Why Epidermophyton does not infect hair?

When I was a student, my mycology class, the class of dermatophytes used to be more interesting than other classes of mycology.

It’s not only because the dermatophytes are common in 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 of dermatophytes used to intrigued me.

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.

·         The genus Trichophyton is capable of invading the hair, skin and nails.

·         The genus Epidermophyton involves the skin and nails only.

·         The genus Microsporum involves only the hair and the skin.

But I used to forget the things and information used to get interchanged between species, landing me with the wrong answer. I tried to make mneomonics at that time, but could not interesting mneomonics also.

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.