Candida albicans infection Disease in Pakistan

Candida albicans infection Disease in Pakistan

Candida albicans is a type of yeast that is commonly used as a model organism for biology. It is generally referred to as a dimorphic fungus since it grows both as yeast and filamentous cells. However it has several different morphological phenotypes that are discussed more in detail in the chapter on Morphology. It is a common member of human gut flora and does not seem to proliferate outside mammalian hosts. It is detectable in the gastrointestinal tract and mouth in 40­60% of healthy adults. It is usually a commensal organism, but can become pathogenic in immunocompromised individuals under a variety of conditions. C. albicans, together with C. tropicalis, C. parapsilosis and C. glabrata, is responsible for 50–90% of all cases of candidiasis in humans. A mortality rate of 40% has been reported for patients with systemic candidiasis due to C. albicans. Estimates range from 2800 to 11200 deaths caused annually in the USA due to C. albicans causes candidiasis. C. albicans was for a long time considered an obligate diploid organism without a haploid stage. This is however not the case. Next to a haploid stage C. albicans can also exist in a tetraploid stage. The latter is formed when diploid C. albicans cells mate when they are in the opaque form. The diploid genome size is approximately 29Mb and up to 70% of the protein coding genes have not yet been characterized. C. albicans is easily cultured in the lab and can be studied both in vivo as in vitro.

Depending on the media different studies can be done as the media influences the morphological state of C. albicans. A special type of medium is CHROMagar™ Candida which can be used to identify different species of candida. Etymology Candida albicans can be seen as a tautology. Candida comes from the latin word candidus, meaning white. Albicans itself is the present participle of the latin word albicō, meaning becoming white. This leads to white becoming white, making it a tautology. It is often shortly referred to as thrush, candidiasis or candida. More than hundred synonyms have been used to describe C. albicans. Over 200 species have been described within the candida genus. The oldest reference to thrush, most likely caused by C. albicans, dates back to 400 B.C. in Hippocrates' work Of the Epidemics describing oral candidiasis. The most commonly used strains to study C. albicans are the WO­1 and SC5314 strains. The WO­1 strain is known to switch between white­opaque form with higher frequency while the SC5314 strain is the strain used for gene sequence reference. One of the most important features of the C. albicans genome is the high heterozygosity. At the base of this heterozygosity lies the occurrence of numeric and structural chromosomal rearrangements and changes as means of generating genetic diversity by chromosome length polymorphisms, reciprocal translocations, chromosome deletions, Nonsynonymous single­base polymorphisms and trisomy of individual chromosomes. These karyotypic alterations lead to changes in the phenotype, which is an adaptation strategy of this fungus. These mechanisms are further being explored with the availability of the complete analysis of the C. albicans genome. An unusual feature of the Candida genus is that in many of its species the CUG codon, which normally specifies leucine, specifies serine in these species. This is an unusual example of a departure from the standard genetic code, and most such departures are in start codons or, for eukaryotes, mitochondrial genetic codes. This alteration may, in some environments, help these Candida species by inducing a permanent stress response, a more generalized form of the heat shock response. However this different codon usage makes it more difficult to study C. albicans protein­protein interactions in the model organism S. cerevisiae.

To overcome this problem a C. albicans specific two­hybrid system was developed. The genome of C. albicans is highly dynamic, contributed by the different CUG translation, and this variability has been used advantageously for molecular epidemiological studies and population studies in this species. The genome sequence has allowed for identifying the presence of a parasexual cycle in C. albicans. This study of the evolution of sexual reproduction in six Candida species found recent losses in components of the major meiotic crossover­formation pathway, but retention of a minor pathway. Morphology C. albicans exhibits a wide range of different morphological phenotypes due to phenotypic switching and bud to hypha transition. The yeast to hyphae transition is a rapid process and induced by environmental factors. Phenotypic switching is spontaneous, happens at lower rates and in certain strains up to seven different phenotypes are known. The best studied switching mechanism is the white to opaque switching . Other systems have been described as well. Two systems were discover by David R. Soll and colleagues. Switching in C. albicans is often, but not always, influenced by environmental conditions such as the level of CO2, anaerobic conditions, medium used and temperature. Yeast to hyphae switching Although often referred to as dimorphic, C. albicans is in fact polyphenic . When cultured in standard yeast laboratory medium, C. albicans grows as ovoid "yeast" cells. However, mild environmental changes in temperature, CO2, nutrients and pH can result in a morphological shift to filamentous growth. Filamentous cells share many similarities with yeast cells. Both cell types seem to play a specific, distinctive role in the survival and pathogenicity of C. albicans. Yeast cells seem to be better suited for the dissemination in the bloodstream while hyphal cells have been proposed as a virulence factor. Hyphal cells are invasive and speculated to be important for tissue penetration, colonization of organs and surviving plus escaping macrophages. The transition from yeast to hyphal cells is termed to be one of the key factors in the virulence of C. albicans, however it is not deemed necessary. When C. albicans cells are grown in a medium that mimics the physiological environment of a human host, they grow as filamentous cells . Candida albicans can also form Chlamydospores, the function of which remains unknown, but it is speculated they play a role in surviving harsh environments as they are most often formed under unfavorable conditions. The cAMP­PKA signaling cascade is crucial for the morphogenesis and an important transcriptional regulator for the switch from yeast like cells to filamentous cells is EFG1. High frequency switching Besides the well studied yeast to hyphae transition other switching systems have been described. One such system is the "high frequency switching" system. During this switching different cellular morphologies are generated spontaneously. This type of switching does not occur en masse, represents a variability system and it happens independently from environmental conditions. The strain 3153A produces at least seven different colony morphologies. In many strains the different phases convert spontaneously to the other at a low frequency. The switching is reversible, and colony type can be inherited from one generation to another. While several genes that are expressed differently in different colony morphologies have been identified, some recent efforts focus on what might control these changes. Further, whether a potential molecular link between dimorphism and phenotypic switching occurs is a tantalizing question. Being able to switch through so many different phenotypes makes C. albicans able to grow in different environments and this both as a commensal as a pathogen. The white to opaque switching is an epigenetic switching system. Phenotypic switching is often used to refer to white­opaque switching, which consists of two phases: one that grows as round cells in smooth, white colonies and one that is rod­like and grows as flat, grey colonies . This switch from white cells to opaque cells is important for the virulence and the mating process of C. albicans as the opaque form is the mating competent form, being a million times more efficient in mating compared to the white type. This switching between white and opaque form is regulated by the WOR1 regulator which is controlled by the mating type locus repressor that inhibits the expression of WOR1. Besides the white and opaque phase there is also a third one: the gray phenotype. This phenotype shows the highest ability to cause cutaneous infections. The white, opaque and gray phenotypes form a tristable phenotypic switching system. Since it is often difficult to differentiate between white, opaque and gray cells phloxine B, a dye, can be added to the medium. A potential regulatory molecule in the white to opaque switching is Efg1p, a transcription factor found in the WO­1 strain that regulates dimorphism, and more recently has been suggested to help regulate phenotypic switching. Efg1p is expressed only in the white and not in the gray cell­type, and overexpression of Efg1p in the gray form causes a rapid conversion to the white form. White­GUT switch A very special type of phenotypic switch is the white­GUT switch . GUT cells are extremely adapted to survival in the digestive tract by metabolic adaptations to available nutrients in the digestive tract. The GUT cells live as commensal organisms and outcompete other phenotypes

The transition from white to GUT cells is driven by passage through the gut where environmental parameters trigger this transition by increasing the WOR1 expression. Role in disease Candida is found worldwide but most commonly compromises immunocompromised individuals diagnosed with serious diseases such as HIV and cancer. Candida are ranked as one of the most common groups of organisms that cause nosocomial infections. Especially high risk individuals are patients that have recently undergone surgery, a transplant or are in the Intensive Care Units, Candida albicans infections is the top source of fungal infections in critically ill or otherwise immuncompromised patients. These patients predominantly develop oropharyngeal or thrush candidiasis, which can lead to malnutrition and interfere with the absorption of medication. Methods of transmission include mother to infant through childbirth, people­to­people acquired infections that most commonly occur in hospital settings where immunocompromised patients acquire the yeast from healthcare workers and has a 40% incident rate. Men can become infected after having sex with a woman that has an existing vaginal yeast infection. Some distinguishing features also include, discharge, dry and, red appearance of vaginal mucosa or skin. Candida continues to be the fourth most commonly isolated organism in bloodstream infections. Superficial and local infections It commonly occurs, as a superficial infection, on mucous membranes in the mouth or vagina. Once in their life around 75% of women will suffer from vulvovaginal candidiasis and about 90% of these infections are caused by C. albicans. It however may also affect a number of other regions. For example, higher prevalence of colonization of C. albicans was reported in young individuals with tongue piercing, in comparison to unpierced matched individuals. To infect host tissue, the usual unicellular yeast­like form of C. albicans reacts to environmental cues and switches into an invasive, multicellular filamentous form, a phenomenon called dimorphism. In addition, an overgrowth infection is considered superinfection, usually applied when an infection become opportunistic and very resistant to antifungals. It then becomes suppressed by antibiotics. The infection is prolonged when the original sensitive strain is replaced by the antibiotic­resistant strain. Candidiasis is known to cause GI symptoms particularly in immunocompromised patients or those receiving steroids or antibiotics. Recently, there is emerging literature that an overgrowth of fungus in the small intestine of non­immunocompromised subjects may cause unexplained GI symptoms. Small intestinal fungal overgrowth is characterized by the presence of excessive number of fungal organisms in the small intestine associated with gastrointestinal symptoms. The most common symptoms observed in these patients were belching, bloating, indigestion, nausea, diarrhea, and gas. The underlying mechanism that predisposes to SIFO is unclear. Further studies are needed; both to confirm these observations and to examine the clinical relevance of fungal overgrowth. Systemic infections Systemic fungal infections including those by C. albicans have emerged as important causes of morbidity and mortality in immunocompromised patients . C. albicans often forms biofilms inside the body. Such C. albicans biofilms may form on the surface of implantable medical devices or organs. In these biofilms it is often found together with Staphylococcus aureus. Such multispecies infections lead to higher mortalities. In addition hospital­acquired infections by C. albicans have become a cause of major health concerns. Although Candida albicans is the most common cause of candidemia, there has been a decrease in the incidence and an increases isolation of non­albicans species of Candida in recent years. Preventive measures include keeping a healthy lifestyle including good nutrition, proper nutrition, and careful antibiotic use. Economical implications Given the fact that candidiasis is the fourth most frequent hospital acquired infection worldwide it leads to immense financial implications. Approximately 60000 cases of systemic candidiasis each year in the USA alone lead up to a cost to be between $2­4 6/12/2017 billion. The total costs for candidiasis are among the highest compared to other fungal infections due to the high prevalence. The immense costs are partly explained by a longer stay in the intensive care unit or hospital in general. An extended stay for up to 21 more days compared to non infected patients is not uncommon. Proteins important for pathogenesis Hwp1 Hwp1 stands for Hyphal wall protein 1. Hwp1 is a mannoprotein located on the surface of the hyphae in the hyphal form of Candida albicans. Hwp1 is a mammalian transglutaminase substrate. This host enzyme allows Candida albicans to attach stably to host epithelial cells. Adhesion of Candida albicans to host cells is an essential first step in the infection process for colonization and subsequent induction of mucosal infection. Slr1 RNA­binding protein Slr1 was recently discovered to play a role in instigating the hyphal formation and virulence in C. albicans. Candidalysin Candidalysin is a cytolytic 31­amino acid α­helical peptide toxin that is released during hyphal formation. It contributes to virulence during mucosal infections. Genetic and genomic tools Due to his nature as a model organism, being an important human pathogen and the alternative codon usage, several specific projects and tools have been created to study C. albicans. ORFeome project Every predicted ORF has been created in a gateway adapted vector and made publicly available. The vectors can be propagated in E.coli and grown on LB+gentamicin medium. This way every ORF is readily available in an easy to use vector. Using the gatewaysystem it is possible to transfer the ORF of interest to any other gateway adapted vector for further studies of the specific ORF. Microarrays Both DNA and protein microarrays were designed to study DNA expression profiles and antibody production in patients against C. albicans cell wall proteins. GRACE library Using a tetracycline­regulatable promoter system a gene replacement and conditional expression library was created for 1152 genes. By using the regulatable promoter and having deleted 1 of the alleles of the specific gene it was possible to discriminate between nonessential and essential genes. Of the tested 1152 genes 567 showed to be essential. The knowledge on essential genes can be used to discover novel antifungals. Application in engineering Candida albicans has been used in combination with carbon nanotubes to produce stable electrically conductive bio­nano­composite tissue materials that have been used as temperature sensing elements Treatment Treatment commonly includes: amphotericin B, echinocandin, or fluconazole for systemic infections Nystatin for oral and esophageal infections Clotrimazole for skin and genital yeast infections Important to note is that similar to antibiotic resistance, resistance to many anti­fungals is becoming a big problem. New anti­fungals have to be developed to cope with this problem since only a limited number of anti­fungals are available.