Development of arthrospores of Trichophyton mentagrophytes

Comprehensive Assessment of the Virulence Factors sub 3, sub 6 and mcpA in the Zoonotic Dermatophyte Trichophyton benhamiae Using FISH and qPCR

Skin infections by keratinophilic fungi are commonly referred to as dermatophytosis and represent a major health burden worldwide. Although patient numbers are on the rise, data on virulence factors, their function and kinetics are scarce. We employed an ex vivo infection model based on guinea pig skin explants (GPSE) for the zoonotic dermatophyte Trichophyton (T.) benhamiae to investigate kinetics of the virulence factors subtilisin (sub) 3, sub 6, metallocarboxypeptidase A (mcpA) and isocitrate lyase (isol) at gene level for ten days. Fluorescence in situ hybridization (FISH) and quantitative polymerase chain reaction (qPCR) were used to detect and quantify the transcripts, respectively. Kingdom-spanning, species-specific and virulence factor-specific probes were successfully applied to isolated fungal elements showing inhomogeneous fluorescence signals along hyphae. Staining results for inoculated GPSE remained inconsistent despite thorough optimization. qPCR revealed a significant increase of sub 3- and mcpA-transcripts toward the end of culture, sub 6 and isol remained at a low level throughout the entire culture period. Sub 3 is tightly connected to the de novo formation of conidia during culture. Since sub 6 is considered an in vivo disease marker. However, the presented findings urgently call for further research on the role of certain virulence factors during infection and disease.

Towards a Standardized Procedure for the Production of Infective Spores to Study the Pathogenesis of Dermatophytosis

Dermatophytoses are superficial infections of human and animal keratinized tissues caused by filamentous fungi named dermatophytes. Because of a high and increasing incidence, as well as the emergence of antifungal resistance, a better understanding of mechanisms involved in adhesion and invasion by dermatophytes is required for the further development of new therapeutic strategies. In the last years, several in vitro and in vivo models have emerged to study dermatophytosis pathogenesis. However, the procedures used for the growth of fungi are quite different, leading to a highly variable composition of inoculum for these models (microconidia, arthroconidia, hyphae), thus rendering difficult the global interpretation of observations. We hereby optimized growth conditions, including medium, temperature, atmosphere, and duration of culture, to improve the sporulation and viability and to favour the production of arthroconidia of several dermatophyte species, including Trichophyton rubrum and Trichophyton benhamiae. The resulting suspensions were then used as inoculum to infect reconstructed human epidermis in order to validate their ability to adhere to and to invade host tissues. By this way, this paper provides recommendations for dermatophytes culture and paves the way towards a standardized procedure for the production of infective spores usable in in vitro and in vivo experimental models.

Dimorphism in Neopseudocercosporella capsellae, an Emerging Pathogen Causing White Leaf Spot Disease of Brassicas

White leaf spot pathogen: Neopseudocercosporella capsellae causes significant damage to many economically important Brassicaceae crops, including oilseed rape through foliar, stem, and pod lesions under cool and wet conditions. A lack of information on critical aspects of the pathogen's life cycle limits the development of effective control measures. The presence of single-celled spores along with multi-celled conidia on cotyledons inoculated with multi-celled conidia suggested that the multi-celled conidia were able to form single-celled spores on the host surface. This study was designed to demonstrate N. capsellae morphological plasticity, which allows the shift between a yeast-like single-celled phase and the multi-celled hyphal phase. Separate experiments were designed to illustrate the pathogen's morphological transformation to single-celled yeast phase from multi-celled hyphae or multi-celled macroconidia in-vitro and in-planta. Results confirmed the ability of N. capsellae to switch between two morphologies (septate hyphae and single-celled yeast phase) on a range of artificial culture media (in-vitro) or in-planta on the host surface before infection occurs. The hyphae-to-yeast transformation occurred through the production of two morphologically distinguishable blastospore (blastoconidia) types (meso-blastospores and micro-blastospores), and arthrospores (arthroconidia).

Fungal morphogenesis

Morphogenesis in fungi is often induced by extracellular factors and executed by fungal genetic factors. Cell surface changes and alterations of the microenvironment often accompany morphogenetic changes in fungi. In this review, we will first discuss the general traits of yeast and hyphal morphotypes and how morphogenesis affects development and adaptation by fungi to their native niches, including host niches. Then we will focus on the molecular machinery responsible for the two most fundamental growth forms, yeast and hyphae. Last, we will describe how fungi incorporate exogenous environmental and host signals together with genetic factors to determine their morphotype and how morphogenesis, in turn, shapes the fungal microenvironment.

Matricellular protein Cfl1 regulates cell differentiation

Like higher eukaryotic cells in tissues, microbial cells in a community act in concert in response to environmental stimuli. They coordinate gene expression and their physiological and morphological states through intercellular communication mediated by matricellular signals. The adhesion protein Cfl1 was recently shown to be a matricellular signal in regulating morphogenesis and biofilm formation in the eukaryotic microbe Cryptococcus neoformans. Cfl1 is naturally highly expressed in the hyphal subpopulation during the mating colony development. Some Cfl1 proteins are cleaved and released to the ECM (extracellular matrix). The released exogenous Cfl1 activates Cryptococcus cells to express their endogenous Cfl1, to undergo filamentation, and to form structured biofilm colonies. In this study, we demonstrate that the N-terminal signal peptide and the novel conserved cysteine-rich SIGC domain at the C-terminus are critical for the adherence property and the signaling activity of this multifunctional protein. The investigation of this fungal matricellular signaling network involving Cfl1 and the master regulator of morphogenesis Znf2 provides a foundation to further elucidate intercellular communication in microbial development.

The link between morphotype transition and virulence in Cryptococcus neoformans

Cryptococcus neoformans is a ubiquitous human fungal pathogen. This pathogen can undergo morphotype transition between the yeast and the filamentous form and such morphological transition has been implicated in virulence for decades. Morphotype transition is typically observed during mating, which is governed by pheromone signaling. Paradoxically, components specific to the pheromone signaling pathways play no or minimal direct roles in virulence. Thus, the link between morphotype transition and virulence and the underlying molecular mechanism remain elusive. Here, we demonstrate that filamentation can occur independent of pheromone signaling and mating, and both mating-dependent and mating-independent morphotype transition require the transcription factor Znf2. High expression of Znf2 is necessary and sufficient to initiate and maintain sex-independent filamentous growth under host-relevant conditions in vitro and during infection. Importantly, ZNF2 overexpression abolishes fungal virulence in murine models of cryptococcosis. Thus, Znf2 bridges the sex-independent morphotype transition and fungal pathogenicity. The impacts of Znf2 on morphological switch and pathogenicity are at least partly mediated through its effects on cell adhesion property. Cfl1, a Znf2 downstream factor, regulates morphogenesis, cell adhesion, biofilm formation, and virulence. Cfl1 is the first adhesin discovered in the phylum Basidiomycota of the Kingdom Fungi. Together with previous findings in other eukaryotic pathogens, our findings support a convergent evolution of plasticity in morphology and its impact on cell adhesion as a critical adaptive trait for pathogenesis.

Although morphogenesis and virulence are commonly associated in many eukaryotic pathogens, the nature of such association is often unknown. For example, Cryptococcus neoformans, a fungal pathogen that causes cryptococcal meningitis, typically undergoes morphological transition between the yeast and the filamentous form during mating. However, molecules that are critical for mating do not directly impact fungal virulence. Thus, the nature of the long observed association between morphotype and virulence in this microbe remains elusive despite decades of effort. Here we demonstrate that constitutively activated pheromone signaling is insufficient to drive morphological transition under mating-suppressing conditions, including those relevant to host physiology. Rather, we demonstrate that sex-independent morphological switching is driven by the transcription factor Znf2 and this regulator controls the ability of this fungus to cause disease. Znf2 governs Cryptococcus morphotype and virulence potential at least partly through its effects on cell surface proteins. One novel adhesin Cfl1functions downstream of Znf2 and it orchestrates morphological switch, cell adhesion, biofilm formation, and pathogenicity. Thus, cell adhesion at least partly underlies the link between morphological transition and pathogenicity in C. neoformans. Our findings provide a platform for further elucidation of the impact of morphotype on virulence in this ubiquitous pathogen. The discovery of Cfl1 and other novel adhesins in Cryptococcus could lay a foundation for the development of vaccines or alternative therapies to combat the fatal diseases caused by this fungus.

Disseminated dermal Trichophyton rubrum infection - an expression of dermatophyte dimorphism?

We present a case of disseminated dermal infection caused by Trichophyton rubrum (T. rubrum). This rare variant of dermatophytosis has an atypical clinical and histopathological presentation and occurs exclusively in immunosuppressed patients. The large, broad, pleomorphic hyphae with scattered budding arthrospores in this variant of T. rubrum infection are unusual and may represent expression of dermatophyte dimorphism previously described in vitro.

Arthroconidia production in Trichophyton rubrum and a new ex vivo model of onychomycosis

The dermatophyte fungus Trichophyton rubrum often produces arthroconidia in vivo, and these cells are thought to be involved in pathogenesis, and, in shed skin scales, to act as a source of infection. The purpose of this study was (i) to examine the environmental and iatrogenic factors which affect arthroconidiation in T. rubrum in vitro, (ii) to look at arthroconidia formation in a large number of clinical isolates of T. rubrum and (iii) to develop a new model for the study of arthroconidia formation in nail tissue. Arthroconidia production was studied in T. rubrum grown on a number of media and under conditions of varying pH, temperature and CO(2) concentration. The effect of the presence of antifungals and steroids on arthroconidia formation was also examined. Nail powder from the healthy toenails of volunteers was used as a substrate for arthroconidial production. On Sabouraud dextrose agar in the presence of 10 % CO(2) plus air, arthroconidial formation occurred optimally at 37 degrees C and pH 7.5, and was maximal at 10 days. Most isolates of T. rubrum showed a similar level of arthroconidial production, and only two out of 50 strains were unable to produce arthroconidia. Subinhibitory levels of some antifungals and betamethasone resulted in the stimulation of arthroconidia formation. Arthroconidial production in ground nail material also occurred under the same optimal conditions, but took longer to reach maximal levels (14 days). These in vitro and ex vivo results provide a useful basis for the understanding of arthroconidium formation in vivo in infected tissues such as nails.

Characterization of the Mucor circinelloides life cycle by on-line image analysis

AIMS

The life cycle of the dimorphic fungus Mucor circinelloides was studied in a temperature-controlled flow-through cell, which constitutes an ideal tool when following the development of individual cells, with a view to understanding the growth and differentiation processes occurring in and between the different morphological forms of the organism.

METHODS AND RESULTS

Mycelial growth and the transformation of hyphae into chains of arthrospores were characterized by image analysis techniques and described quantitatively. The influence of the nature (glucose and xylose) and concentration of the carbon source on specific growth rate and hyphal growth unit length were studied. The organism branched more profusely on xylose than on glucose while the specific growth rates determined were rather similar. Methods were developed to study the yeast-like growth phase of M. circinelloides in the flow-through cell, and combined with fluorescent microscopy which allowed new insights to bud formation. Additionally, numbers and distribution of nuclei in arthrospores, hyphae and yeasts were studied.

CONCLUSIONS

The results give essential information on the morphological development of the organism.

SIGNIFICANCE AND IMPACT OF STUDY

Development of any industrial process utilizing this organism will be dependent on the information obtained here for effective process optimization.

Development of the primer set for the detection of the hsp60 gene in Trichophyton mentagrophytes cDNA

Three sequences of hsp60 from Saccharomyces cerevisiae, Schizosaccharomyces pombe and Histoplasma capsulatum were compared. Local multiple alignment of these sequences allowed the selection of two oligonucleotides suitable as primers for the polymerase chain reaction. This primer set was used for the amplification of a part of the hsp60 gene from cDNA of Trichophyton mentagrophytes and S. cerevisiae. Similar fragments detected in both PCR's imply the possible future use of the developed primer set for the detection of the hsp60 gene in other fungal species.

Detection of antigens in mycelial and in arthroconidial phases of Trichophyton mentagrophytes

Protein pattern changes were investigated in the filamentous fungus Trichophyton mentagrophytes during the morphological transition induced by increased temperature and higher CO2 partial pressure in cultivation atmosphere. The differences between the mycelial and the arthroconidial phase were characterized by SDS-PAGE and by immunodetection with mouse polyclonal antibodies. The components found by Western blotting in mycelia were 88, 86, 32, 29, 19.5, 18.5 kDa, in arthroconidia 108, 92, 88, 66, 56, 41, 39, 19.5 kDa. The results suggest the participation of some heat shock associated proteins of T. mentagrophytes in host immune response against mycotic infection.

UV susceptibility and negative phototropism of dermatophytes

High doses of ultraviolet radiation (UVR) have well-known inhibitory effects upon dermatophytes. In the present study, the effect of repetitive low doses of UVR on mycelial growth of dermatophytes was tested. Pellets of Trichophyton rubrum, T. mentagrophytes and Microsporum canis were placed between two thin layers of Sabouraud glucose agar. Obverse, reverse or both sides of these 'sandwich' agars were irradiated for 10 days twice daily with 0.13 or 0.17 J cm-2 UVB. To simulate microaerophilic conditions, one or both agar sides were covered by transparent airtight plastic lids. In addition, T. rubrum was also grown as usual on plates of Sabouraud glucose agar without any covering, and irradiated on its obverse side twice daily with UVA (13.5 J cm-2), UVB (0.17, 0.34 or 0.69 J cm-2) or infrared light, or once only with 3.8-15.1 J cm-2 UVB. As a result, thallus diameters of all strains were found to be reduced by repetitive UVB irradiation under both aerobic and microaerophilic growth conditions. T. rubrum was unaffected by infrared irradiation, responded with an increased pigmentation to UVA (13.5 J cm-2 twice daily) and was inhibited by a single dose of 15.1 J cm-2 UVB. Negative phototropism of dermatophytes is a new observation. It may be biologically relevant as a mechanism to evade harmful doses of UVR.

Models of cell differentiation in conidial fungi

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Formation and germination of fungal arthroconidia

A large variety of fungi are known to produce asexual spores known as arthroconidia. These propagules are formed by segmentation of existing hyphae and may form by several mechanisms. The specific processes of formation may lead to acropetal, basipetal, or random formation of endoarthroconidia or exoarthroconidia. The development of arthroconidia is a survival response to the depletion of nutrients or other environmental stresses and, in at least one case, is accompanied by the excretion of high levels of antibiotics. Arthroconidia are generally not as resistant to physical factors as are other fungal reproductive structures and are also susceptible to a variety of antibiotics and disinfectants. Arthroconidia are produced by some medically important fungi and in some cases, such as coccidioidomycosis, they are the principal means of dissemination. Germination of arthroconidia in complex media generally does not require specific "activation" events.

Formation and ultrastructure of Mucor rouxii arthrospores

The formation of arthrospores in Mucor rouxii was studied by transmission and scanning electron microscopy and light microscopy. The arthrospores formed in a random manner in terminal and internal regions of the hyphae. The earliest appearance of the arthrospores was seen by scanning electron microscopy as compartments delineated by double ridges. These ridges probably corresponded to the site of septal wall formation. The elongated compartments varied considerably in size. As the arthrospores matured, they tended to separate as a result of a gradual change in the shape of the arthrospores to a nearly spherical form and also as the result of simultaneous degradation of the outermost cell wall layer. The mature arthrospores were surrounded by a complex cell wall consisting of at least three distinct layers in addition to the original hyphal cell wall. Crystal-like structures were seen in the cytoplasm of some of the arthrospores in addition to the usual organelles such as mitochondria, nuclei, and ribosomes. Septum formation by centripetal cell wall growth from the lateral hyphal wall was documented by transmission electron microscopy. However, evidence was also found which suggested that not all internal cell wall development in the fungal hyphae during arthrosporogenesis necessarily led to the formation of mature arthrospores.

Effect of occlusion on Trichophyton mentagrophytes infections in guinea pigs

The effect of occlusion and griseofulvin on the duration and severity of acute dermatophytosis was assessed in guinea pigs. Sixty guinea pigs given standard Trichophyton mentagrophytes infections were divided into 5 groups: (A) control, (B) intermittent wet occlusion, (C) continuous-wet occlusion, (D) griseofulvin, (E) griseofulvin-continuous-wet occlusion. Lesions were largest in the control group, smallest in the griseofulvin-continuous-occlusion group, and of intermediate area in the other groups. The degree of inflammation and alopecia was not affected by occlusion, but was markedly reduced or eliminated with griseofulvin. Healing of the lesions occurred more rapidly in the griseofulvin-continuous-occlusion group than in any other group. This study suggests that either occlusion of the inoculated site or oral griseofulvin markedly reduces the expected area of the fungal lesion, and occlusion together with griseofulvin shortens the duration of the lesion.

Scanning electron microscopy of experimental Trichophyton mentagrophytes infections in guinea pig skin

Trichophyton mentagrophytes invasion of guinea pig skin was examined by scanning electron microscopy. Biopsies were obtained daily for 12 days from experimental infection sites. Dermatophyte invasion, examined in detail by scanning electron microscopy of cross-sectioned, prefixed skin was evidenced by: the appearance of hyphae within the stratum corneum; follicular invasion by hyphae, which remained initially within the follicle wall; emergence of the hyphae from the wall into the follicular canal; proliferation of the fungus down the follicle, with furrowing of the follicle wall and hair shaft cuticle; penetration of hyphae into the hair shaft by subcuticular and transcuticular routes; and massive peripilar hyphal proliferation with arthrosporogenesis. A three-dimensional perception of the invasion sequence of a dermatophyte in guinea pig skin was obtained by scanning electron microscopy.

Survival and resistance of Trichophyton mentagrophytes arthrospores

The effects of several physical and chemical agents on the survival of Trichophyton mentagrophytes arthrospores were investigated. Although arthrospores of this dermatophyte were highly resistant to chilling and freezing, they were extremely susceptible to moderate heat (above 50 degrees C) and desiccation. This high susceptibility could be significantly reduced when they were dried in the presence of exogenous proteins. These arthrospores were markedly susceptible to glutaraldehyde. They appeared to be significantly more resistant than their hyphal counterparts to common antimycotics such as clotrimazole, griseofulvin, miconazole nitrate, and nystatin. Clinical and epidemiological implications of these observations are discussed.