Phenotypic diversity and correlation between white-opaque switching and the CAI microsatellite locus in Candida albicans

Variation in transcription regulator expression underlies differences in white-opaque switching between the SC5314 reference strain and the majority of Candida albicans clinical isolates

Candida albicans, a normal member of the human microbiome and an opportunistic fungal pathogen, undergoes several morphological transitions. One of these transitions is white-opaque switching, where C. albicans alternates between 2 stable cell types with distinct cellular and colony morphologies, metabolic preferences, mating abilities, and interactions with the innate immune system. White-to-opaque switching is regulated by mating type; it is repressed by the a1/α2 heterodimer in a/α cells, but this repression is lifted in a/a and α/α mating type cells (each of which are missing half of the repressor). The widely used C. albicans reference strain, SC5314, is unusual in that white-opaque switching is completely blocked when the cells are a/α; in contrast, most other C. albicans a/α strains can undergo white-opaque switching at an observable level. In this paper, we uncover the reason for this difference. We show that, in addition to repression by the a1/α2 heterodimer, SC5314 contains a second block to white-opaque switching: 4 transcription regulators of filamentous growth are upregulated in this strain and collectively suppress white-opaque switching. This second block is missing in the majority of clinical strains, and, although they still contain the a1/α2 heterodimer repressor, they exhibit a/α white-opaque switching at an observable level. When both blocks are absent, white-opaque switching occurs at very high levels. This work shows that white-opaque switching remains intact across a broad group of clinical strains, but the precise way it is regulated and therefore the frequency at which it occurs varies from strain to strain.

Investigating in vivo Mycobacterium avium subsp. paratuberculosis microevolution and mixed strain infections

Mycobacterium avium subsp. paratuberculosis (MAP) causes Johne's Disease (JD) in ruminants, which is responsible for significant economic loss to the global dairy industry. Mixed strain infection (MSI) refers to the concurrent infection of a susceptible host with genetically distinct strains of a pathogen, whereas within-host changes in an infecting strain leading to genetically distinguishable progeny is called microevolution. The two processes can influence host-pathogen dynamics, disease progression and outcomes, but not much is known about their prevalence and impact on JD. Therefore, we obtained up to 10 MAP isolates each from 14 high-shedding animals and subjected them to whole-genome sequencing. Twelve of the 14 animals examined showed evidence for the presence of MSIs and microevolution, while the genotypes of MAP isolates from the remaining two animals could be attributed solely to microevolution. All MAP isolates that were otherwise isogenic had differences in short sequence repeats (SSRs), of which SSR1 and SSR2 were the most diverse and homoplastic. Variations in SSR1 and SSR2, which are located in ORF1 and ORF2, respectively, affect the genetic reading frame, leading to protein products with altered sequences and computed structures. The ORF1 gene product is predicted to be a MAP surface protein with possible roles in host immune modulation, but nothing could be inferred regarding the function of ORF2. Both genes are conserved in Mycobacterium avium complex members, but SSR1-based modulation of ORF1 reading frames seems to only occur in MAP, which could have potential implications on the infectivity of this pathogen. IMPORTANCE Johne's disease (JD) is a major problem in dairy animals, and concerns have been raised regarding the association of Mycobacterium avium subsp. paratuberculosis (MAP) with Crohn's disease in humans. MAP is an extremely slow-growing bacterium with low genome evolutionary rates. Certain short sequence repeats (SSR1 and SSR2) in the MAP chromosome are highly variable and evolve at a faster rate than the rest of the chromosome. In the current study, multiple MAP isolates with genetic variations such as single-nucleotide polymorphisms, and more noticeably, diverse SSRs, could simultaneously infect animals. Variations in SSR1 and SSR2 affect the products of the respective genes containing them. Since multiple MAP isolates can infect the same animal and the possibility that the pathogen undergoes further changes within the host due to unstable SSRs, this could provide a compensative mechanism for an otherwise slow-evolving pathogen to increase phenotypic diversity for overcoming host responses.

Molecular genotyping of Candida albicans isolated from different sites may trace the etiological infection routes: Observation in China

BACKGROUND

Invasive candidiasis is a growing concern worldwide, especially in immunocompromised patients, including ICU patients.

OBJECTIVES

As Candida albicans is the leading cause of candidaemia, it is important to investigate the evolution of C. albicans in patients with candidaemia.

METHODS

We analysed 238 strains of C. albicans isolated from different body sites. Antifungal susceptibility testing, CAI loci genotyping and multilocus sequence typing (MLST) of all isolates were performed. The relationships among the total isolates that differed in sequence at only one of the seven housekeeping gene loci were analysed using eBURST.

RESULTS

Multilocus sequence typing analysis in 238 isolates by combining seven housekeeping alleles revealed 175 diploid sequence types, in which 84 were newly identified. eBURST analysis for these data recognised 19 clonal complexes (CCs) and 79 singletons. Besides, seventy-three CAI genotypes were identified. Blood isolates showed maximum genotypes (49), and the dominant genotypes were CAI 17-21 and CAI 21-21. Oral isolates possessed 25 CAI genotypes, and the dominant genotypes were CAI 17-21 and CAI 21-21 as well. Since isolates with CAI allele numbers <30 showed easier transmission, CAI 17-21 and CAI 21-21 were the most frequently transmitted. Finally, the CAI genotypes were classified into six groups.

CONCLUSIONS

This work revealed the oral and blood strains isolated from the patients with candidaemia in ICU shared the identical dominant CAI genotypes. Our data expanded the C. albicans MLST database and helped with understanding the evolution and spread of invasive candidiasis.

Genetic diversity assessed using PFGE, MLP and MLST in Candida spp. candidemia isolates obtained from a Brazilian hospital

Candida spp. are the main causative agents of invasive fungal infections in immunocompromised patients. Candidemia has attributable mortality rates of 15 to 35% and increases hospitalisation time and costs, thus making this disease a public health concern. This study aimed to use pulsed-field gel electrophoresis (PFGE), microsatellite length polymorphism (MLP) and multilocus sequence typing (MLST) to analyse the genetic relationships among 65 Candida spp. bloodstream isolates, including 35 Candida albicans, 15 Candida glabrata and 15 Candida tropicalis isolates, all of which were obtained from patients in a Brazilian hospital. Moreover, patient clinical data were assessed. All techniques resulted in high discriminatory indexes. C. albicans and C. tropicalis isolates showed high genetic variability, while C. glabrata isolates had relatively low genetic variability. Moreover, a cluster of C. glabrata isolates was identified in a hospital unit. New MLST sequence types, diploid sequence types and alleles are described. Relationships were not observed between the molecular typing results and clinical characteristics. The molecular typing of clinical strains increases our understanding of candidemia epidemiology and promotes the development of strategies that can reduce the incidence of this disease. Moreover, this study is the first to combine these techniques to genotype these three species in Brazil.

Pathogenesis and Clinical Relevance of Candida Biofilms in Vulvovaginal Candidiasis

The ability of Candida spp. to form biofilms is crucial for its pathogenicity, and thus, it should be considered an important virulence factor in vulvovaginal candidiasis (VVC) and recurrent VVC (RVVC). Its ability to generate biofilms is multifactorial and is generally believed to depend on the site of infection, species and strain involved, and the microenvironment in which the infection develops. Therefore, both cell surface proteins, such as Hwp1, Als1, and Als2, and the cell wall-related protein, Sun41, play a critical role in the adhesion and virulence of the biofilm. Immunological and pharmacological approaches have identified the NLRP3 inflammasome as a crucial molecular factor contributing to host immunopathology. In this context, we have earlier shown that Candida albicans associated with hyphae-secreted aspartyl proteinases (specifically SAP4-6) contribute to the immunopathology of the disease. Transcriptome profiling has revealed that non-coding transcripts regulate protein synthesis post-transcriptionally, which is important for the growth of Candida spp. Other studies have employed RNA sequencing to identify differences in the 1,245 Candida genes involved in surface and invasive cellular metabolism regulation. In vitro systems allow the simultaneous processing of a large number of samples, making them an ideal screening technique for estimating various physicochemical parameters, testing the activity of antimicrobial agents, and analyzing genes involved in biofilm formation and regulation (in situ) in specific strains. Murine VVC models are used to study C. albicans infection, especially in trials of novel treatments and to understand the cause(s) for resistance to conventional therapeutics. This review on the clinical relevance of Candida biofilms in VVC focuses on important advances in its genomics, transcriptomics, and proteomics. Moreover, recent experiments on the influence of biofilm formation on VVC or RVVC pathogenesis in laboratory animals have been discussed. A clear elucidation of one of the pathogenesis mechanisms employed by Candida biofilms in vulvovaginal candidiasis and its applications in clinical practice represents the most significant contribution of this manuscript.

Association between CAI microsatellite, multilocus sequence typing, and clinical significance within Candida albicans isolates

Candida albicans bloodstream infection (BSI) is epidemiologically important because of its increasing frequency and serious outcome. Strain typing and delineation of the species are essential for understanding the phylogenetic relationship and clinical significance. Microsatellite CAI genotyping and multilocus sequence typing (MLST) were performed on 285 C. albicans bloodstream isolates from patients in Chang Gung Memorial Hospital at Linkou (CGMHL), Taiwan from 2003 to 2011. Data regarding demographics, comorbidities, risk factors, and clinical outcomes were recorded within adult patients with C. albicans BSI. Both CAI genotyping and MLST yielded comparable discriminatory power for C. albicans characterization. Besides, the distribution of CAI repetition showed a satisfactory phylogenetic association, which could be a good alternative method in the molecular phylogenetics of C. albicans and epidemiological studies. As for the clinical scenario, clade 17 isolates with CAI alleles either possessing 29 or more repetitions were related to higher 14-day and 30-day mortality, and shorter median survival days.

A population shift between two heritable cell types of the pathogen Candida albicans is based both on switching and selective proliferation

Differentiated cell types often retain their characteristics through many rounds of cell division. A simple example is found in Candida albicans, a member of the human microbiota and also the most prevalent fungal pathogen of humans; here, two distinct cell types (white and opaque) exist, and each one retains its specialized properties across many cell divisions. Switching between the two cell types is rare in standard laboratory medium (2% glucose) but can be increased by signals in the environment, for example, certain sugars. When these signals are removed, switching ceases and cells remain in their present state, which is faithfully passed on through many generations of daughter cells. Here, using an automated flow cytometry assay to monitor white-opaque switching over 96 different sugar concentrations, we observed a wide range of opaque-to-white switching that varied continuously across different sugar compositions of the medium. By also measuring white cell proliferation rates under each condition, we found that both opaque-to-white switching and selective white cell proliferation are required for entire populations to shift from opaque to white. Moreover, the switching frequency correlates with the preference of the resulting cell type for the growth medium; that is, the switching is adjusted to increase in environments that favor white cell proliferation. The widely adjustable, all-or-none nature of the switch, combined with the long-term heritability of each state, is distinct from conventional forms of gene regulation, and we propose that it represents a strategy used by C. albicans to efficiently colonize different niches of its human host.