The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida; Antifungal immunity; MicrobiotaCándida; Inmunidad antimicótica; MicrobiotaCàndida; Immunitat antifúngica; MicrobiotaCandida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients

The pathogenic and colonization potential of Candida africana

The Candida albicans population displays high genetic diversity illustrated by 18-well differentiated genetic clusters. Cluster 13, also known as Candida africana, is an outlying cluster and includes strains first described as atypical C. albicans isolates of vaginal origin, showing apparent tropism for the female genital tract. In our study, we combined in vitro, and in vivo models to explore the colonization and pathogenic potential of C. africana. We report that C. africana has similar fitness to C. albicans when it comes to colonization of the oral and vaginal mucosa, however it has decreased fitness in gastro-intestinal colonization and systemic infection. Interestingly, despite high population homogeneity, our in vitro data highlighted for the first time a variability in terms of growth rate, biofilm formation and filamentation properties between C. africana strains. Overall, our data lays the foundations for exploring specific features of C. africana that might contribute to its apparent niche restriction

CandidaDB: A genome database for Candida albicans pathogenomics

CandidaDB is a database dedicated to the genome of the most prevalent systemic fungal pathogen of humans, Candida albicans. CandidaDB is based on an annotation of the Stanford Genome Technology Center C.albicans genome sequence data by the European Galar Fungail Consortium. CandidaDB Release 2.0 (June 2004) contains information pertaining to Assembly 19 of the genome of C.albicans strain SC5314. The current release contains 6244 annotated entries corresponding to 130 tRNA genes and 5917 protein-coding genes. For these, it provides tentative functional assignments along with numerous pre-run analyses that can assist the researcher in the evaluation of gene function for the purpose of specific or large-scale analysis. CandidaDB is based on GenoList, a generic relational data schema and a World Wide Web interface that has been adapted to the handling of eukaryotic genomes. The interface allows users to browse easily through genome data and retrieve information. CandidaDB also provides more elaborate tools, such as pattern searching, that are tightly connected to the overall browsing system. As the C.albicans genome is diploid and still incompletely assembled, CandidaDB provides tools to browse the genome by individual supercontigs and to examine information about allelic sequences obtained from complementary contigs. CandidaDB is accessible at http://genolist.pasteur.fr/CandidaDB.Sequence data from C.albicans were obtained from the Stanford Genome Technology Center (http://www.sequence. stanford.edu/group/candida). Sequencing of C.albicans was accomplished with the support of the NIDR and the Burroughs Wellcome Fund. This work was supported by grants from the European Commission (QLK2-2000-00795; MCRTN-CT-2003-504148; ‘Galar Fungail Consortium’) to A.J.P.B., C.E., A.D., J.E., C.G., B.H., F.M.K., J.P.M. and R.S. and the Ministere de la Recherche et de la Technologie (PRFMMIP ‘Re´seau Infections Fongiques’) to C.E. and C.G. F.T. was supported by the Institut Pasteur Strategic Horizontal Program on Anopheles gambiae. N.M. was supported by a fellowship of the Junta de Castilla y Leon and by grants DGCYT (PM-98-0317 and BIO 2002-02124) to A.D. R.S. was supported in part by grants from the Spanish Ministerio de Ciencia y Tecnologia (BMC2003- 01023) and Agencia Valenciana de Ciencia i Tecnologia de la Generalitat Valenciana (Grupos 03/187)

Gymnemic acids inhibit hyphal growth and virulence in Candida albicans

Candida albicans is an opportunistic and polymorphic fungal pathogen that causes mucosal, disseminated and invasive infections in humans. Transition from the yeast form to the hyphal form is one of the key virulence factors in C. albicans contributing to macrophage evasion, tissue invasion and biofilm formation. Nontoxic small molecules that inhibit C. albicans yeast-to-hypha conversion and hyphal growth could represent a valuable source for understanding pathogenic fungal morphogenesis, identifying drug targets and serving as templates for the development of novel antifungal agents. Here, we have identified the triterpenoid saponin family of gymnemic acids (GAs) as inhibitor of C. albicans morphogenesis. GAs were isolated and purified from Gymnema sylvestre leaves, the Ayurvedic traditional medicinal plant used to treat diabetes. Purified GAs had no effect on the growth and viability of C. albicans yeast cells but inhibited its yeast-to-hypha conversion under several hypha-inducing conditions, including the presence of serum. Moreover, GAs promoted the conversion of C. albicans hyphae into yeast cells under hypha inducing conditions. They also inhibited conidial germination and hyphal growth of Aspergillus sp. Finally, GAs inhibited the formation of invasive hyphae from C. albicans-infected Caenorhabditis elegans worms and rescued them from killing by C. albicans. Hence, GAs could be useful for various antifungal applications due to their traditional use in herbal medicine

Hidden killers: persistence of opportunistic fungal pathogens in the human host.

International audienceOpportunistic fungal pathogens are responsible for life-threatening systemic infections in immunocompromized individuals. Yet, they are also able to persist in immunocompetent individuals through different strategies. This review explores recent advances in our understanding of several survival strategies: the establishment of a commensal relationship between yeast of the genus Candida and the host; the formation of biofilms that allow microbes in these communities to be protected from chemical and cellular attacks; and the persistence of airborne pathogens within macrophages following primary infection. While research has concentrated on deciphering virulence factors of pathogenic fungi, additional understanding of how fungal pathogens persist in healthy hosts might help us design new strategies to prevent the transition from harmless interactions to devastating infections

Candida albicans biofilms: building a heterogeneous, drug-tolerant environment.

International audienceFungi are able to form biofilms on medical implants, causing serious infections. A better understanding of fungal biofilm formation is necessary to develop tools for detection or prevention and to identify new antifungal strategies. This review explores recent advances in the characterization at the molecular level of fungal biofilms, especially those formed by the yeast Candida albicans: the identification of complex transcriptional networks that control their formation; the pivotal role of the extracellular matrix in biofilm antifungal tolerance; and the knowledge gained on the physiology of biofilm cells and heterogeneity within these communities. These findings may help develop new, targeted therapeutic strategies

Tracing the Origin of Invasive Fungal Infections

International audienceInvasive fungal infections are a major cause of mortality in immunocompromised patients. By using high-resolution sequencing, Zhai et al. provide insight into translocation of Candida strains from the gut mycobiota to the bloodstream of transplanted patients. Microbiota-driven diagnostic methods could rapidly emerge for preventing deadly fungal infections

Use of CRISPR-Cas9 To Target Homologous Recombination Limits Transformation-Induced Genomic Changes in Candida albicans

International audienceMost of our knowledge relating to molecular mechanisms of human fungal pathogenesis in Candida albicans relies on reverse genetics approaches, requiring strain engineering. DNA-mediated transformation of C. albicans has been described as highly mutagenic, potentially accentuated by the organism's genome plasticity, including the acquisition of genomic rearrangements, notably upon exposure to stress. The advent of CRISPR-Cas9 has vastly accelerated the process of genetically modifying strains, especially in diploid (such as C. albicans) and polyploid organisms. The effects of unleashing this nuclease within the genome of C. albicans are unknown, although several studies in other organisms report Cas9-associated toxicity and off-target DNA breaks. Upon the construction of a C. albicans strain collection, we took the opportunity to compare strains which were constructed using CRISPR-Cas9-free and CRISPR-Cas9-dependent transformation strategies, by quantifying and describing transformation-induced loss-of-heterozygosity and hyperploidy events. Our analysis of 57 strains highlights the mutagenic effects of transformation in C. albicans, regardless of the transformation protocol, but also underscores interesting differences in terms of genomic changes between strains obtained using different transformation protocols. Indeed, although strains constructed using the CRISPR-Cas9-free transformation method display numerous concomitant genomic changes randomly distributed throughout their genomes, the use of CRISPR-Cas9 leads to a reduced overall number of genome changes, particularly hyperploidies. Overall, in addition to facilitating strain construction by reducing the number of transformation steps, the CRISPR-Cas9-dependent transformation strategy in C. albicans appears to limit transformation-associated genome changes.IMPORTANCE Genome editing is essential to nearly all research studies aimed at gaining insight into the molecular mechanisms underlying various biological processes, including those in the opportunistic pathogen Candida albicans The adaptation of the CRISPR-Cas9 system greatly facilitates genome engineering in many organisms. However, our understanding of the effects of CRISPR-Cas9 technology on the biology of C. albicans is limited. In this study, we sought to compare the extents of transformation-induced genomic changes within strains engineered using CRISPR-Cas9-free and CRISPR-Cas9-dependent transformation methods. CRISPR-Cas9-dependent transformation allows one to simultaneously target both homologs and, importantly, appears less mutagenic in C. albicans, since strains engineered using CRISPR-Cas9 display an overall decrease in concomitant genomic changes

Bioluminescent fungi for real-time monitoring of fungal infections.

International audienceNovel luciferase reporters have been developed that allow real-time monitoring of infections by the fungal pathogens Candida albicans and Aspergillus fumigatus. Although these reporters still suffer limitations in the context of invasive infections, they provide unprecedented tools to monitor superficial infections and the efficacy of antifungal drugs or vaccines. In particular, the sensitivity and ease of detection of the cell-surface Gaussia princeps luciferase developed for C. albicans should make it a powerful tool for functional genomics studies in this and other pathogenic fungi