Dermatophytes: Their taxonomy, ecology and pathogenicity

structures with typical arrangement on hyphae, chlamydospores, spirals, antler-shaped hyphae (chandeliers), nodular organs, pectinate organs and racquet hyphae [2,4]. In addition, some physiological characteristics based on nutritional requirements [4-8]such as vitamin deficiency can be used to identify some dermatophytes. Most dermatophyte colonies develop forms and pigmentation which can allow a presumptive identification of that dermatophyte species. The appearance of a fungus colony depends on the medium used, but for comparative purposes Sabouraud dextrose agar (SDA) medium is conventionally used to obtain colonies which can be compared to others reported in the literature [4]. Ajello [4] lists five important colony characteristics to look for in presumptive identification of a dermatophyte culture when it is one to three weeks old: (1) rate of growth (2) general topography (flat, heaped, regularly or irregularly folded) (3) texture (yeast-like, glabrous, powdery, granular, velvety or cottony) (4) surface pigmentation and (5) reverse pigmentation. Based on the above criteria, particularly on differences in conidial morphology, dermatophyte species can be classified into three genera within the Fungi Imperfecti (or Deuteromycotina) namely: Epidermophyton, Microsporum, and Trichophyton [2]. The studies of Cole and Samson [9] have shown that the ontogeny of the holothallic conidia of Microsporum and Trichophyton is essentially the same. Their only difference is the macroconidial cell-wall thickness and presence of echinulations in Microsporum species which are absent in Trichophyton species [10-11]. However, there has been some controversy in the broad classification of some dermatophytes. Benedek [12] felt that the genus Achorion should have been retained [2] as was Epidermophyton simply because of its “established usage” and that Emmon’s proposal cannot be considered a natural classification. The proposed system [13,14] distinguished the genera Epidermophyton, Microsporum, Trichophyton, Microides and Keratinomyces. Ajello [15] rejected Vanbreuseghem’s genus Microides based on the similarity in morphology of M. interdigitalis to T. mentagrophytes. He proposed that M. interdigitalis should be considered a variety of T. mentagrophytes, to be known as T. mentagrophytes var. interdigitale while Emmons [2] considered T. interdigitale to be a synonym of T. mentagrophytes. Corresponding address: Dr. Mukoma F. Simpanya Department of Biological Sciences, University of Botswana, P/Bag 0022, Gaborone, Botswana Fax: +267 355 2784; E-mail: [email protected]