MICROBIOLOGY

• Dermatophytes: filamentous fungi that colonize and digest keratinized structures such as the stratum corneum of skin, hair and nails.
  
  • Cause of superficial infections at those sites with local inflammation and damage.
  • Deeper invasions of the dermis and subcutaneous tissues are rare.
• Dozens of species. T. rubrum is the most commonly isolated organism, but infection microbiology varies by geographic locale and exposure history.
  • Trichophyton (e.g. T. rubrum, T. interdigitale, T. mentagrophytes, T. tonsurans, T. verrucosum)
    • Important emerging dermatophytes
      • T. indotineae has emerged as an essential cause of skin infections in South Asia and has increasingly spread to other regions, including the United Kingdom and Germany. Infection can spread easily from person to person, and resistance to terbinafine is common.
      • T. mentagrophytes genotype VII (TMVII): Associated with sexual transmission among men who have sex with men and with sex tourism to SE Asia.
      • T. rubrum that is resistant to terbinfaine: Increasingly prelvalent
  • Microsporum (e.g. M. canis, M. persicolor)
  • Epidermophyton (E. floccosum).
  • Other species: Arthroderma, Ctenomyces, Guarromyces, Lophophyton, Nannizzia, Paraphyton.
• Certain species tend to infect specific structures:
  • Trichophyton--hair, nails and skin
  • Microsporum--hair and skin
  • Epidermophyton--skin and nails
• Dermatophytes can also be classified according to usual habitat: anthropophilic (humans, e.g., T. rubrum, T. tonsurans, E. floccosum), zoophilic (cats, dogs, e.g., M. canis, T. verrucosum and M. persicolor), geophilic (soil, e.g., M. gypseum).
• Acquisition of dermatophyte spores: can occur via direct contact with a human or animal carrier or indirectly from contaminated surfaces, including household items such as clothing, towels, bedding and combs.
  • Sexual transmission of Trichophyton species is increasingly recognized, leading to a condition termed tinea genitals, which includes infection at sites including the penis, scrotum, gluteal region, inguinal region and elsewhere.
• Terbinafine resistance in T. rubrum, T. interdigitale and especially T. indotineae, with associated clinical failure, is an emerging problem in multiple countries, most notably in India.
• Diagnostics (see more below)
  • Conventional methods such as culture, morphology, growth requirements, biochemical testing and direct microscopy are the mainstay of microbiological identification.
  • Advanced methods: nucleic acid amplification (e.g., PCR), MALDI-TOF-MS,