response. The living tissues are not invaded because
dermatophytes cannot grow at 37°C. Nevertheless,
the irritation caused by dermatophytes often leads to
scratching and further damage to the tissues, enabling
bacteria to invade. Scratching also leads to the shed-
ding of dermatophyte-infested skin and hair, so the
shared use of “scratching posts” by animals can lead
to transmission of the infection. Similarly, in human
populations communal facilities such as bathing areas,changing rooms, and domestic carpets can provide
a source of inoculum. This indirect transmission is
thought to be more important than direct host-to-host
transmission. Even the role of the conidial stages is
debatable, because these are not always found on
infected tissues while they are attached to the host, but
pigmented hyphae and vegetative arthrospores (thick-
walled spores formed by hyphal fragmentation) can be
found in these tissues.THE MOULDS OF MAN 325
Fig. 16.2(a) Spindle-shaped macroconidia, and microconidia, of Microsporumspp. (b) Enzyme-mediated penetration
of a hair by hyphae of Trichophyton mentagrophytes. (Reproduced by courtesy of the Canadian National Centre for Mycology;
http://www2.provlab.ab.ca/bugs/webbug/mycology/dermhome.htm)(a) (b)Fig. 16.3(a) Flattened fronds of hyphae of a dermatophyte, growing in planes of weakness within a stratified substrate
such as skin flakes. (b) Diagram of a perforating organ, similar to that in Fig. 16.2b.FB4eC16 04/20/2005 02:53PM Page 325