A Microbial Biorealm page on the genus Penicillium marneffei
Higher order taxa
Description and significance
Penicillium marneffei is a unicellular organism with round to oval celled fungus, 4 to 8 micrometers in diameter [1, 4]. These cells can actually divide by cross wall formation within macrophages, or form extracellular elongated cells. The special feature of Penicillium marneffei compared to other penicillia is its thermal dimorphism. This capability to grow as a mycelium at 25 degrees C and as a yeast at 37 degrees C is the organism’s main virulence factor. 
The history and mode of transmission of the organism remains unclear. The natural habitat of the fungus and its exact route of transmission have not been described . Soil exposure, often during the rainy season, has been suggested to be a serious risk factor. Also, many isolates from bamboo rats and humans were shown to share identical multi-locus genotypes. These data show that either transmission of P. marneffei can occur from rodents to humans or rodents and humans are coinfected from common environmental sources. The main route of infection is thought to be through inhalation of conidia into the lungs, where it can then disseminate via a hematogenous route to other body locations, especially the liver. 
Penicillium marneffei is a rising pathogenic fungus that can cause a fatal systemic mycosis in patients infected with human immunodeficiency virus (HIV). P. marneffei infection is endemic in tropical Asia, especially Thailand, northeastern India, China, Hong Kong, Vietnam, and Taiwan. It was first isolated from the hepatic lesions of a bamboo rat (Rhizomys sinensis) that had been maintained in captivity for experimental infections at the Pasteur Institute of Indochina, Dalat, South Vietnam, in 1956. The fungus was named Penicillium marneffei, in honor of Hubert Merneffe, who was the director of the Pasteur Institute of Indochina . After discovery of the infection in wild rodents, two cases of human infections were reported. The first case was laboratory acquired. The second, the first known natural human infection, was reported in 1973 . The mortality rate of patients with P. marneffei infection has been extremely high. 
Penicillium marneffei is believed to be haploid, with at least six large chromosomes . The genome size of P. marneffei has been estimated to be in the range of 17.8 to 26.2 Mb . The completed mitochondrial genome shows that it is circular, contains 35,438 nucleotides, and has a GC content of 24%. This 35 kb mitochondrial genome contains the genes encoding ATP synthase subunits, cytochrome oxidase subunits, apocytochrome b, reduced nicotinamide adenine dinucleotide ubiquinone oxireductase subunits, ribosomal protein of the small ribosomal subunit, 28 tRNAs, and large ribosomal RNAs . The mitochondrial genome of P. marneffei is more closely related to those of molds than to those of yeasts .
Cell structure and metabolism
Penicillium marneffei is a round-shaped fungus. The distinctive central transverse septum, by the process of fission, is special to P. marneffei. Also, budding and cysts forms are absent in P. marneffei infection .
Fungal cell types can be categorized into one of two groups; those that show polarized apical growth and those that exhibit isotrophic growth. A dimorphic fungi is able to switch between these two types. Dimorphic switching requires fungal cells to undergo changes in polarized growth in response to environmental stimuli and during cell differentiation . Penicillium marneffei, an opportunistic human fungal, is the only known Penicillium species that shows temperature-dependent dimorphic growth .
When the temperature is below 25 degrees Celsius, the fungus growth is as multinucleate mycelia with the formation of septate, and branched hyphae. They are also capable of undergoing asexual reproduction, producing uninucleate spores on complex multicullular structures called conidiophores. The 25-37 degrees Celsius temperature shift activates the arthroconidiation programme, where cell and nuclear division become coupled, double septa are deposited between hyphal cells and cell separation occurs along the septal plane to free singal yeast cells. When the temperature is at 37 degrees Celsius on aritificial medium or in human tissue, the fungus grows in a yeast-like form with the formation of fission arthroconidium cells, where the cell divides by fission to make two uninucleate cells. The germinated conidia produce hyphae that are usually shorter in length and wider in diameter than those produced at 25 degrees Celsius. Also, at 37 degrees Celsius, the hyphae are more highly branched than at 25 degrees Celsius. The fission yeast cells signify the parasitic form of P. marneffei. This mold to yeast change or phase transition is a diagnostic characteristic of P. marneffei. On the contrary, when switching yeast from 37 degrees Celsius to 25 degrees Celsius, yeast cells elongate, cell and nuclear division uncouple to form multinucleate compartments and grow with a filamentous pattern [1, 2].
The basic ecology of this pathogen remains enigmatic. The main issue is whether the human disease, penicilliosis marneffei, occurs as a consequence of animal or environmental transmission. In other words, the ecological reservoir(s) of human peniciliiosis marneffei remains unknown .
Penicillium marneffei seems to be a primary pulmonary pathogen that disseminates to other internal organs by hematogenous spread . It usually infects immunocomprised individuals and may cause fatal penicilliosis marneffei, a prevalent systematic mycosis of AIDS patients in Southeast Asia. Its pathogenicity seems to be intimately related to the dimorphic transition from a mycelial phase in the environment to a yeast phase in the human host .
The capability of a fungus to cause infection is simply an accidental encounter in its life cycle, but the fungus needs to survive and replicate in the host to cause disease . During infection, P. marneffei yeast cells exist intracellularly in macrophages. To cope with nutrient deprivation during the infection process, a number of pathogens employ the glyoxylate cycle to utilize fatty acids as carbon sources . Infection by P. marneffei appears to begin following phagocytosis of inhaled conidia by host alveolar macrophages. As a facultative intracellular pathogen, P. marneffei survives and replicates as a yeast inside the phagosome. Within the phagosome, P. marneffei must protect itself from the host defense mechanisms, such as the ROS and reactive nitrogen intermediate .
Common manifestations of disseminated P. marneffei infection in AIDS patients are fever, anemia, weight loss, lymphadenopathy, hepatosplenomegaly, respiratory signs, and skin lesions. Paitents who don’t receive the appropriate antifungal treatment have a poor prognosis . Early diagnosis is important, and untreated infections are usually fatal. Response to anti-fungal therapy is good if the treatment is started early, and amphotericin B and itraconazole are generally effective, even though patients may need secondary anti-fungal prophylaxis for life after their first treatment. Primary prophylaxis with itraconazole can prevent systemic P. marneffei infection in patients with advanced HIV infection, and secondary prophylaxis can prevent relapses during remission .
Application to Biotechnology
Melanin are multifunctional compunds found in diverse species and have been implicated in virulence in human fungal pathogens. Penicillium marneffei conidia and yeast cells can produce melanin o melanin-like compunds in viro and yeast cells can synthesize pigment in vivo. This pigment may actually play some role in the virulence of P. marneffei. P. marneffei also produces and secretes a brick-red diffusible water-soluble pigment, during growth at temperatures below 30 degrees Celsius. It generally diffuses into commonly used media like Sabouraud dextrose agar and malt extract agar, and doesn't produce pigment in observable amounts when the fungus is cultured at temperatures higher than 30 degrees Celsius. This red pigment was found to have some structural similarities with the copper-colored pigment produced by Penicillium herquei, because both pigments contain the phenalene carbon framework [13, 14].
Recently, pigment production by fungi has drawn a lot of attention of scientists. The unique pigment production by this fungus may help to better understand certain biological aspects of this pathogenic fungus .
The triazole anti-fungal agent, voriconzole, was used as therapy for systematic Penicillium marneffei infections in patients with advanced HIV infections. Patients were treated in the hospital setting with intravenous voriconazole (6 mg/kg every 12 hours on Day 1 and then 4 mg/kg every 12 hours for at least 3 days, after which patients could switch to oral therapy at 200 mg twice a day) or as outpatients with oral voriconazole (400 mg twice a day on Day 1 and then 200 mg twice a day) for a maximum of 12 weeks. At the end of therapy, eight of nine evaluable patients had favorable response to therapy, based on mycological and clinical findings. Treatment with voriconazole was well tolerated, with no discontinuations caused by drug-related adverse events. The results of this study show that voriconazole is an effective, well-tolerated, and covenient choice for the treatment of systemic infections with P. marneffei .
Superoxide dismutase (SOD) is an enzyme that converts superoxide radicals into hydrogen peroxide and oxygen molecules. SOD has been shown to contribute to the virulence of many human-pathogenic fungi through its ability to neutralize toxic levels of reactive oxygen species generated by the host. sodA, a copper zinc superoxide dismutase gene, was isolated and characterized from the important human pathogenic fungus, P. marneffei. Differential expression of the soda gene in P. marneffei was demonstrated by semi-quantitative RT-PCR. The sodA transcript accumulated in conidia, expression was downregulated in the mycelia phase, and transcript expression was upregulated in the yeast phase as well as during macrophage infection. This higher expression of sodA during macrophage infection indicate that this gene may play an important role in stress responses and in the adaptation of P. marneffei to the internal macrophage environment .
The pathology of P. marneffei seems to be related to the dimorphic transition from a mycelial phase in the environment to a yeast phase in the human host. To further understand the molecular mechanisms in the switching process of P. marneffei, differential gene expression analysis was carried out between the mycelial phase and the yeast phase by suppression subtractive hybridization. Five hundred genes from forward and reverse-subtracted cDNA libraries were screened by cDNA array dot blotting and 43 genes were identified to be differentially expressed. It is believed that these genes determine some of the major characteristics of dimorphism in P. marneffei .
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15. Penicillium marneffei, genus NCBI reference: http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=37727&lvl=3&p=mapview&p=has_linkout&p=blast_url&p=genome_blast&lin=f&keep=1&srchmode=1&unlock
Edited by Tiffanie Chan, student of Rachel Larsen