Penicillium species are usually regarded as unimportant in terms of causing human disease. Penicillium marneffei, discovered in 1956, is different. This is the only known thermally dimorphic species of Penicillium, and it can cause a lethal systemic infection (penicilliosis) with fever and anaemia similar to disseminated cryptococcosis.
The fact that Penicillium marneffei is thermally dimorphic, is a relevant clue when trying to identify it. However, it should be kept in mind that other human-pathogenic fungi are thermally dimorphic as well. Cultures should be done from bone marrow, skin, blood and sputum samples.
Plating samples out onto two Sabouraud agar plates, then incubating one at 30°C and the other at 37°C, should result in two different morphologies. A mold-form will grow at 30°C, and a yeast-form at 37°C.
Mycelial colonies will be visible on the 30°C plate after two days. Growth is initially fluffy and white and eventually turns green and granular after sporulation has occurred. A soluble red pigment is produced, which diffuses into the agar, causing the reverse side of the plate to appear red or pink. The periphery of the mold may appear orange-coloured, and radial sulcate folds will develop.
Under the microscope, the mold phase will look like a typical Penicillium, with hyaline, septate and branched hyphae; the conidiophores are located both laterally and terminally. Each conidiophore gives rise to three to five phialides, where chains of lemon-shaped conidia are formed.
On the 37°C plate, the colonies grow as yeasts. These colonies can be cerebriform, convoluted, or smooth. There is a decreased production in pigment, the colonies appearing cream/light-tan/light-pink in colour. Microscopically, sausage-shaped cells are mixed with hyphae-like structures. As the culture ages, segments begin to form. The cells divide by binary fission, rather than budding. The cells are not yeast cells, but rather arthroconidia. Culturing isn't the only method of diagnosis. A skin scraping can be prepared, and stained with Wright's stain. Many intracellular and extracellular yeast cells with crosswalls are suggestive of P. marneffei infection. Smears from bone marrow aspirates may also be taken; this is regarded as the most sensitive method. These samples can be stained with the Giemsa stain. Histological examination can also be done on skin, bone marrow or lymph nodes.
The patient's history also is a diagnostic help. If they have traveled to Southeast Asia and are HIV-positive, then there is an increased risk of them having penicilliosis.
Antigen testing of urine and serum, and PCR amplification of specific nucleotide sequences have been tried, with high sensitivity and specificity. Rapid identification of penicilliosis is sought, as rapid treatment is critical. Treatment should be provided as soon as penicilliosis is suspected.
Abstract on P. marneffei epidemiology and diagnosis