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Why Epidermophyton does not infect hair?

During college life, the  lecture of  dermatophytes used to be more interesting than other classes of mycology.
It’s not only because the dermatophytes are most common problems affecting all people (and of course students: whom we used to tease with the name of fungal disease eg. MS Dandruff, Mr. Tinea capitis), but also the nature of infection caused by dermatophytes used to intrigued me.
Conidia of dermatophytes
Do you know what dermatophytes are?
Dermatophytes are keratinophilic and keratinolytic fungi. They can affect all keratinized areas of the body (hair, skin and nails)
I used to rote learn the facts about Trichophyton, Epidermophyton and Microsporum because these facts were mostly important during the viva sessions
  1. The genus Trichophyton is capable of invading the hair, skin and nails (All three so tri).
  2. The genus Epidermophyton involves the skin and nails only (Not Hair).
  3. The genus Microsporum involves only the hair and the skin (Not Nail).
But  remembering above mention facts was daunting and i used to come with wrong answers.  I tried to make mneomonics at that time, but could not make interesting mneomonics.

Mneomonics to remember which dermatophyte does not affect what:

  1. Trichophyton: Infects all three (tri)- i.e. Skin, Nail and Hair
  2. Epidermophyton does not affect Hair (REMEMBER “E” is close to ‘H’ alphabetically), Similarly
  3. Microsporum does not affect Nails as ‘M’ is close to ‘N’
Two questions used to strike my mind always, but at that time I never got satisfactory answers about them. These were;
1.       Why Microsporum does not infect nails?
2.       Why Epidermophyton does not infect hair?
And today I am a lecturer but I have not yet got satisfactory answers for the above mentioned questions. I am well versed with all the species, nature of infections causing by these species, but I am still searching the best explanation.