Culture, Fungus

A sterile hyphae result indicates the absence of asexually produced spores (conidia). Lack of conidia complicates classificaton of a mold. Thus, a report of sterile hyphae is consistent with presence of either a dermatophyte or a contaminant (some nondermatophytes can infect nails). If hyphae were not seen on direct examination of tissue, hair, or nails, then a contaminant is more likely. A report of sterile hyphae excludes Coccidioides immitis/posadasii, because arthroconidia are expressed in their mature cultures. Molecular testing could be considered for sterile hyphae encountered from culture of specimens from normally sterile sites such as blood or tissues.

Zygomycetes produce fragile, haphazardly-branching hyphae in tissue and these are easily disrupted during biopsy and specimen processing, producing sterile cultures. More than half of zygomycetes tissue cultures produce no growth.¹

To improve chances of growth, submit larger biopsies rather than smaller biopsies, and limit refrigeration during transport if at all possible. Tissue pieces need to be carefully minced (never ground) in the microbiology laboratory. The minced blocks should be gently placed on the surface of mycology plated media.

Dermatophytes are molds that are able to digest keratin from hair, skin, or nails. Living tissue is generally not involved and the host reaction to fungal products produces clinical symptoms. Major genera of dermatophytes are Microsporum, Trichophyton,and Epidermophyton.² Other yeasts and molds infect skin and nails in addition to dermatophytes.

No. Candida albicans is normally found in stool; thus, a stool culture for Candida has no medical significance. Stool cultures for fungus should be resisted. Fungal enteritis is rare and usually is only encountered in patients with severe systemic disease.

A patient may ask for this test because popular belief holds that systemic candidiasis is a cause of chronic fatigue; this, however, is devoid of medical fact.

The following fungi are considered emerging pathogens:

• Aspergillus terreus,the only Aspergillus species that can readily produce positive blood cultures owing to its ability to produce conidia within tissues. Aspergillus terreus is resistant to amphotericin B.
• Cladophialophora bantiana,a neurotropic mold found to infect patients without predisposing disease.
• Cryptococcus gattii, which produces disease identical to that of Cryptococcus neoformans but may infect patients without obvious immunocompromise.³