Diversity and ontogeny of Cryptococcus neoformans var. grubii originating from South Africa

[摘要] ENGLISH ABSTRACT: Cryptococcus neoformans (Sanfelice) Vuillemin is an opportunistic fungal pathogen responsible for causing meningitis predominantly in immuno-compromised individuals, particularly in those suffering from human immuno virus (HIV) acquired immuno-deficiency syndrome (AIDS). Two main varieties are known, C. neoformans var. neoformans (serotype D) and C. neoformans var. grubii (serotype A), as well as a hybrid variety, C. neoformans (serotype AD). These serotypes may belong to one of two mating types, namely mating type a (MATa), or mating type alpha (MATα). Several molecular typing methods were previously developed to classify C. neoformans into four major genotypic groups, namely VNI, VNII, VNIII and VNIV. In addition to clinical samples, these yeasts are known to occur in a wide diversity of environmental habitats including soil, avian guano, trees and decaying wood.The study had two main objectives, firstly to obtain an indication of the prevailing C. neoformans genotypes occurring within the HIV positive and AIDS populations of South Africa and to obtain an indication of its distribution within the environment, particularly within the Western Cape Province, South Africa. Secondly, to examine whether C. neoformans is able to grow and interact with selected microbes when cultured on woody debris and to determine if C. neoformans is capable of producing its ontogenic stages on this woody debris.Despite attempts to isolate C. neoformans from 256 environmental samples originating from a variety of habitats in South Africa, a total of only four isolates were obtained from the environment. None were isolated from environmental sources in the Western Cape Province, South Africa. The four environmental C. neoformans var. grubii strains isolated from soil in the North West province of South Africa, and 32 clinicalC. neoformans strains originating from the Gauteng and Western Cape provinces of South Africa were subsequently identified and characterized. Strains were identified by sequencing the internal transcribed spacer (ITS) region of the ribosomal gene cluster, while serotypes and mating types were confirmed using polymerase chain reaction (PCR) primers. The genotype of each strain was determined by employing three PCR-based typing techniques, namely PCR fingerprinting using the mini-satellite M13, microsatellite(GACA)4 and random amplified polymorphic DNA (RAPD) analysis, as well asrestriction fragment length polymorphism (RFLP) analysis of the phopholipase B1 gene.A total of 97 % of the strains were identified as C. neoformans var. grubii (serotype A),while only one strain was identified as C. neoformans var. neoformans (serotype D). Allstrains were found to be MATα and haploid. The majority of strains grouped intogenotype VNI (75.6 %), seven strains represented genotype VNII (21.2 %), while onlyone strain represented genotype VNIV (3 %). These results are in accordance withprevious and current literature stating that C. neoformans var. grubii (serotype A, MATα,VNI) is responsible for the majority of cryptococcal infections.Using plate assays, all the C. neoformans strains were screened for wooddegrading enzymes. All strains tested positive for cellulase activity, 6 % of strains testedpositive for laccase production at 22 ºC, but no strains were able to degrade xylan.Subsequently, three C. neoformans var. grubii strains, originating from clinical andenvironmental samples, all representing the same genotype (VNI) and mating type(MATα), were evaluated for growth on Acacia mearnsii and Eucalyptus camaldulensisdebris. While minimal differences were noted between strains, those cultured on A.mearnsii yielded significantly higher cell counts. Finally, all strains were mated onAcacia mearnsii and Eucalyptus camaldulensis debris, as well as V8 juice and yeastcarbon base (YCB) agar to determine whether C. neoformans strains were capable ofboth dikaryotic and monokaryotic fruiting when cultured on woody debris. A total of19 %, 6 %, 42 % and 72 % of the C. neoformans strains were able to mate when crossedon A. mearnsii and E. camaldulensis debris, V8 juice and YCB agar, respectively.Monokaryotic fruiting was observed in 3 %, 3 % and 3 % of strains when C. neoformanswas cultured on Acacia mearnsii, Eucalyptus camaldulensis debris and YCB,respectively. This may be the first observation of C. neoformans in a hyphal phase whencultured on medium comprised solely of woody debris, the perceived natural habitat ofthis yeast.

[发布机构] Stellenbosch University