The molecular pathogenicity of Fusarium keratitis: a fungal transcriptional regulator promotes hyphal penetration of the cornea

In vitro and ex vivo models of microbial keratitis: Present and future

Microbial keratitis (MK) is an infection of the cornea, caused by bacteria, fungi, parasites, or viruses. MK leads to significant morbidity, being the fifth leading cause of blindness worldwide. There is an urgent requirement to better understand pathogenesis in order to develop novel diagnostic and therapeutic approaches to improve patient outcomes. Many in vitro, ex vivo and in vivo MK models have been developed and implemented to meet this aim. Here, we present current in vitro and ex vivo MK model systems, examining their varied design, outputs, reporting standards, and strengths and limitations. Major limitations include their relative simplicity and the perceived inability to study the immune response in these MK models, an aspect widely accepted to play a significant role in MK pathogenesis. Consequently, there remains a dependence on in vivo models to study this aspect of MK. However, looking to the future, we draw from the broader field of corneal disease modelling, which utilises, for example, three-dimensional co-culture models and dynamic environments observed in bioreactors and organ-on-a-chip scenarios. These remain unexplored in MK research, but incorporation of these approaches will offer further advances in the field of MK corneal modelling, in particular with the focus of incorporation of immune components which we anticipate will better recapitulate pathogenesis and yield novel findings, therefore contributing to the enhancement of MK outcomes.

Fusarium Keratitis From a Comprehensive Eye Health Care Facility in South India: Molecular Characterization by MALDI-TOF Versus PCR Sequencing, Species Complex Distribution, and Clinical Correlation

PURPOSE

Fusarium keratitis possesses significant diagnostic and therapeutic challenges. Medically relevant Fusaria belong to various species complexes and show prominent differences in their antifungal susceptibility profile which may influence the clinical outcome. Rapid diagnostic methods are warranted for precise identification of species complexes for prompt initiation of correct antifungals. The aim of the study was to compare between matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) and PCR sequencing for correct species-level identification and to analyze the clinical outcome among different Fusarium species complexes.

METHODS

Twenty-nine culture-proven Fusarium keratitis cases were included in this study. A phylogenetic tree was constructed after TEF1α gene sequencing and isolates were subjected to MALDI-TOF MS, followed by database expansion and identification. Clinical outcome and risk association among species complexes were analyzed retrospectively.

RESULTS

Maximum likelihood phylogeny categorized 68.9% isolates as Fusarium solani species complex (FSSC), 17.2% as Fusarium dimerum species complex (FDSC), followed by 13.7% as Fusarium fujikuroi species complex (FFSC). With extended database, MALDI-TOF MS could correctly speciate 96.5% (28/29) isolates. Previous antibiotic usage (P = 0.034) and preoperative antifungal treatment with natamycin, voriconazole, or ketoconazole (P = 0.025) were significantly higher in the FSSC group. The patients in the FFSC group had a significantly longer duration of symptoms at the time of clinical presentation to the clinic (15 days vs. 5 days, P = 0.030). Among 11 patients with a clinically poor outcome, 9 (31%) had FSSC infection.

CONCLUSIONS

Patients infected with the FSSC had more aggressive infection with poor prognosis. MALDI-TOF MS can serve as the best alternative method to conventional molecular identification with reduced turnaround time, which may help the ophthalmologists to consider the appropriate antifungals or early surgical intervention for improved outcome.

Development of topical chitosan/ β-glycerophosphate-based hydrogel loaded with levofloxacin in the treatment of keratitis: An ex-vivo study

Staphylococcus species are responsible for most cases of post-operative endophthalmitis. Topical ocular drug was applied for post-operative infection prevention, but the way of delivery encounters many challenges in terms of patient's compliance, drug efficacy, and drug penetration. We used the levofloxacin-loaded chitosan/gelatin/β-glycerophosphate hydrogel sustained releasing system with good in vitro anti-bacterial efficacy and biocompatibility, which we had previously designed, for ex vivo keratitis model to test the preclinical drug efficacy and to determine drug level in the anterior chamber of the eye. The result showed that the ex-vivo corneal keratitis model with S. aureus infection revealed mild opacity over the central cornea with stromal infiltrate, but without obvious stromal infiltration post levofloxacin-loaded hydrogel treatment after 24 h of infection. Quantification of viable bacteria showed a significant anti-bacterial activity. The histological evidence also showed no visible S. aureus after levofloxacin-loaded hydrogel treatment, with a significant anti-inflammatory effect. We also examined the drug concentration in the aqueous humor 24 h after instilling one drop of the levofloxacin-loaded hydrogel. The concentration achieved to a desired drug level. These results suggested that by the ex-vivo model, levofloxacin-loaded hydrogel can be applied for treatment in post-operative endophthalmitis or keratitis after the ophthalmic surgery.

Detection of singlet oxygen luminescence for experimental corneal rose bengal photodynamic antimicrobial therapy

Rose bengal photodynamic antimicrobial therapy (RB-PDAT) treats corneal infection by activating rose bengal (RB) with green light to produce singlet oxygen (¹O₂). Singlet oxygen dosimetry can help optimize treatment parameters. We present a ¹O₂ dosimeter for detection of ¹O₂ generated during experimental RB-PDAT. The system uses a 520 nm laser and an InGaAs photoreceiver with bandpass filters to detect ¹O₂ luminescence during irradiation. The system was validated in RB solutions and ex vivo in human donor eyes. The results demonstrate the feasibility of ¹O₂ dosimetry in an experimental model of RB-PDAT in the cornea.

Experimental Models for Fungal Keratitis: An Overview of Principles and Protocols

Fungal keratitis is a potentially blinding infection of the cornea that afflicts diverse patient populations worldwide. The development of better treatment options requires a more thorough understanding of both microbial and host determinants of pathology, and a spectrum of experimental models have been developed toward this end. In vivo (animal) models most accurately capture complex pathological outcomes, but protocols may be challenging to implement and vary widely across research groups. In vitro models allow for the molecular dissection of specific host cell–fungal interactions, but they do so without the appropriate environmental/structural context; ex vivo (corneal explant) models provide the benefits of intact corneal tissue, but they do not provide certain pathological features, such as inflammation. In this review, we endeavor to outline the key features of these experimental models as well as describe key technical variations that could impact study design and outcomes.

Efflux pump inhibition controls growth and enhances antifungal susceptibility of Fusarium solani species complex

Aim: To evaluate the inhibition of efflux pumps by using promethazine (PMZ) as a strategy to control Fusarium solani species complex (FSSC). Materials & methods: The susceptibility of FSSC strains to PMZ and the interaction between PMZ and antifungals were evaluated. The efflux pump activity was confirmed by flow cytometry with rhodamine 6G. Finally, PMZ was tested against FSSC biofilms. Results: PMZ inhibited FSSC planktonic growth and showed synergism with antifungals. PMZ reduced R6G efflux and inhibited cell adhesion, impaired the development of biofilms and disrupted mature biofilms. PMZ-challenged biofilms showed increased sensitivity to amphotericin B. Conclusion: The study provides indirect evidence of the occurrence of efflux pumps in FSSC and opens a perspective for this target in the control of fusariosis.

Fusarium: The versatile pathogen

Fusarium is an emerging human opportunistic pathogen of growing importance, especially among immunosuppressed haematology patients due to an increased incidence of disseminated infections over the past two decades. This trend is expected only to continue due to the advances in medical and surgical technologies that will prolong the lives of the severely ill, making these patients susceptible to rare opportunistic infections. Production of mycotoxins, enzymes such as proteases, angio-invasive property and an intrinsically resistant nature, makes this genus very difficult to treat. Fusarium is frequently isolated from the cornea and less commonly from nail, skin, blood, tissue, Continuous Ambulatory Peritoneal Dialysis (CAPD) fluid, urine and pleural fluid. Conventional microscopy establishes the genus, but accurate speciation requires multilocus sequence typing with housekeeping genes such as internal transcribed spacer, translation elongation factor-1α and RPB1 and 2 (largest and second largest subunits of RNA polymerase), for which expansive internet databases exist. Identifying pathogenic species is of epidemiological significance, and the treatment includes immune reconstitution by granulocyte-colony-stimulating factor, granulocyte macrophage-colony-stimulating factor and a combination of the most active species - specific antifungals, typically liposomal amphotericin-B and voriconazole. However, patient outcome is difficult to predict even with in vitro susceptibility with these drugs. Therefore, prevention methods and antifungal prophylaxis have to be taken seriously for these vulnerable patients by vigilant healthcare workers. The current available literature on PubMed and Google Scholar using search terms 'Fusarium', 'opportunistic invasive fungi' and 'invasive fusariosis' was summarised for this review.

Ex vivo Caprine Model to Study Virulence Factors in Keratitis

Purpose:

To develop an infectious keratitis model using caprine (goat) corneas and to investigate the expression of virulence factors during infection.

Methods:

Goat eyes were surface-sterilized and dissected, and the corneas were placed on an agarose-gelatin solid support (0.5% in phosphate-buffered saline) in a 12-well culture plate containing 10% fetal bovine serum-supplemented culture medium for 3 weeks. Cell viability tests (trypan blue and MTT) were performed on the cultured corneas. Corneas were infected with Pseudomonas aeruginosa and Fusarium solani separately. Infection progression was observed via histological analysis and hematoxylin and eosin (H-E) staining. For Pseudomonas-infected corneas, expression of eight virulence genes (exoS, exoT, exoY, alpR, prpL, lasA, lasB, and algD) was determined via quantitative real-time PCR (qRT-PCR) at 48-h and 72-h time-points. For Fusarium-infected corneas, expression of five proteases (C7Z0E6, C7ZFW9, C7Z7U2, C7ZNV5, and C7YY94) was quantified via qRT-PCR at 2, 4, and 8 days after infection. Protease from infected corneas was detected via gelatin zymography.

Results:

Goat corneas with a viable epithelium could be maintained for 15 days. Pseudomonas infection progressed rapidly, and complete corneal degradation was observed on day 4 after infection. Fusarium infection progressed more slowly. Histological analysis and H-E staining of Fusarium-infected cornea revealed mycelia penetrating all layers of the cornea. qRT-PCR revealed expression of all eight virulence factors, and statistically significant difference in expression of prpL and alpR in Pseudomonas-infected corneas. Expression of C7ZNV5 was highest in Fusarium-infected corneas.

Conclusion:

Goat corneas can be used to evaluate the expression of virulence factors involved in Pseudomonas and Fusarium infection.

Ex vivo rabbit and human corneas as models for bacterial and fungal keratitis

PURPOSE

In the study of microbial keratitis, in vivo animal models often require a large number of animals, and in vitro monolayer cell culture does not maintain the three-dimensional structure of the tissues or cell-to-cell communication of in vivo models. Here, we propose reproducible ex vivo models of single- and dual-infection keratitis as an alternative to in vivo and in vitro models.

METHODS

Excised rabbit and human corneoscleral rims maintained in organ culture were infected using 10⁸ cells of Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans or Fusarium solani. The infection was introduced by wounding with a scalpel and exposing corneas to the microbial suspension or by intrastromal injection. Post-inoculation, corneas were maintained for 24 and 48 h at 37 °C. After incubation, corneas were either homogenised to determine colony-forming units (CFU)/cornea or processed for histological examination using routine staining methods. Single- and mixed-species infections were compared.

RESULTS

We observed a significant increase in CFU after 48 h compared to 24 h with S. aureus and P. aeruginosa. However, no such increase was observed in corneas infected with C. albicans or F. solani. The injection method yielded an approximately two- to 100-fold increase (p < 0.05) in the majority of organisms from infected corneas. Histology of the scalpel-wounded and injection models indicated extensive infiltration of P. aeruginosa throughout the entire cornea, with less infiltration observed for S. aureus, C. albicans and F. solani. The models also supported dual infections.

CONCLUSIONS

Both scalpel wounding and injection methods are suitable for inducing infection of ex vivo rabbit and human cornea models. These simple and reproducible models will be useful as an alternative to in vitro and in vivo models for investigating the detection and treatment of microbial keratitis, particularly when this might be due to two infective organisms.

Fusarium keratitis: genotyping, in vitro susceptibility and clinical outcomes

PURPOSE

To determine differences in the clinical characteristics and antifungal susceptibility patterns among molecularly characterized ocular Fusarium sp isolates.

METHODS

Fifty-eight isolates of Fusarium sp obtained from 52 eyes of 52 patients were retrieved from the Ocular Microbiology Laboratory of the Bascom Palmer Eye Institute and grown in pure culture. These isolates were characterized based on DNA sequence analysis of the ITS1/2 and ribosomal deoxyribonucleic acid regions. Antifungal susceptibilities were determined for each isolate using broth microdilution methods, and the corresponding medical records were reviewed to determine the clinical outcomes.

RESULTS

Fusarium solani isolates had significantly higher values of minimum inhibitory concentration for 90% isolates (MIC90) with voriconazole than F. non-solani organisms (16 and 4 μg/mL, respectively). Isolates of F. solani also exhibited a significantly longer time to cure (65 vs. 40.5 days), a worse follow-up best-corrected visual acuity (20/118 vs. 20/36), and an increased need for urgent surgical management (7 vs. 0 penetrating keratoplasties) when compared with those of F. non-solani.

CONCLUSIONS

This is the first report to examine the correlation between ocular genotyped Fusarium sp and clinical outcomes. It supports the overall worse prognosis of F. solani versus F. non-solani isolates, including higher voriconazole resistance by the former. The clinical implementation of molecular-based diagnostics and antifungal efficacy testing may yield important prognostic and therapeutic information that could improve the management of fungal ocular infections.

Adaptation to pH and role of PacC in the rice blast fungus Magnaporthe oryzae

Fungi are known to adapt to pH partly via specific activation of the Pal signaling pathway and subsequent gene regulation through the transcription factor PacC. The role of PacC in pathogenic fungi has been explored in few species, and each time its partaking in virulence has been found. We studied the impact of pH and the role of PacC in the biology of the rice pathogen Magnaporthe oryzae. Conidia formation and germination were affected by pH whereas fungal growth and appressorium formation were not. Growth in vitro and in planta was characterized by alkalinization and ammonia accumulation in the surrounding medium. Expression of the MoPACC gene increased when the fungus was placed under alkaline conditions. Except for MoPALF, expression of the MoPAL genes encoding the pH-signaling components was not influenced by pH. Deletion of PACC caused a progressive loss in growth rate from pH 5 to pH 8, a loss in conidia production at pH 8 in vitro, a loss in regulation of the MoPALF gene, a decreased production of secreted lytic enzymes and a partial loss in virulence towards barley and rice. PacC therefore plays a significant role in M. oryzae’s biology, and pH is revealed as one component at work during interaction between the fungus and its host plants.