Impact of Resistance to Fluconazole on Virulence and Morphological Aspects of Cryptococcus neoformans and Cryptococcus gattii Isolates

Temporal trends in antifungal susceptibility of Cryptococcus neoformans isolates from a reference laboratory in the United States, 2011-2021

BACKGROUND

There are no established clinical breakpoints for antifungal agents against Cryptococcus species; however, epidemiological cut-off values can help distinguish wild-type (WT) isolates without any acquired resistance from non-WT strains, which may harbour resistance mechanisms.

PATIENTS/METHODS

We describe the trends of antifungal MICs and percentages of WT C. neoformans species complex (CNSC) isolates processed in our reference laboratory from November 2011 to June 2021. There were only nine isolates in 2011, thus, we included them in the year 2012 for data analysis. Clinical data is also described when available.

RESULTS

We identified 632 CNSC, the majority collected from blood (n = 301), cerebrospinal fluid (n = 230), and respiratory (n = 71) sources. The overall percentage of WT isolates for amphotericin B (AMB), 5-flucytosine, and fluconazole was 77%, 98%, and 91%, respectively. We noticed a statistically significant change in the percentage of AMB WT isolates over the years, with 98% of isolates being WT in 2012 compared to 79% in 2021 (p < .01). A similar change was not observed for other antifungal agents. Clinical data was available for 36 patients, primarily non-HIV immunocompromised patients with disseminated cryptococcosis. There were no statistically significant differences in the clinical characteristics and outcomes between patients with WT (58.3%) versus non-WT (41.7%) isolates, but we noticed higher mortality in patients infected with an AMB non-WT CNSC isolate.

CONCLUSIONS

We observed an increase in the percentage of AMB non-WT CNSC isolates in the past decade. The clinical implications of this finding warrant further evaluation in larger studies.

Advanced approach for antifungal susceptibility and characterization of resistance properties in clinical and environmental isolates of Cryptococcus species complex

BACKGROUND

Meningitis due to Cryptococcus neoformans/gattii is a fatal infection affecting immunocompromised population worldwide. Amphotericin B (AmB), fluconazole (FLC) and 5-flucytosine are the drugs of choice to treat the infection. We studied antifungal susceptibility pattern of clinical and environmental cryptococcal species using newer approach and analyze their resistant characteristics.

METHODS

Eighty clinical (54 C. neoformans and 26 C. gattii) and 18 environmental (14 C. neoformans and 4 C. gattii) isolates were subjected to antifungal susceptibility testing by automated (VITEK2C) method. Minimum inhibitory concentrations (MIC) were analyzed statistically. Genomic DNA of FLC resistant isolates was extracted and amplified to detect presence of CnAFR1 gene.

RESULTS

C. neoformans showed 1.85% and 21.4% AmB resistance, and 1.85% and 28.5% FLC- resistance, whereas C. gattii showed 25% and 50% FLC-resistance among clinical and environmental isolates respectively. MIC values were significantly (p < 0.05) different for the isolates from 2 sources. CnAFR1 gene sequence analysis revealed phylogenetic relationship among the resistant isolates.

CONCLUSIONS

This pioneering study provides an insight into the sensitivity patterns of clinical and environmental cryptococcal isolates from south India. The recent emergence of AmB-resistance may transpire as a challenge for the clinicians. As the clinical and environmental isolates are phylogenetically evolved from CnAFR1 gene of Filobasidiella neoformans, the resistance is most probably an inherent attribute. This study emphasizes the need for speciation and antifungal susceptibility testing of cryptococcal isolates from clinical sources to institute appropriate antifungal therapy and to reduce the mortality and morbidity.

Experimental and in-host evolution of triazole resistance in human pathogenic fungi

The leading fungal pathogens causing systemic infections in humans are Candida spp., Aspergillus fumigatus, and Cryptococcus neoformans. The major class of antifungals used to treat such infections are the triazoles, which target the cytochrome P450 lanosterol 14-α-demethylase, encoded by the ERG11 (yeasts)/cyp51A (molds) genes, catalyzing a key step in the ergosterol biosynthetic pathway. Triazole resistance in clinical fungi is a rising concern worldwide, causing increasing mortality in immunocompromised patients. This review describes the use of serial clinical isolates and in-vitro evolution toward understanding the mechanisms of triazole resistance. We outline, compare, and discuss how these approaches have helped identify the evolutionary pathways taken by pathogenic fungi to acquire triazole resistance. While they all share a core mechanism (mutation and overexpression of ERG11/cyp51A and efflux transporters), their timing and mechanism differs: Candida and Cryptococcus spp. exhibit resistance-conferring aneuploidies and copy number variants not seen in A. fumigatus. Candida spp. have a proclivity to develop resistance by undergoing mutations in transcription factors (TAC1, MRR1, PDR5) that increase the expression of efflux transporters. A. fumigatus is especially prone to accumulate resistance mutations in cyp51A early during the evolution of resistance. Recently, examination of serial clinical isolates and experimental lab-evolved triazole-resistant strains using modern omics and gene editing tools has begun to realize the full potential of these approaches. As a result, triazole-resistance mechanisms can now be analyzed at increasingly finer resolutions. This newfound knowledge will be instrumental in formulating new molecular approaches to fight the rapidly emerging epidemic of antifungal resistant fungi.

Cryptococcus spp. and Cryptococcosis: focusing on the infection in Brazil

Cryptococcosis is a global fungal infection caused by the Cryptococcus neoformans/Cryptococcus gattii yeast complex. This infection is acquired by inhalation of propagules such as basidiospores or dry yeast, initially causing lung infections with the possibility of progressing to the meninges. This infection mainly affects immunocompromised HIV and transplant patients; however, immunocompetent patients can also be affected. This review proposes to evaluate cryptococcosis focusing on studies of this mycosis in Brazilian territory; moreover, recent advances in the understanding of its virulence mechanism, animal models in research are also assessed. For this, literature review as realized in PubMed, Scielo, and Brazilian legislation. In Brazil, cryptococcosis has been identified as one of the most lethal fungal infections among HIV patients and C. neoformans VNI and C. gattii VGII are the most prevalent genotypes. Moreover, different clinical settings published in Brazil were described. As in other countries, cryptococcosis is difficult to treat due to a limited therapeutic arsenal, which is highly toxic and costly. The presence of a polysaccharide capsule, thermo-tolerance, production of melanin, biofilm formation, mechanisms for iron use, and morphological alterations is an important virulence mechanism of these yeasts. The introduction of cryptococcosis as a compulsory notification disease could improve data regarding incidence and help in the management of these infections.

In Vitro and In Vivo Effect of Peptides Derived from 14-3-3 Paracoccidioides spp. Protein

Background: Paracoccidioidomycosis (PCM) is a chronic disease that causes sequelae and requires prolonged treatment; therefore, new therapeutic approaches are necessary. In view of this, three peptides from Paracoccidioides brasiliensis 14-3-3 protein were selected based on its immunogenicity and therapeutic potential. Methods: The in vitro antifungal activity and cytotoxicity of the 14-3-3 peptides were evaluated. The influence of the peptides in immunological and survival aspects was evaluated in vivo, using Galleria mellonella and the expression of antimicrobial peptide genes in Caenorhabditis elegans. Results: None of the peptides were toxic to HaCaT (skin keratinocyte), MRC-5 (lung fibroblast), and A549 (pneumocyte) cell lines, and only P1 exhibited antifungal activity against Paracoccidioides spp. The peptides could induce an immune response in G. mellonella. Moreover, the peptides caused a delay in the death of Paracoccidioides spp. infected larvae. Regarding C. elegans, the three peptides were able to increase the expression of the antimicrobial peptides. These peptides had essential effects on different aspects of Paracoccidioides spp. infection showing potential for a therapeutic vaccine. Future studies using mammalian methods are necessary to validate our findings.

Minocycline and Fluconazole Have a Synergistic Effect Against Cryptococcus neoformans Both in vitro and in vivo

In recent decades, the incidence of Cryptococcus neoformans infection, which causes cryptococcosis, has consistently increased. Fluconazole (FLU) is frequently used in the treatment of this disease, mainly in the immunocompromised population, and long-term therapy usually produces drug resistance. Research on antifungal sensitizers has gained attention as a possible means of overcoming this drug resistance. Minocycline (MINO) has an inhibitory effect in vitro on FLU-resistant Candida albicans, and the combination of MINO and FLU has a synergistic effect on FLU-resistant C. albicans. A synergistic effect of MINO/FLU has been reported against C. neoformans, but this effect has not been evaluated on FLU-resistant isolates. This study aimed to investigate the interaction of MINO and FLU against FLU-resistant C. neoformans both in vitro and in vivo. We found that the combination of MINO and FLU had a synergistic effect on FLU-resistant C. neoformans in vitro. For all FLU-resistant strains, the minimum inhibitory concentration (MIC) of FLU decreased significantly when used in combination with MINO, dropping from >128 μg/ml down to 4–8 μg/ml. Additionally, MINO and FLU had a synergistic effect on both susceptible and resistant C. neoformans biofilms, in which the MIC of FLU decreased from >256 μg/ml down to 4–16 μg/ml. Compared with FLU alone, the combination of MINO with FLU prolonged the survival rate of Galleria mellonella larvae infected with FLU-resistant C. neoformans, and also significantly decreased the fungal burden of infected larvae and reduced the tissue damage and destruction caused by FLU-resistant C. neoformans. These findings will contribute to the discovery of antifungal agents and may yield a new approach for the treatment of cryptococcosis caused by FLU-resistant C. neoformans.

In vivo Activity of Copper(II), Manganese(II), and Silver(I) 1,10-Phenanthroline Chelates Against Candida haemulonii Using the Galleria mellonella Model

Candida haemulonii is an emerging opportunistic pathogen resistant to most antifungal drugs currently used in clinical arena. Metal complexes containing 1,10-phenanthroline (phen) chelating ligands have well-established anti-Candida activity against different medically relevant species. This study utilized larvae of Galleria mellonella, a widely used model of in vivo infection, to examine C. haemulonii infection characteristics in response to different copper(II), manganese(II), and silver(I) chelates containing phen, which had demonstrated potent anti-C. haemulonii activity in a previous study. The results showed that C. haemulonii virulence was influenced by inoculum size and incubation temperature, and the host G. mellonella immune response was triggered in an inoculum-dependent manner reflected by the number of circulating immune cells (hemocytes) and observance of larval melanization process. All test chelates were non-toxic to the host in concentrations up to 10 μg/larva. The complexes also affected the G. mellonella immune system, affecting the hemocyte number and the expression of genes encoding antifungal and immune-related peptides (e.g., inducible metalloproteinase inhibitor protein, transferrin, galiomycin, and gallerimycin). Except for [Ag₂(3,6,9-tdda)(phen)₄].EtOH (3,6,9-tddaH₂ = 3,6,9-trioxoundecanedioic acid), all chelates were capable of affecting the fungal burden of infected larvae and the virulence of C. haemulonii in a dose-dependent manner. This work shows that copper(II), manganese(II), and silver(I) chelates containing phen with anti-C. haemulonii activity are capable of (i) inhibiting fungal proliferation during in vivo infection, (ii) priming an immune response in the G. mellonella host and (iii) affecting C. haemulonii virulence.

Efficacy of voriconazole in vitro and in invertebrate model of cryptococcosis

Cryptococcosis is a common opportunistic infection in patients with advanced HIV infection and may also affect immunocompetent patients. The available antifungal agents are few and other options are needed for the cryptococcosis treatment. In this work, we first analyzed the virulence of twelve C. neoformans and C. gattii strains assessing capsule thickness, biofilms formation, and survival and morbidity in the invertebrate model of Galleria mellonella and then we evaluated the antifungal activity of voriconazole (VRC) in vitro and in vivo also using G. mellonella. Our results showed that all Cryptococcus spp. isolates were able to produce capsule and biofilms, and were virulent using G. mellonella model. The VRC has inhibitory activity on planktonic cells with MIC values ranging from 0.03 to 0.25 μg/mL on Cryptococcus spp.; and these isolates were more tolerant to fluconazole (ranging from 0.25 to 16 μg/mL), the triazol agent often recommended alone or in combination with amphotericin B in the cryptococcosis therapy. In contrast, mature biofilms were less susceptible to the VRC treatment. The VRC (10 or 20 mg/kg) treatment of infected G. mellonella larvae significantly increased the larval survival when compared to the untreated group for the both Cryptococcus species and significantly decreased the fungal burden and dissemination in the larval tissue. Our findings corroborate with the literature data, supporting the potential use of VRC as an alternative for cryptococcosis treatment. Here, we emphasize the use of G. mellonella larval model as an alternative animal model for studies of antifungal efficacy on mycosis, including cryptococcosis.

New antifungal 4-chloro-3-nitrophenyldifluoroiodomethyl sulfone reduces the Candida albicans pathogenicity in the Galleria mellonella model organism

Candida albicans represents an interesting microorganism to study complex host-pathogen interactions and for the development of effective antifungals. Our goal was to assess the efficacy of 4-chloro-3-nitrophenyldifluoroiodomethyl sulfone (named Sulfone) against the C. albicans infections in the Galleria mellonella host model. We assessed invasiveness of CAI4 parental strain and mutants: kex2Δ/KEX2 and kex2Δ/kex2Δ in G. mellonella treated with Sulfone. We determined that KEX2 expression was altered following Sulfone treatment in G. mellonella-C. albicans infection model. Infection with kex2Δ/kex2Δ induced decreased inflammation and minimal fault in fitness of larvae vs CAI4. Fifty percent of larvae died within 4–5 days (P value < 0.0001) when infected with CAI4 and kex2Δ/KEX2 at 10⁹ CFU/mL; survival reached 100% in those injected with kex2Δ/kex2Δ. Larvae treated with Sulfone at 0.01 mg/kg 30 min before infection with all C. albicans tested survived infection at 90–100% vs C. albicans infected-PBS-treated larvae. Hypersensitive to Sulfone, kex2Δ/kex2Δ reduced virulence in survival. KEX2 was down-regulated when larvae were treated with Sulfone: 30 min before and 2 h post-SC5314-wild-type infection respectively. kex2Δ/kex2Δ was able to infect larvae, but failed to kill host when treated with Sulfone. Sulfone can be used to prevent or treat candidiasis. G. mellonella facilitates studding of host-pathogen interactions, i.e., testing host vs panel of C. albicans mutants when antifungal is dosed.

Genotypic diversity and clinical outcome of cryptococcosis in renal transplant recipients in Brazil

Genotypic diversity and fluconazole susceptibility of 82 Cryptococcus neoformans and Cryptococcus gattii isolates from 60 renal transplant recipients in Brazil were characterized. Clinical characteristics of the patients and prognostic factors were analysed. Seventy-two (87.8%) isolates were C. neoformans and 10 (12.2%) were C. gattii. VNI was the most common molecular type (40 cases; 66.7%), followed by VNII (9 cases; 15%), VGII (6 cases; 10%), VNB (4 cases; 6.7%) and VNI/II (1 case; 1.7%). The isolates showed a high genetic diversity in the haplotype network and six new sequence types were described, most of them for VNB. There was a bias towards skin involvement in the non-VNI population (P = .012). VGII isolates exhibited higher fluconazole minimum inhibitory concentrations compared to C. neoformans isolates (P = 0.008). The 30-day mortality rate was 38.3%, and it was significantly associated with fungemia and absence of headache. Patients infected with VGII had a high mortality rate at 90 days (66.7%). A variety of molecular types produce disease in renal transplant recipients in Brazil and highlighted by VGII and VNB. We report the clinical appearance and impact of the molecular type, fluconazole susceptibility of the isolates, and clinical characteristics on patient outcome in this population.

Recent Advances in the Use of Galleria mellonella Model to Study Immune Responses against Human Pathogens

The use of invertebrates for in vivo studies in microbiology is well established in the scientific community. Larvae of Galleria mellonella are a widely used model for studying pathogenesis, the efficacy of new antimicrobial compounds, and immune responses. The immune system of G. mellonella larvae is structurally and functionally similar to the innate immune response of mammals, which makes this model suitable for such studies. In this review, cellular responses (hemocytes activity: phagocytosis, nodulation, and encapsulation) and humoral responses (reactions or soluble molecules released in the hemolymph as antimicrobial peptides, melanization, clotting, free radical production, and primary immunization) are discussed, highlighting the use of G. mellonella as a model of immune response to different human pathogenic microorganisms.

Discovering a new class of antifungal agents that selectively inhibits microbial carbonic anhydrases

Infections caused by pathogens resistant to the available antimicrobial treatments represent nowadays a threat to global public health. Recently, it has been demonstrated that carbonic anhydrases (CAs) are essential for the growth of many pathogens and their inhibition leads to growth defects. Principal drawbacks in using CA inhibitors (CAIs) as antimicrobial agents are the side effects due to the lack of selectivity toward human CA isoforms. Herein we report a new class of CAIs, which preferentially interacts with microbial CA active sites over the human ones. The mechanism of action of these inhibitors was investigated against an important fungal pathogen, Cryptococcus neoformans, revealing that they are also able to inhibit CA in microbial cells growing in vitro. At our best knowledge, this is the first report on newly designed synthetic compounds selectively targeting β-CAs and provides a proof of concept of microbial CAs suitability as an antimicrobial drug target.

The status of cryptococcosis in Latin America

Cryptococcosis is a life-threatening fungal infection caused by the encapsulated yeasts Cryptococcus neoformans and C. gattii, acquired from the environment. In Latin America, as occurring worldwide, C. neoformans causes more than 90% of the cases of cryptococcosis, affecting predominantly patients with HIV, while C. gattii generally affects otherwise healthy individuals. In this region, cryptococcal meningitis is the most common presentation, with amphotericin B and fluconazole being the antifungal drugs of choice. Avian droppings are the predominant environmental reservoir of C. neoformans, while C. gattii is associated with several arboreal species. Importantly, C. gattii has a high prevalence in Latin America and has been proposed to be the likely origin of some C. gattii populations in North America. Thus, in the recent years, significant progress has been made with the study of the basic biology and laboratory identification of cryptococcal strains, in understanding their ecology, population genetics, host-pathogen interactions, and the clinical epidemiology of this important mycosis in Latin America.

Automated Measurement of Cryptococcal Species Polysaccharide Capsule and Cell Body

The purpose of this technique is to provide a consistent, accurate, and manageable process for large numbers of polysaccharide capsule measurements. First, a threshold image is generated based on intensity values uniquely calculated for each image. Then, circles are detected based on contrast between the object and background using the well-established Circle Hough Transformation (CHT) algorithm. Finally, the detected cell capsules and bodies are matched according to center coordinates and radius size, and data is exported to the user in a manageable spreadsheet. The advantages of this technique are simple but significant. First, because these calculations are performed by an algorithm rather than a human both accuracy and reliability are increased. There is no decline in accuracy or reliability regardless of how many samples are analyzed. Second, this approach establishes a potential standard operating procedure for the Cryptococcus field instead of the current situation where capsule measurement varies by lab. Third, given that manual capsule measurements are slow and monotonous, automation allows rapid measurements on large numbers of yeast cells that in turn facilitates high throughput data analysis and increasingly powerful statistics. The major limitations of this technique come from how the algorithm functions. First, the algorithm will only generate circles. While Cryptococcus cells and their capsules take on a circular morphology, it would be difficult to apply this technique to non-circular object detection. Second, due to how circles are detected the CHT algorithm can detect enormous pseudo-circles based on the outer edges of several clustered circles. However, any misrepresented cell bodies caught within the pseudo-circle can be easily detected and removed from the resulting data sets. This technique is meant for measuring the circular polysaccharide capsules of Cryptococcus species based on India Ink bright field microscopy; though it could be applied to other contrast based circular object measurements.

Environmental Triazole Induces Cross-Resistance to Clinical Drugs and Affects Morphophysiology and Virulence of Cryptococcus gattii and C. neoformans

Cryptococcus gattii and Cryptococcus neoformans are environmental fungi that cause cryptococcosis, which is usually treated with amphotericin B and fluconazole. However, therapeutic failure is increasing because of the emergence of resistant strains. Because these species are constantly isolated from vegetal materials and the usage of agrochemicals is growing, we postulate that pesticides could be responsible for the altered susceptibility of these fungi to clinical drugs. Therefore, we evaluated the influence of the pesticide tebuconazole on the susceptibility to clinical drugs, morphophysiology, and virulence of C. gattii and C. neoformans strains. The results showed that tebuconazole exposure caused in vitro cross-resistance (CR) between the agrochemical and clinical azoles (fluconazole, itraconazole, and ravuconazole) but not with amphotericin B. In some strains, CR was observed even after the exposure ceased. Further, tebuconazole exposure changed the morphology, including formation of pseudohyphae in C. neoformans H99, and the surface charge of the cells. Although the virulence of both species previously exposed to tebuconazole was decreased in mice, the tebuconazole-exposed colonies recovered from the lungs were more resistant to azole drugs than the nonexposed cells. This in vivo CR was confirmed when fluconazole was not able to reduce the fungal burden in the lungs of mice. The tolerance to azoles could be due to increased expression of the ERG11 gene in both species and of efflux pump genes (AFR1 and MDR1) in C. neoformans Our study data support the idea that agrochemical usage can significantly affect human pathogens present in the environment by affecting their resistance to clinical drugs.

In Vitro Antifungal Susceptibility of Environmental Isolates of Cryptococcus spp. from the West Region of Cameroon

Background

Cryptococcus neoformans is responsible of cryptococcosis, a life-threatening infection that affects healthy and immunocompromised individuals. It is the first cause of adult acute meningitis in some sub-Saharan African countries with a mortality rate of about 100% in cases of inappropriate therapy. This study aimed at examining the occurrence and the antifungal patterns of Cryptococcus isolates from pigeon droppings and bat guanos in the west region of Cameroon.

Methods

A total of 350 samples were randomly collected from three selected localities of west region of Cameroon. The identification was performed based on capsule production assessed by Indian ink preparation. Additional tests performed were urea broth, glycine and tryptophan assimilation tests. The antifungal susceptibility test was performed by the broth microdilution method.

Results

Mycological analysis led to the identification of 98 isolates, of which 57 isolates of C. neoformans var. gattii and 41 isolates of C. neoformans var. neoformans. All the isolates showed resistance to antifungals tested except nystatin which showed MIC mean values ranging between 0.5 µg/mL and 0.65 µg/mL.

Conclusion

The prevalence of C. neoformans in pigeons and bats excreta in the west region of Cameroon is 28.57 %. C. neoformans var. gattii and C. neoformans var. neoformans are the main serotypes. Isolates found to be resistant to fluconazole and ketoconazole. Our results emphasize the need for further study on the molecular epidemiology in comparison with clinical isolates.