Basic Mycology Ppt

[Leonides Suttle]

Mycologyhas impacted your life, I guarantee it. Whether it was that penicillin you took to get better or those cultivated mushrooms you ate for dinner yesterday, you have mycology to thank. Mycologists are scientists who dedicate their careers to all things fungi.

The fungus Talaromyces marneffei was described by Professor Gabriel Segretain in 1959, originally as a member of the genus Penicillium. As early as 60 years ago, its peculiarity in exhibiting temperature-dependent morphological dimorphism, its characteristic ability to secrete diffusing red pigment during the mycelial phase and its pathogenicity have already been recognised.Six decades have passed, and our understanding on this intriguing fungus has improved. Apart from the clinical aspect, we have gained a glimpse on its taxonomy, animal or environmental source(s), mechanism of thermal dimorphism, molecular genetics, virulence as well as pathogenesis. However, we are still on our way to get out of the talaromycosis mist. A lot more collective endeavour on T. marneffei research is needed to solve the jigsaw puzzle.

This year marks the 60th anniversary of the official description of the thermally dimorphic fungus, Talaromyces marneffei (previously called Penicillium marneffei), which is an important fungal pathogen endemic in Southeast Asia especially to HIV-positive patients, by Professor Gabriel Segretain [1]. This fungus was first isolated in 1955 by Capponi and Sureau from laboratory Chinese bamboo rats (Rhizomys sinensis), which are native in the Central Highlands of Vietnam, at Institut Pasteur de Dalat [2], where it caused fatal spontaneous disseminated infection involving the reticuloendothelial system in three of the rats. The pathogenicity of T. marneffei was further demonstrated by Segretain, Capponi and Sureau in various animal models, including mice, rats, hamsters and guinea pigs [2, 3].

Given the high mortality rate of T. marneffei infection in untreated patients, especially in immunocompromised patients, it is important to identify the possible sources of talaromycosis. In particular, the fact that HIV patients who have travel histories to endemic areas could also become infected by T. marneffei implies that short-term exposure to the source(s) of the fungus is sufficient to trigger infection and that the infection source(s) should originate from an environment contacted by tourists [37]. Studies beginning from the 1980s identified a number of bamboo rats species, including Chinese bamboo rats, hoary bamboo rats (R. pruinosus), large bamboo rats (R. sumatrensis) and lesser bamboo rats (Cannomys badius), as the natural carriers of T. marneffei in southern China, central and northern Thailand as well as India [9, 19, 38,39,40,41,42,43,44,45]. The fungus could also be recovered from the faeces of bamboo rats as well as soils from their burrows [40, 43, 45, 46]. Apart from bamboo rats and their associated soil samples, recently the fungus has been detected from nasal swabs of outdoor dogs in Chiang Mai, Thailand, as well, although fungal culture was not successful [37]. A case-control study in 1997 demonstrated that exposure to or consumption of bamboo rats did not constitute a risk factor for talaromycosis [47]. There was also no significant association between exposure to bamboo thickets or forests, where the rodents reside, to T. marneffei infection [47]. Instead, it was shown that recent occupational exposure to animals or plants, probably involving soil, was associated with talaromycosis [47]. Indeed, a previous environmental study detected T. marneffei from soil samples collected outside the habitats of bamboo rats, including a bat cave, an elephant camp and surroundings of a Buddhist Temple using molecular methods [48]. Unfortunately, viable culture of the fungus could still not be obtained from these positive soil samples [48]. These findings suggested that an environmental reservoir, likely soil, should exist for T. marneffei and humans and bamboo rats, and possibly dogs, may acquire the infection from this common environmental source. Interestingly, although earlier studies in Thailand and Vietnam showed that the incidence of T. marneffei infection increased during rainy season [49, 50], a later study in Vietnam found that humidity, but not rainfall, temperature nor wind, was the environmental predictor of T. marneffei hospital admission [51]. It was suspected that humidity may facilitate the environmental reservoir of T. marneffei to expand, favour fungal growth or aid in conidia release into the environment [51]. Further efforts are in need to determine the environmental source(s) of this fungus and to connect this/these environmental reservoir(s) with the animal hosts and humans such that the route of infection acquisition can be identified or confirmed.

Temperature-dependent morphological dimorphism is one of the characteristics of T. marneffei. Indeed, this fungus is the only known Talaromyces species exhibiting such a feature. Although the change in morphology has been well studied during phase transition [52], the underlying molecular mechanism still warrants much further investigation. From 2000 onwards, a number of genes have been identified to be involved in morphogenesis and during phase transition, including abaA [53], brlA [54], cflA [55], cflB [56], drkA [57], gasA [58], gasC [59], hgrA [60], madsA [61, 62], myoB [63], pakA [64], pakB [65], PmHHK1 [66], rasA [67], rfxA [68], rttA [69], sadA [70], sakA [71, 72], slnA [57], sskA [73], stuA [74], tupA [54] and yakA [75], by loss-of-function studies. The majority of these genes are related to transcriptional regulation [53, 54, 60, 62, 68, 74] or G-protein signalling [55, 56, 58, 59, 67] or encode kinases [57, 64,65,66, 71, 72, 75]; the functions of these genes were recently thoroughly reviewed by Andrianopoulos and his colleagues [76, 77]. Other than these genes, differentially expressed gene analyses by suppression subtractive hybridisation [78], microarray [79, 80] or RNA-sequencing [61, 81] identified groups of genes that are signature to phase transition. In addition to protein-coding genes, small RNA-sequencing also revealed 24 microRNA-like small RNA (milRNA) candidates which were more abundantly expressed during the hyphal stage than the yeast phase [82]. However, further functional studies are needed to authenticate the participations of these genes in dimorphic switching. Moreover, how T. marneffei detects the temperature stimulus and transduces the environmental signals to effect morphological change is still unclear. Elucidation of the molecular mechanism for this could help reveal potential drug targets for stopping dimorphic switching to the yeast phase in vivo so as to prevent the fungus from evading host immune response by hiding intracellularly.

The advancement of various omics technologies in the twenty-first century has allowed more in-depth and sophisticated characterisation of the mycology of T. marneffei. The first draft genome of the fungus, based on the original ex-type strain ATCC 18224T isolated by Capponi and Sureau, was sequenced using the Sanger strategy. It was around 28 Mb in length and was released online in 2007 [83]. A few years later, using a similar approach the draft genome for a second strain (PM1), isolated in Hong Kong, was published [84]. Subsequent second- and third-generation sequencing technologies further improved this draft genome [61]. Earlier this year, the genome of T. marneffei, based on a Chinese clinical strain TM4, was completed with the help of optical mapping; it was revealed that the T. marneffei genome consisted of eight chromosomes [85]. The availability of the whole genome sequences of T. marneffei has boosted further research on this fungus, especially in the past decade; knowledge generated from the fungal genomes as well as additional transcriptomic, proteomic and metabolomic studies was discussed in detail in a recent review [86]. One example was that the pigmentation phenomenon observed for T. marneffei could now be explained, with the molecular identities of the various pigments uncovered [87,88,89,90].

Following 60 years of effort, the mist of T. marneffei has somehow faded away. We have gained some knowledge on its taxonomy, source(s), underlying mechanism of thermal dimorphism, molecular genetics, virulence and pathogenesis. However, the jigsaw puzzle is still far from completion. Continuous collective endeavour is needed to uncover the T. marneffei mystery so that this fungus, currently recognised as one of the ten most feared fungi in the world [117], no longer poses threats to humanity.

This work was supported by the General Research Fund (No. 17104318), Research Grants Council, University Grants Committee, Hong Kong; Health and Medical Research Fund (No. HKM-15-M07 [commissioned project]), Food and Health Bureau, Government of the Hong Kong Special Administrative Region, Hong Kong; Seed Fund for Basic Research, The University of Hong Kong, Hong Kong; and the Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, Ministry of Education, China.

Patrick C. Y. Woo has provided scientific advisory/laboratory services for Gilead Sciences, Incorporated, International Health Management Associates, Incorporated, Merck & Corporation, Incorporated, and Pfizer, Incorporated. The other authors report no conflicts of interest. The funding sources had no role in study design, data collection, analysis, interpretation or writing of the report. The authors alone are responsible for the content and the writing of the manuscript.

Evaluation of Education: Courses in basic botany, plant anatomy or morphology, cytology, histology, genetics, taxonomy or systematics, algology, mycology, ethnobotany, and those dealing with specific problems of a botanical nature or with specific groups of plants are qualifying. The courses required for mycologists are specific and must have been in mycology.

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