Fungal Culture

Fungal cultures are used to evaluate for suspected fungal disease (eg, candidiasis).

Normal findings: No growth in 24 days ^[1]

A positive culture result must be interpreted with caution. Endogenous fungal flora and contaminants must be identified, particularly when the specimen is skin or its appendages.

Specimens are collected using a sterile technique and are transported in a sterile container within 2 hours. If the transportation is delayed, specimens should be stored at 4°C. ^[2]

Swabs are not recommended except for obtaining samples from a mucosal surface in the setting of suspected candidiasis. Sodium polyanethol sulfonate (SPS) is used to collect the samples because it is an anticoagulant that inhibits bacteria-destroying proteins and prevents phagocytosis.

When obtaining scalp specimens, multiple hairs should be plucked and the scalp should be scraped. Nail and skin lesions should be wiped off with 70% alcohol. Nail clippings or scrapings from beneath the nails or advancing margins should be collected in a clean container. ^[2]

A yellow-top container (SPS anticoagulants) is recommended for blood and bone marrow specimens. ^[3]

All samples must be sent in a sealed, leak-proof container marked with a biohazard sticker to comply with Occupational Safety and Health Administration (OSHA) safety standards. It is important to adhere to biosafety and regulatory considerations in handling and mailing medically important fungi in a culture collection. ^[4]

Description

Fungi are either saprophytic or parasitic in nature, as they lack chlorophyll and cannot produce their own carbohydrate. Of the 50,000 known species of fungi, only 100-150 species of yeast and molds cause disease in humans. Humans are generally resistant to fungal infections; nevertheless, inhalation of spores of some of the dimorphic fungi produces illness ranging from mild to severe disseminated disease. Immunocompromised hosts are susceptible to illness from many usually innocuous fungi. ^[5]

When clinically significant fungal infection is suspected, fungal cultures are usually obtained. Symptomatic fungal infections may be dermatophytes involving the skin and its appendages (nail; eg, tinea capitis), systemic mycosis (histoplasmosis), or opportunistic mycosis (eg, aspergillosis).

First, specimens should be directly examined, which may provide important information regarding identification of the pathogenic organism, obviating the need for culture, which is much more time consuming and expensive. Moreover, a direct examination result provides a rapid report to the clinician, which may allow early institution of therapy, since easily identifiable morphological characteristics of a fungus on a direct examination may establish a diagnosis. Approximately 90% of fungal infections are due to dermatophytes, which do not usually require fungal culture for diagnosis. ^[5]

Depending on the specimen and suspected pathogen, inoculation media may vary. Specimens for routine fungal culture should be first inoculated in nonselective media such as Sabouraud dextrose brain heart infusion (BHI) agar or plain BHI agar. A blood-containing media such as BHI blood agar improves the sensitivity or recovery of dimorphic fungi. Selective media such as inhibitory mold agar is usually used for contaminated specimens. Antibiotics are used to make the media selective. Nonselective media should always be inoculated with selective media, as some organisms may be inhibited by the antibiotics. ^[2]

Fungi that require special media include Cryptococcus neoformans (bird seed agar), Candida (chromogenic agar for its differentiation of isolates), dermatophytes (dermatophytes test medium), and Malassezia furfur (need long-chain fatty acid supplementation). ^[2]

Inoculated media should be incubated at 25-30°C in room air for up to 4 weeks. Systemic dimorphic fungi should be incubated at 35-37°C. Fastidious organisms should be incubated up to 8 weeks. ^[2] Candida species usually grow well in aerobic bacterial culture media; growth should be finalized after a week of incubation, as turnaround time for yeast is about 7 days. However, extra time may be needed for isolation and identification of isolates. ^[2]

Indication/Applications

To detect fungal bloodstream infection, fungal blood cultures are performed. Fungal blood cultures are indicated in patients with malignancies, those with trauma, those receiving broad-spectrum antibiotics, those with HIV infection or other immunocompromised conditions, or those who are in the critical condition (eg, sepsis, hypotension, multi-organ failure).

Other fungal cultures are obtained from the tissue involved by the suspected fungi (eg, skin scrapings or nail clippings in cases of dermatophytes infecting the skin and nails).

Considerations

Empiric treatment should be started, as results of the fungal culture may not be available when the decision is made to begin therapy. In some cases, unnecessary treatment may be instituted for endogenous candidal species or mold contaminants if growth is interpreted as clinically significant. It is a common pitfall, and clinicians should be wary of it.

Detection of antigens such as Histoplasma antigen in urine or cryptococcal antigen (India ink is less sensitive than cryptococcal antigen for meningitis) in blood or cerebrospinal fluid may be useful in determining a diagnosis. Clinical information such as animal exposure, area of residence, travel history, and immune status should be factored into the decision making in requesting fungal cultures. Histopathological and immunological testing may aid in the diagnosis of invasive fungal infections. ^[2] Gram stain and wet mount (direct microscopy) may help diagnose oral or vaginal candidiasis.

Aspergillus species are rarely isolated with blood culture, even in the face of systemic infection.