Evidence that the human pathogenic fungus Cryptococcus neoformans var. grubii may have evolved in Africa.
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2011-05-11
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Most of the species of fungi that cause disease in mammals, including Cryptococcus neoformans var. grubii (serotype A), are exogenous and non-contagious. Cryptococcus neoformans var. grubii is associated worldwide with avian and arboreal habitats. This airborne, opportunistic pathogen is profoundly neurotropic and the leading cause of fungal meningitis. Patients with HIV/AIDS have been ravaged by cryptococcosis--an estimated one million new cases occur each year, and mortality approaches 50%. Using phylogenetic and population genetic analyses, we present evidence that C. neoformans var. grubii may have evolved from a diverse population in southern Africa. Our ecological studies support the hypothesis that a few of these strains acquired a new environmental reservoir, the excreta of feral pigeons (Columba livia), and were globally dispersed by the migration of birds and humans. This investigation also discovered a novel arboreal reservoir for highly diverse strains of C. neoformans var. grubii that are restricted to southern Africa, the mopane tree (Colophospermum mopane). This finding may have significant public health implications because these primal strains have optimal potential for evolution and because mopane trees contribute to the local economy as a source of timber, folkloric remedies and the edible mopane worm.
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Litvintseva, Anastasia P, Ignazio Carbone, Jenny Rossouw, Rameshwari Thakur, Nelesh P Govender and Thomas G Mitchell (2011). Evidence that the human pathogenic fungus Cryptococcus neoformans var. grubii may have evolved in Africa. PLoS One, 6(5). p. e19688. 10.1371/journal.pone.0019688 Retrieved from https://hdl.handle.net/10161/11064.
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Thomas Greenfield Mitchell
Among patients with AIDS, leukemia or other cancers, organ or bone marrow transplants, and similar immunocompromising risk factors, the incidence of opportunistic mycoses and the number of different fungal pathogens are increasing dramatically. For many of these fungi, the definition of a species and the recognition of pathogen are highly problematic. Conventional methods of identification are based on morphological and physiological characteristics and are often time-consuming, difficult to interpret, and inconsistent. This laboratory is using DNA-based methods to (i) identify fungal pathogens, (ii) resolve taxonomic issues, (iii) facilitate epidemiological studies, (iv) recognize strains with clinically relevant phenotypes, such as resistance to antifungal drugs, (v) elucidate the origin(s) of diversity and the population genetics of the major pathogens, and (vi) explore functional genomics to identify virulence factors. We have developed reliable methods to genotype strains and are analyzing gene sequences to clarify the phylogeny of controversial taxa.
To conduct rigorous population studies of Candida albicans, we developed single-locus markers based on polymorphisms of PCR products. Genotypic frequencies and segregation patterns at these loci have confirmed that C. albicans is diploid and suggest that some form of recombination occurs in this "asexual" yeast. To investigate whether separate populations of C. albicans exist in disparate geographical locations, we compared strains collected from healthy and HIV-infected persons in U.S. and Brazil. Although a number of different genotypes were recognized at each location, the same multilocus genotype was prevalent among the clinical isolates, indicating a remarkable homogeneity among these populations.
We are using DNA-based methods to compare global isolates of Cryptococcus neoformans from patients with AIDS and other sources, to analyze the distribution and relatedness of strains, to identify genotypes of clinical importance, and to create linkage map of this pathogen. To determine the source of C. neoformans in patients, we developed a genetic markers to investigate the structure of clinical and environmental populations. With analysis of quantitative trait loci, specific genotypes will be identified that represent clones that have significantly diverged with respect to clinically relevant phenotypes, including susceptibility to antifungal drugs and the expression of virulence factors. We are investigating genomic evolution and phenotypic variation in natural populations of C. neoformans. These approaches will correlate genotypes with pathobiological phenotypes, leading to beneficial and predictive information about the epidemiology, diagnosis and prognosis of cryptococcosis in patients with AIDS.
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