Diagnosis of Fungi infection

1. Skin scrapings suspected to contain dermatophytes or pus from a lesion can be

mounted in KOH on a slide and examined directly under a microscope.

2. Skin testing (dermal hypersentivity) used to be popular as a diagnostic tool, but

this use is now discouraged because skin test may interfere with serological

studies by causing false positive results. It may still be used to evaluate the

patient’s immunity as well as a population exposure index in epidemiological

studies.

3. Serology may be helpful when it is applied to a specific fungal disease: there are

no screening antigens for “fungi” in general. Because fungi are poor antigen, the

efficacy of serology varies with different fungal infections. The serologic test will

be discussed under each mycosis. The most common serological tests for fungi

are based on latex agglutination, double immunodiffusion, complement fixation

and enzyme immunoassays. While latex agglutination may favor the detection of

IgM antibodies, double immunodiffusion and complement fixation usually detect

IgG antibodies. Some EIA tests are being developed to detect both IgG and IgM

antibodies. There are some tests which can detect specific fungal antigens, but

they are just coming into general use.

4. Direct fluorescent microscopy may be used for identification, even on non-viable

cultures or on fixed tissue secton. The reagents for this test are difficult to obtain.

5. Biopsy and histopathology. A biopsy may be very useful for the identification and

as a source of the tissue-invading fungi. Usually the Gomori methenamine silver

(GMS) stain is used to reveal the organism which stain black against a green

background. The H&E stain does not always tint the organism, but it will stain the

inflammatory cells.

6. Culture. A definite diagnosis requires a culture and identification. Pathogenic

fungi are usually grown on Saboouraud dextrose agar. It has a slightly acidic pH

(5,6), cyclohezamide, penicillin, streptomycin or other inhibitory antibiotic are

often added to prevent bacterial contamination and overgrowth. Two cultures are

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inoculated and incubated separately at 250C and 370C to reveal dimorphism. The

cultures are examined macroscopically and microscopically. They are not

considered negative for growth until ahter 4 weeks of incubation.

Reference:

Betsy, T and Keogh, J: (2005) Microbiology demystified. Published by the McGraw-Hill

Companies, New York, USA.