Effect of reduced oxygen on the antifungal susceptibility of clinically relevant aspergilli

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

The filamentous fungus Aspergillus fumigatus is an ubiquitous mold that can cause invasive pulmonary infections in immunocompromised patients. Within the lung, A. fumigatus forms biofilms that can enhance resistance to antifungals and immune defenses, highlighting the importance of defining the mechanisms underlying biofilm development and associated emergent properties. A. fumigatus biofilms display a morphology and architecture that is distinct from bacterial and yeast biofilms. Moreover, A. fumigatus biofilms display unique characteristics in the composition of their extracellular matrix (ECM) and the regulatory networks governing biofilm formation. This review will discuss our current understanding of the form and function of A. fumigatus biofilms, including the unique components of ECM matrix, potential drug resistance mechanisms, the regulatory networks governing A. fumigatus biofilm formation, and potential therapeutics targeting these structures.

Novel Insights into Aspergillus fumigatus Pathogenesis and Host Response from State-of-the-Art Imaging of Host-Pathogen Interactions during Infection

Aspergillus fumigatus spores initiate more than 3,000,000 chronic and 300,000 invasive diseases annually, worldwide. Depending on the immune status of the host, inhalation of these spores can lead to a broad spectrum of disease, including invasive aspergillosis, which carries a 50% mortality rate overall; however, this mortality rate increases substantially if the infection is caused by azole-resistant strains or diagnosis is delayed or missed. Increasing resistance to existing antifungal treatments is becoming a major concern; for example, resistance to azoles (the first-line available oral drug against Aspergillus species) has risen by 40% since 2006. Despite high morbidity and mortality, the lack of an in-depth understanding of A. fumigatus pathogenesis and host response has hampered the development of novel therapeutic strategies for the clinical management of fungal infections. Recent advances in sample preparation, infection models and imaging techniques applied in vivo have addressed important gaps in fungal research, whilst questioning existing paradigms. This review highlights the successes and further potential of these recent technologies in understanding the host-pathogen interactions that lead to aspergillosis.

Synthesis, Biological Evaluation, and Structure-Activity Relationships of 4-Aminopiperidines as Novel Antifungal Agents Targeting Ergosterol Biosynthesis

The aliphatic heterocycles piperidine and morpholine are core structures of well-known antifungals such as fenpropidin and fenpropimorph, commonly used as agrofungicides, and the related morpholine amorolfine is approved for the treatment of dermal mycoses in humans. Inspired by these lead structures, we describe here the synthesis and biological evaluation of 4-aminopiperidines as a novel chemotype of antifungals with remarkable antifungal activity. A library of more than 30 4-aminopiperidines was synthesized, starting from N-substituted 4-piperidone derivatives by reductive amination with appropriate amines using sodium triacetoxyborohydride. Antifungal activity was determined on the model strain Yarrowia lipolytica, and some compounds showed interesting growth-inhibiting activity. These compounds were tested on 20 clinically relevant fungal isolates (Aspergillus spp., Candida spp., Mucormycetes) by standardized microbroth dilution assays. Two of the six compounds, 1-benzyl-N-dodecylpiperidin-4-amine and N-dodecyl-1-phenethylpiperidin-4-amine, were identified as promising candidates for further development based on their in vitro antifungal activity against Candida spp. and Aspergillus spp. Antifungal activity was determined for 18 Aspergillus spp. and 19 Candida spp., and their impact on ergosterol and cholesterol biosynthesis was determined. Toxicity was determined on HL-60, HUVEC, and MCF10A cells, and in the alternative in vivo model Galleria mellonella. Analysis of sterol patterns after incubation gave valuable insights into the putative molecular mechanism of action, indicating inhibition of the enzymes sterol C14-reductase and sterol C8-isomerase in fungal ergosterol biosynthesis.

Fungal biofilm architecture produces hypoxic microenvironments that drive antifungal resistance

Human fungal infections may fail to respond to contemporary antifungal therapies in vivo despite in vitro fungal isolate drug susceptibility. Such a discrepancy between in vitro antimicrobial susceptibility and in vivo treatment outcomes is partially explained by microbes adopting a drug-resistant biofilm mode of growth during infection. The filamentous fungal pathogen Aspergillus fumigatus forms biofilms in vivo, and during biofilm growth it has reduced susceptibility to all three classes of contemporary antifungal drugs. Specific features of filamentous fungal biofilms that drive antifungal drug resistance remain largely unknown. In this study, we applied a fluorescence microscopy approach coupled with transcriptional bioreporters to define spatial and temporal oxygen gradients and single-cell metabolic activity within A. fumigatus biofilms. Oxygen gradients inevitably arise during A. fumigatus biofilm maturation and are both critical for, and the result of, A. fumigatus late-stage biofilm architecture. We observe that these self-induced hypoxic microenvironments not only contribute to filamentous fungal biofilm maturation but also drive resistance to antifungal treatment. Decreasing oxygen levels toward the base of A. fumigatus biofilms increases antifungal drug resistance. Our results define a previously unknown mechanistic link between filamentous fungal biofilm physiology and contemporary antifungal drug resistance. Moreover, we demonstrate that drug resistance mediated by dynamic oxygen gradients, found in many bacterial biofilms, also extends to the fungal kingdom. The conservation of hypoxic drug-resistant niches in bacterial and fungal biofilms is thus a promising target for improving antimicrobial therapy efficacy.

Cryptic species of Aspergillus section Terrei display essential physiological features to cause infection and are similar in their virulence potential in Galleria mellonella

Aspergillus species account for the majority of invasive mold infections in immunocompromised patients. Most commonly, members of the Aspergillus section Fumigati are isolated from clinical material, followed by isolates belonging to section Terrei. The section Terrei contains 16 accepted species. Six species were found to be of clinical relevance and studied for differences in growth adaptability and virulence potential. Therefore, a set of 73 isolates (22 A. terreus s.s., 8 A. alabamensis, 27 A. citrinoterreus, 2 A. floccosus, 13 A. hortai, and 1 A. neoafricanus) was studied to determine differences in (a) germination kinetics, (b) temperature tolerance, (c) oxygen stress tolerance (1% O₂), and (d) a combination of the latter two. Virulence potential of phialidic (PC) and accessory conidia (AC) was studied in G. mellonella larvae, using survival as read out. Further, the formation of AC was evaluated in larval tissue. All isolates were able to grow at elevated temperature and hypoxia, with highest growth and germination rates at 37°C. A. terreus s.s., A. citrinoterreus, and A. hortai exhibited highest growth rates. Virulence potential in larvae was inoculum and temperature dependent. All species except A. floccosus formed AC and germination kinetics of AC was variable. Significantly higher virulence potential of AC was found for one A. hortai isolate. AC could be detected in larval tissue 96 h post infection. Based on these findings, cryptic species of section Terrei are well adapted to the host environment and have similar potential to cause infections.

Therapeutic Challenges of Non-Aspergillus Invasive Mold Infections in Immunosuppressed Patients

While Aspergillus spp. remain the major cause of invasive mold infections in hematologic cancer patients and transplant recipients, other opportunistic molds, such as Mucorales, Fusarium, and Scedosporium spp. are increasingly encountered in an expanding population of patients with severe and prolonged immunosuppression. High potential for tissue invasion and dissemination, resistance to multiple antifungals and high mortality rates are hallmarks of these non-Aspergillus invasive mold infections (NAIMIs). Assessment of drug efficacy is particularly difficult in the complex treatment scenarios of NAIMIs. Specifically, correlation between in vitro susceptibility and in vivo responses to antifungals is hard to assess, in view of the multiple, frequently interrelated factors influencing outcomes, such as pharmacokinetic/pharmacodynamic parameters determining drug availability at the site of infection, the net state of immune suppression, delay in diagnosis, or surgical debulking of infectious foci. Our current therapeutic approach of NAIMIs should evolve toward a better integration of the dynamic interactions between the pathogen, the drug and the host. Innovative concepts of experimental research may consist in manipulating the host immune system to induce a specific antifungal response or targeted drug delivery. In this review, we discuss the challenges in the management of NAIMIs and provide an update about the latest advances in diagnostic and therapeutic approaches.

Antifungal resistance in Aspergillus terreus: A current scenario

Invasive aspergillosis caused by intrinsically resistant non-fumigatus Aspergillus species displays a poor outcome in immunocompromised patients. The polyene antifungal amphotericin B (AmB) remains to be "gold standard" in the treatment of invasive fungal infections. Aspergillus terreus is innately resistant to AmB, in vivo and in vitro. Till now, the exact mode of action in polyene resistance is not well understood. This review highlights the underlying molecular basis of AmB resistance in A. terreus, displaying data obtained from AmB susceptible A. terreus and AmB resistant A. terreus strains. The effect of AmB on main cellular and molecular functions such as fungal respiration and stress response pathways will be discussed in detail and resistance mechanisms will be highlighted. The fungal stress response machinery seems to be a major player in the onset of AmB resistance in A. terreus.

Aspergillus terreus: Novel lessons learned on amphotericin B resistance

The polyene antifungal amphotericin B (AmB) exerts a powerful and broad activity against a vast array of fungi and in general displays a remarkably low rate of antimicrobial resistance. Aspergillus terreus holds an exceptional position among the Aspergilli due to its intrinsic AmB resistance, in vivo and in vitro. Until now, the underlying mechanisms of polyene resistance were not well understood. This review will highlight the molecular basis of A. terreus and AmB resistance recently gained and will display novel data on the mode of action of AmB. A main focus is set on fundamental stress response pathways covering the heat shock proteins (Hsp) 90/Hsp70 axis, as well as reactive oxygen species detoxifying enzymes in response to AmB. The effect on main cellular functions such as fungal respiration will be addressed in detail and resistance mechanisms will be highlighted. Based on these novel findings we will discuss new molecular targets for alternative options in the treatment of invasive infections due to A. terreus.

Treatment of Infections Due to Aspergillus terreus Species Complex

The Aspergillus terreus species complex is found in a wide variety of habitats, and the spectrum of diseases caused covers allergic bronchopulmonary aspergillosis, Aspergillus bronchitis and/or tracheobronchitis, and invasive and disseminated aspergillosis. Invasive infections are a significant cause of morbidity and mortality mainly in patients with hematological malignancy. The section Terrei covers a total of 16 accepted species of which most are amphotericin B resistant. Triazoles are the preferred agents for treatment and prevention of invasive aspergillosis. Poor prognosis in patients with invasive A. terreus infections seems to be independent of anti-Aspergillus azole-based treatment.

Regulation of Sterol Biosynthesis in the Human Fungal Pathogen Aspergillus fumigatus: Opportunities for Therapeutic Development

Sterols are a major component of eukaryotic cell membranes. For human fungal infections caused by the filamentous fungus Aspergillus fumigatus, antifungal drugs that target sterol biosynthesis and/or function remain the standard of care. Yet, an understanding of A. fumigatus sterol biosynthesis regulatory mechanisms remains an under developed therapeutic target. The critical role of sterol biosynthesis regulation and its interactions with clinically relevant azole drugs is highlighted by the basic helix loop helix (bHLH) class of transcription factors known as Sterol Regulatory Element Binding Proteins (SREBPs). SREBPs regulate transcription of key ergosterol biosynthesis genes in fungi including A. fumigatus. In addition, other emerging regulatory pathways and target genes involved in sterol biosynthesis and drug interactions provide additional opportunities including the unfolded protein response, iron responsive transcriptional networks, and chaperone proteins such as Hsp90. Thus, targeting molecular pathways critical for sterol biosynthesis regulation presents an opportunity to improve therapeutic options for the collection of diseases termed aspergillosis. This mini-review summarizes our current understanding of sterol biosynthesis regulation with a focus on mechanisms of transcriptional regulation by the SREBP family of transcription factors.

Galleria mellonella as a host model to study Aspergillus terreus virulence and amphotericin B resistance

The aim of this study was to investigate if the alternative in vivo model Galleria mellonella can be used (i) to determine differences in pathogenicity of amphotericin B (AMB) resistant and susceptible A. terreus isolates, (ii) to evaluate AMB efficacy in vivo (iii) and to correlate outcome to in vitro susceptibility data. Larvae were infected with 2 A. terreus AMB resistant (ATR) and 3 AMB susceptible (ATS) isolates and survival rates were correlated to physiological attributes and killing ability of larval haemocytes. Additionally, infected larvae were treated with different concentrations of L-AMB. Haemocyte density were ascertained to evaluate the influence of L-AMB on the larval immune cells. Larvae were sensitive to A. terreus infection in an inoculum-size and temperature dependent manner. In vitro susceptibility to L-AMB correlated with in vivo outcome of antifungal treatment, defining an AMB susceptible strain cluster of A. terreus. Susceptibility to L-AMB increased virulence potential in the larval model, but this increase was also in accordance with faster growth and less damage caused by larval haemocytes. L-AMB treatment primed the larval immune response by increasing haemocyte density. G. mellonella provides a convenient model for the in vivo screening of A. terreus virulence and treatment options, contributing to the generation of a hypothesis that can be further tested in refined experiments in mammalian models.

Blocking Hsp70 enhances the efficiency of amphotericin B treatment against resistant Aspergillus terreus strains

The polyene antifungal amphotericin B (AmB) is widely used to treat life-threatening fungal infections. Even though AmB resistance is exceptionally rare in fungi, most Aspergillus terreus isolates exhibit an intrinsic resistance against the drug in vivo and in vitro. Heat shock proteins perform a fundamental protective role against a multitude of stress responses, thereby maintaining protein homeostasis in the organism. In this study, we elucidated the role of heat shock protein 70 (Hsp70) family members and compared resistant and susceptible A. terreus clinical isolates. The upregulation of cytoplasmic Hsp70 members at the transcriptional as well as translational levels was significantly higher with AmB treatment than without AmB treatment, particularly in resistant A. terreus isolates, thereby indicating a role of Hsp70 proteins in the AmB response. We found that Hsp70 inhibitors considerably increased the susceptibility of resistant A. terreus isolates to AmB but exerted little impact on susceptible isolates. Also, in in vivo experiments, using the Galleria mellonella infection model, cotreatment of resistant A. terreus strains with AmB and the Hsp70 inhibitor pifithrin-μ resulted in significantly improved survival compared with that achieved with AmB alone. Our results point to an important mechanism of regulation of AmB resistance by Hsp70 family members in A. terreus and suggest novel drug targets for the treatment of infections caused by resistant fungal isolates.