The dermatophytes are among the most frequently observed organisms in biomedicine, yet there has never been stability in the taxonomy, identification and naming of the approximately 25 pathogenic species involved. Since the identification of these species is often epidemiologically and ethically important, the difficulties in dermatophyte identification are a fruitful topic for modern molecular biological investigation, done in tandem with renewed investigation of phenotypic characters. Molecular phylogenetic analyses such as multilocus sequence typing have had to be tailored to accommodate differing the mechanisms of speciation that have produced the dermatophytes that are commonly seen today. Even so, some biotypes that were unambiguously considered species in the past, based on profound differences in morphology and pattern of infection, appear consistently not to be distinct species in modern molecular analyses. Most notable among these are the cosmopolitan bane of nails and feet, Trichophyton rubrum, and the endemic African agent of childhood tinea capitis, Trichophyton soudanense, which are effectively inseparable in all analyses. The molecular data require some reinterpretation of results seen in conventional phenotypic tests, but in most cases, phylogenetic insight is readily integrated with current laboratory testing procedures.

/pdf/the-new-species-concept-in-dermatophytes-a-polyphasic-5e6jdjg4t7.pdf

The New Species Concept in Dermatophytes—a Polyphasic Approach

Dermatophytes are keratinolytic fungi responsible for a large variety of diseases that can affect glabrous skin, nails and hair. In many cases, the diagnosis is not clinically obvious, and mycological analysis is required. This includes both direct microscopic examination and cultures. First of all, clinical specimens have to be sampled according to localization and characteristics of the lesions. Direct microscopic examination is usually performed using clearing reagents (KOH or Amman’s chloral-lactophenol), but its sensitivity may be greatly enhanced by the use of stains or fluorochromes such as Congo red or Calcofluor white. Histological analysis is an efficient method, but it is constraining for the patients and, as direct examination, it does not allow precise identification of the pathogen. Cultures are therefore needed, and specific culture media may be used to overcome the growth of rapidly growing contaminating moulds which may hamper the recovery of dermatophytes. Identification at the species level which may be useful to initiate an appropriate treatment or for setting prophylactic measures, relies on macroscopic and microscopic morphology. Subcultures on culture media which stimulate conidiation and, for some species, the production of pigments, are often necessary. Additionally, in case of atypical isolates, some biochemical or physiological tests may be performed such as the search for urease activity or the in vitro hair perforation test. However, their contribution to species identification is rather limited, and progress is still needed for the development of biochemical or immunological tests allowing an accurate identification at the species level, pending for the availability of molecular biology-based kits.

Conventional Methods for the Diagnosis of Dermatophytosis

Superficial fungal infections constitute some of the most common infectious conditions and include dermatophytosis (tinea corporis, tinea capitis, tinea pedis, and tinea unguium) and pityriasis versicolor, as well as rarer disorders like tinea nigra and black and white piedra. The etiologic agents of dermatophytosis are classified, along with some nonpathogenic relatives, in three genera: Trichophyton, Microsporum, and Epidermophyton. Dermatophytes are keratinophilic fungi that are capable of invading the keratinous tissues of living animals. They are grouped into three categories based on host preference and natural habitat: (i) anthropophilic species almost exclusively infect humans, with animals being rarely infected; (ii) geophilic species are soil-associated organisms that can occasionally cause infections in humans and other animals; and (iii) zoophilic species are essentially pathogens of nonhuman mammals, although animal-to-human transmission is not uncommon. This chapter describes the taxonomy, epidemiology, transmission, and clinical significance of dermatophytes. The methods of diagnosis of dermatophyte infections, including specimen collection, transport, and processing; the microscopic examination and culture of clinical specimens from suspected cases of dermatophytosis; and the conventional (phenotypic) and molecular methods of identification of the causative agents, are discussed in detail. Attention is also given to the clinical presentation, diagnosis, and identification of the causative agents for those superficial infections in which the causative fungi colonize the cornified layers of the epidermis or the suprafollicular portion of the hair (tinea versicolor, tinea nigra, black piedra, and white piedra).

Trichophyton, Microsporum, Epidermophyton, and agents of superficial mycoses.

Dermatophytes include fungal species that infect humans, as well as those that also infect other animals or only grow in the environment. The dermatophyte species Trichophyton rubrum is a frequent cause of skin infection in immunocompetent individuals. While members of the T. rubrum species complex have been further categorized based on various morphologies, their population structure and ability to undergo sexual reproduction are not well understood. In this study, we analyze a large set of T. rubrum and T. interdigitale isolates to examine mating types, evidence of mating, and genetic variation. We find that nearly all isolates of T. rubrum are of a single mating type, and that incubation with T. rubrum “morphotype” megninii isolates of the other mating type failed to induce sexual development. While the region around the mating type locus is characterized by a higher frequency of SNPs compared to other genomic regions, we find that the population is remarkably clonal, with highly conserved gene content, low levels of variation, and little evidence of recombination. These results support a model of recent transition to asexual growth when this species specialized to growth on human hosts.

https://www.genetics.org/content/genetics/208/4/1657.full.pdf

Whole-Genome Analysis Illustrates Global Clonal Population Structure of the Ubiquitous Dermatophyte Pathogen Trichophyton rubrum

Sexual reproduction is a rich source of genetic variation and commonly observed among fungi. Basically two different modes of sexual reproduction are observed in fungi, namely heterothallism where two compatible mating types are required to undergo mating and homothallism in which the organism is self-fertile. The genomic region governing the process of sexual reproduction and sex determination is called the mating type (MAT) locus. In filamentous ascomycetes including dermatophytes, the MAT locus harbors two different transcription factor genes in two different mating types. This review focuses on sexual reproduction and the structure of the MAT locus in dermatophytes. The reproductive modes and the observed mating types are summarized for different phylogenetic clades of dermatophytes. In addition, the question of whether or not unisexual reproduction, an interesting form of homothallism, may be the sexual reproduction mode especially in anthropophilic dermatophytes is raised.

Sexual Reproduction in Dermatophytes.

In addition to its requirement for histidine, Trichophyton megninii can be readily differentiated from certain other dermatophytes, particularly Trichophyton rubrum, by its "+" mating type and a positive urease test on urea-indole broth.

Contribution to our knowledge of Trichophyton megninii

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Contribution to our knowledge of Trichophyton megninii