central conidiation genes (brlA, abaA, and wetA) and their upstream activators (flbC and flbD). Restoration of brlA expression to the wild-type level by flbC or flbD deletion abolished the abnormal submerged conidiation in the creA null mutant but did not restore GAG production. The C-terminal region of CreA was crucial for the suppression of asexual development, and the repressive domain contributed to GAG production. Overall, CreA is involved in GAG production and asexual development in an inverse manner.

Phase-Dependent Differential In Vitro and Ex Vivo Susceptibility of Aspergillus flavus and Fusarium keratoplasticum to Azole Antifungals

Fungal keratitis (FK) is an invasive infection of the cornea primarily associated with Aspergillus and Fusarium species. FK is treated empirically with a limited selection of topical antifungals with varying levels of success. Though clinical infections are typically characterized by a dense network of mature mycelium, traditional models used to test antifungal susceptibility of FK isolates exclusively evaluate susceptibility in fungal cultures derived from asexual spores known as conidia. The purpose of this study was to characterize differences in fungal response when topical antifungal treatment is initiated at progressive phases of fungal development. We compared the efficacy of voriconazole and luliconazole against in vitro cultures of A. flavus and F. keratoplasticum at 0, 24, and 48 h of fungal development. A porcine cadaver corneal model was used to compare antifungal efficacy of voriconazole and luliconazole in ex vivo tissue cultures of A. flavus and F. keratoplasticum at 0, 24, and 48 h of fungal development. Our results demonstrate phase-dependent susceptibility of both A. flavus and F. keratoplasticum to both azoles in vitro as well as ex vivo. We conclude that traditional antifungal susceptibility testing with conidial suspensions does not correlate with fungal susceptibility in cultures of a more advanced developmental phase. A revised method of antifungal susceptibility testing that evaluates hyphal susceptibility may better predict fungal response in the clinical setting where treatment is often delayed until days after the initial insult.

Deciphering the regulatory mechanisms of the cAMP/protein kinase A pathway and their roles in the pathogenicity of Candida auris

The emergence of multidrug-resistant fungal pathogens is a significant concern for global public health. Candida auris poses a considerable threat as a multidrug-resistant fungal pathogen. Our recent study revealed that the adenylyl cyclase Cyr1 and protein kinase A (PKA) pathways play distinct and redundant roles in drug resistance and pathogenicity of C. auris. However, the upstream and negative feedback regulatory mechanisms of C. auris are not yet fully understood. In this study, we discovered that the small GTPase Ras1, along with its nucleotide exchange factor Cdc25 and GTPase-activating protein Ira2, plays a major role in regulating cAMP/PKA-dependent traits, while G-protein-coupled receptor Gpr1 and heterotrimeric G-protein α subunit Gpa2 play a minor role. Pde2 plays a major role in negative feedback regulation of the cAMP/PKA pathway, while Pde1 plays a minor role. Hyperactivation of the Ras/cAMP/PKA pathway by deleting PDE2 or BCY1 renders C. auris cells thermosensitive and susceptible to nutrient deficiency, which leads to attenuated virulence. Our study demonstrates the distinct contributions of hyperactivation of the Ras/cAMP/PKA signaling pathway to C. auris pathogenesis and suggests potential therapeutic targets for C. auris-mediated candidiasis. IMPORTANCE Candida auris is a major concern as a multidrug-resistant fungal pathogen. While our previous studies highlighted the crucial roles of the cAMP/protein kinase A (PKA) pathway in regulating drug resistance, stress responses, morphogenesis, ploidy change, biofilm formation, and pathogenicity in this pathogen, their regulatory mechanism remains unclear. In our study, we provided evidence that the cAMP/PKA signaling pathway in C. auris is primarily governed by the small GTPase RAS rather than a G-protein-coupled receptor. Additionally, we discovered that the negative feedback regulation of cAMP, controlled by phosphodiesterases, is vital for C. auris virulence by promoting resistance to high temperatures and nutrient deficiencies. These findings underscore the diverse pathobiological significance of the Ras/cAMP/PKA signaling pathway in C. auris, shedding light on potential therapeutic targets and strategies for combating this multidrug-resistant fungal pathogen.

Differential Susceptibility of Mixed Polymicrobial Biofilms Involving Ocular Coccoid Bacteria (Staphylococcus aureus and S. epidermidis) and a Filamentous Fungus (Fusarium solani) on Ex Vivo Human Corneas

Biofilms confer several advantages to the organisms associated with them, such as increased resistances to antibacterial and antifungal compounds compared to free living cells. Compared to monomicrobial biofilms involving a single microorganism, biofilms composed of microorganisms affiliated to bacterial and fungal kingdoms are predominant in nature. Despite the predominance of polymicrobial biofilms, and more so mixed polymicrobial biofilms, they are rarely studied. The objective of the current study is to evaluate the potential of ocular bacteria and a filamentous fungus to form monomicrobial and mixed polymicrobial biofilms on synthetic and natural substrates and to monitor their response to antibiotics. In this sense, we demonstrated that the ocular pathogens Staphylococcus aureus, S. epidermidis, and Fusarium solani form monomicrobial and mixed polymicrobial biofilms both on tissue culture polystyrene plates and on ex vivo human corneas from cadavers using confocal microscopy and scanning electron microscopy. Additionally, the mixed polymicrobial biofilms involving the above ocular bacteria and a filamentous fungus were less susceptible to different antibacterials and antifungals in relation to the corresponding control planktonic cells. Further, the MICs to the screened antibacterials and antifungals in polymicrobial biofilms involving a bacterium or a fungus was either increased, decreased, or unchanged compared to the corresponding individual bacterial or fungal biofilm. The results would be useful to the ophthalmologist to plan effective treatment regimens for the eye since these are common pathogens of the eye causing keratitis, endophthalmitis, conjunctivitis, etc.

Benzyl isothiocyanate improves the prognosis of Aspergillus fumigatus keratitis by reducing fungal load and inhibiting Mincle signal pathway

Aspergillus fumigatus keratitis is a potential blinding disease associated with A. fumigatus invasion and excessive inflammatory response. Benzyl isothiocyanate (BITC) is a secondary metabolite with broad antibacterial and anti-inflammatory activity extracted from cruciferous species. However, the role of BITC in A. fumigatus keratitis has not been discovered yet. This study aims to explore the antifungal and anti-inflammatory effects and mechanisms of BITC in A. fumigatus keratitis. Our results provided evidences that BITC exerted antifungal effects against A. fumigatus by damaging cell membranes, mitochondria, adhesion, and biofilms in a concentration-dependent manner. In vivo, fungal load and inflammatory response including inflammatory cell infiltration and pro-inflammatory cytokine expression were reduced in BITC-treated A. fumigatus keratitis. Additionally, BITC significantly decreased Mincle, IL-1β, TNF-α, and IL-6 expression in RAW264.7 cells that stimulated by A. fumigatus or Mincle ligand trehalose-6,6-dibehenate. In summary, BITC possessed fungicidal activities and could improve the prognosis of A. fumigatus keratitis by reducing fungal load and inhibiting the inflammatory response mediated by Mincle.

Battle royale: Immune response on biofilms – host-pathogen interactions

The research interest of the scientific community in biofilm-forming microorganisms is growing due to the problems caused by their infections affecting humans and animals, mainly because of the difficulty of the host immune system in eradicating these microbial complex communities and the increasing antimicrobial resistance rates worldwide. This review describes the virulence factors and their interaction with the microbial communities of four well-known and highly biofilm-forming pathogens, more exactly, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus spp., and Candida spp. The innate and adaptive immune responses caused by the infection with these microorganisms and their evasion to the host immune system by biofilm formation are discussed in the present work. The relevance of the differences in the expression of certain virulence factors and the immune response in biofilm-associated infections when compared to planktonic infections is usually described as the biofilm architecture protects the pathogen and alters the host immune responses, here we extensively discussed these mechanisms.