Moulds (filamentous fungi)

Dermatophytes: These are agents causing cutaneous mycoses. That is, they infect the Keratanised layers of the skin and its integuments – and usually invoke a cellular immune response, causing pathological changes to occur in the deeper layers. There are three genres (need to learn):

• Microsporum spp, Trichophyton spp, Epidermophyton spp.

Clinically, these mycoses manifest as ringworm (note: it is not a worm) or tinea. The lesions are “snake like” or “ringlike” (resembles worm ==> hence name ==> “ringworm”). The source of these infections are from Humans (anthropophilic ==> mild chronic infections ==> difficult to treat), Animals (zoophilic ==> aggressive infection over milder period of time ==> spontaneous healing, respond well to therapy), Soil (geophilic).

Pathogenicity

These infections are usually confined to outer skin layers (keratinised layers). The reason for this is believed to do with keratin being used as a substrate (keratinophilic fungi). Keratinases have been isolated from these fungi, which indicates this. The skin lesion is symmetrical, it has an active edge (spreading edge), and a healing center. The hair in these regions is lost, nails discolour and crumble.

Tinea can be described according to their various anatomical landmarks (i.e.: Tinea capitis: head, Tinea pedis: feet etc). The species is not important to be learnt, but you should realise that some species are exclusively found in only some parts of the world.

Diagnosis

Woods lamp – fluorescent lamp, is used for some species of dermotophyes. In others, scrapings are taken from lesions: hair, nails – treated with an alkali solution (20% KOH) to get rid of the epithelial cells and other debris. Dermatophytes resist this solution, and appear branching. Closer examination reveals spores (round in appearance – Fig 69-12 pp 572 Murrary 3rd Ed) that are within the hair shaft (endothrix) or outside the hair shaft (ectothrix). Microscopic examination of scrapings only confirms the infection as being  fungal. To confirm the type of fungal infection, we need to culture the specimen. Skin contains many bacteria and saprobic fungi as part of its normal flora. So Sabournauds media is not routinely used, as these elements overgrow the actual dermotophytes (slow growing). Other media such as DIM have special antibiotics to inhibit bacterial and fungal growth, allowing dermatophytes to growing within hindrance. Intubation at 25º C normally (2-6 weeks) – treat patient while waiting.

Treatment of dermatophyte infections

Griseofulvin ==>orally administered, only used for dermatophyte infections, interacts with tubulin of susceptible fungi, alters tubulin function (affects cell wall). One problem is that griseofulvin is taken in by other cells also exhibiting tubulin, therefore resistance is due to lack of uptake by susceptible fungi.

Others include: allylamines (terbinafine, naftifine) are topical agents, very active against in vitro dermatophytes (also works for dimorphic fungi ==> expensive drug), affect sterols in cell membrane.

Category: Microbiology Notes