Rapid detection of aneuploidy following the generation of mutants in Candida albicans

Virulence phenotypes result from interactions between pathogen ploidy and genetic background

Studying fungal virulence is often challenging and frequently depends on many contexts, including host immune status and pathogen genetic background. However, the role of ploidy has often been overlooked when studying virulence in eukaryotic pathogens. Since fungal pathogens, including the human opportunistic pathogen Candida albicans, can display extensive ploidy variation, assessing how ploidy impacts virulence has important clinical relevance. As an opportunistic pathogen, C. albicans causes nonlethal, superficial infections in healthy individuals, but life-threatening bloodstream infections in individuals with compromised immune function. Here, we determined how both ploidy and genetic background of C. albicans impacts virulence phenotypes in healthy and immunocompromised nematode hosts by characterizing virulence phenotypes in four near-isogenic diploid and tetraploid pairs of strains, which included both laboratory and clinical genetic backgrounds. We found that C. albicans infections decreased host survival and negatively impacted host reproduction, and we leveraged these two measures to survey both lethal and nonlethal virulence phenotypes across the multiple C. albicans strains. In this study, we found that regardless of pathogen ploidy or genetic background, immunocompromised hosts were susceptible to fungal infection compared to healthy hosts. Furthermore, for each host context, we found a significant interaction between C. albicans genetic background and ploidy on virulence phenotypes, but no global differences between diploid and tetraploid pathogens were observed.

Genotyping Reveals High Clonal Diversity and Widespread Genotypes of Candida Causing Candidemia at Distant Geographical Areas

The objectives of this study were to gain further insight on Candida genotype distribution and percentage of clustered isolates between hospitals and to identify potential clusters involving different hospitals and cities. We aim to genotype Candida spp. isolates causing candidemia in patients admitted to 16 hospitals in Spain, Italy, Denmark, and Brazil. Eight hundred and eighty-four isolates (Candida albicans, n = 534; C. parapsilosis, n = 282; and C. tropicalis, n = 68) were genotyped using species-specific microsatellite markers. CDC3, EF3, HIS3, CAI, CAIII, and CAVI were used for C. albicans, Ctrm1, Ctrm10, Ctrm12, Ctrm21, Ctrm24, and Ctrm28 for C. tropicalis, and CP1, CP4a, CP6, and B for C. parapsilosis. Genotypes were classified as singletons (genotype only found once) or clusters (same genotype infecting two or more patients). Clusters were defined as intra-hospital (involving patients admitted to a single hospital), intra-ward (involving patients admitted to the same hospital ward) or widespread (involving patients admitted to different hospitals). The percentage of clusters and the proportion of patients involved in clusters among species, genotypic diversity and distribution of genetic diversity were assessed. Seven hundred and twenty-three genotypes were detected, 78 (11%) being clusters, most of which (57.7%; n = 45/78) were intra-hospital clusters including intra-ward ones (42.2%; n = 19/45). The proportion of clusters was not statistically different between species, but the percentage of patients in clusters varied among hospitals. A number of genotypes (7.2%; 52/723) were widespread (found at different hospitals), comprising 66.7% (52/78) of clusters, and involved patients at hospitals in the same city (n = 21) or in different cities (n = 31). Only one C. parapsilosis cluster was a widespread genotype found in all four countries. Around 11% of C. albicans and C. parapsilosis isolates causing candidemia are clusters that may result from patient-to-patient transmission, widespread genotypes commonly found in unrelated patients, or insufficient microsatellite typing genetic discrimination.

Amplification of TLO Mediator Subunit Genes Facilitate Filamentous Growth in Candida Spp

Filamentous growth is a hallmark of C. albicans pathogenicity compared to less-virulent ascomycetes. A multitude of transcription factors regulate filamentous growth in response to specific environmental cues. Our work, however, suggests the evolutionary history of C. albicans that resulted in its filamentous growth plasticity may be tied to a change in the general transcription machinery rather than transcription factors and their specific targets. A key genomic difference between C. albicans and its less-virulent relatives, including its closest relative C. dubliniensis, is the unique expansion of the TLO (TeLOmere-associated) gene family in C. albicans. Individual Tlo proteins are fungal-specific subunits of Mediator, a large multi-subunit eukaryotic transcriptional co-activator complex. This amplification results in a large pool of ‘free,’ non-Mediator associated, Tlo protein present in C. albicans, but not in C. dubliniensis or other ascomycetes with attenuated virulence. We show that engineering a large ‘free’ pool of the C. dubliniensis Tlo2 (CdTlo2) protein in C. dubliniensis, through overexpression, results in a number of filamentation phenotypes typically associated only with C. albicans. The amplitude of these phenotypes is proportional to the amount of overexpressed CdTlo2 protein. Overexpression of other C. dubliniensis and C. albicans Tlo proteins do result in these phenotypes. Tlo proteins and their orthologs contain a Mediator interaction domain, and a potent transcriptional activation domain. Nuclear localization of the CdTlo2 activation domain, facilitated naturally by the Tlo Mediator binding domain or artificially through an appended nuclear localization signal, is sufficient for the CdTlo2 overexpression phenotypes. A C. albicans med3 null mutant causes multiple defects including the inability to localize Tlo proteins to the nucleus and reduced virulence in a murine systemic infection model. Our data supports a model in which the activation domain of ‘free’ Tlo protein competes with DNA bound transcription factors for targets that regulate key aspects of C. albicans cell physiology.

The ascomycete fungus Candida albicans is a leading cause of hospital-acquired bloodstream infections in the United States. Due to limited anti-fungal drug options, there is an approximately 40% mortality rate and over 10,000 deaths per year associated with systemic C. albicans infections. It is unknown why C. albicans is the primary cause of systemic Candidiasis, versus related ascomycetes such as Candida dubliniensis. The genomes of C. albicans and C. dubliniensis are remarkably similar, yet C. dubliniensis has reduced virulence and exhibits less phenotypic plasticity. A striking genomic difference between the fungi is the amplification of the TLO (TeLOmere-associated) genes in C. albicans, which encode a fungal-specific subunit of the Mediator co-activator complex. Amplification results in a large pool of ‘free’ (non-Mediator associated) Tlo protein in C. albicans that is absent in C. dubliniensis. Engineering a large ‘free’ pool of Tlo protein in C. dubliniensis, through overexpression, results in phenotypes common in C. albicans, yet typically absent in C. dubliniensis. Tlo proteins contain a potent transcriptional activation domain. Nuclear localization of the Tlo activation domain is necessary and sufficient for the TLO overexpression phenotypes. This study provides a mechanistic explanation for how TLO amplification in C. albicans may enhance its virulence.

Clusters of patients with candidaemia due to genotypes of Candida albicans and Candida parapsilosis: differences in frequency between hospitals

The presence of clusters (identical genotypes infecting different patients) suggests patient-to-patient transmission or a common source for strains. We report the results of a genotyping study based on microsatellite markers of Candida albicans (n = 179) and Candida parapsilosis (n = 76) causing candidaemia, to assess and compare the percentage of patients grouped in clusters during the study period (January 2010 to December 2012). The study was performed in two large tertiary hospitals in Madrid, Spain. We detected 145 C. albicans genotypes (21 in clusters) and 63 C. parapsilosis genotypes (seven in clusters). Clusters involved two to seven patients each. Most of the clusters in the two centres involved two patients for both species, but the number of patients included in each cluster differed between hospitals. Considering both species, the percentage of patients per cluster ranged from 19% to 38% (p < 0.05) in Hospital A and B respectively. Up to 2.9% of genotypes were present in both hospitals. Clusters of C. albicans and C. parapsilosis genotypes causing candidaemia differed between hospitals, suggesting differences in strain transmission. Occasionally, the same genotypes were found in patients admitted to different hospitals located in the same city.

The Mnn2 mannosyltransferase family modulates mannoprotein fibril length, immune recognition and virulence of Candida albicans

The fungal cell wall is the first point of interaction between an invading fungal pathogen and the host immune system. The outer layer of the cell wall is comprised of GPI anchored proteins, which are post-translationally modified by both N- and O-linked glycans. These glycans are important pathogen associated molecular patterns (PAMPs) recognised by the innate immune system. Glycan synthesis is mediated by a series of glycosyl transferases, located in the endoplasmic reticulum and Golgi apparatus. Mnn2 is responsible for the addition of the initial α1,2-mannose residue onto the α1,6-mannose backbone, forming the N-mannan outer chain branches. In Candida albicans, the MNN2 gene family is comprised of six members (MNN2, MNN21, MNN22, MNN23, MNN24 and MNN26). Using a series of single, double, triple, quintuple and sextuple mutants, we show, for the first time, that addition of α1,2-mannose is required for stabilisation of the α1,6-mannose backbone and hence regulates mannan fibril length. Sequential deletion of members of the MNN2 gene family resulted in the synthesis of lower molecular weight, less complex and more uniform N-glycans, with the sextuple mutant displaying only un-substituted α1,6-mannose. TEM images confirmed that the sextuple mutant was completely devoid of the outer mannan fibril layer, while deletion of two MNN2 orthologues resulted in short mannan fibrils. These changes in cell wall architecture correlated with decreased proinflammatory cytokine induction from monocytes and a decrease in fungal virulence in two animal models. Therefore, α1,2-mannose of N-mannan is important for both immune recognition and virulence of C. albicans.

Endemic genotypes of Candida albicans causing fungemia are frequent in the hospital

Genotyping of Candida albicans strains causing candidemia can uncover the presence of endemic genotypes in the hospital. Using a highly reproducible and discriminatory microsatellite marker panel, we studied the genetic diversity of 217 C. albicans isolates from the blood cultures of 202 patients with candidemia (from January 2007 to December 2011). Each isolate represented 1 candidemia episode. Multiple episodes were defined as the isolation of C. albicans in further blood cultures taken ≥7 days after the last isolation in blood culture. Of the 202 patients, 188 had 1 episode, 13 had 2 episodes, and 1 had 3 episodes. Identical genotypes showed the same alleles for all 6 markers. The genotypes causing both episodes were identical in most patients with 2 episodes (11/13; 84.6%). In contrast, 2 different genotypes were found in the patient with 3 episodes, one causing the first and second episodes and the other causing the third episode (isolated 6 months later). We found marked genetic diversity in 174 different genotypes: 155 were unique, and 19 were endemic and formed 19 clusters (2 to 6 patients per cluster). Up to 25% of the patients were infected by endemic genotypes that infected 2 or more different patients. Some of these endemic genotypes were found in the same unit of the hospital, mainly neonatology, whereas others infected patients in different wards.