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Thesis (PhD (Microbiological Pathology) --University of Limpopo (Medunsa Campus), 2010 / Background: Azoles and polyenes are antifungal agents used for treatment and/or prophylaxis of C. albicans infections, and a high increase in antifungal resistance in clinical isolates of C. albicans in HIV/AIDS patients has been reported. Five genetic clades were described among C. albicans isolates using DNA fingerprinting methods (clades I, II, III, SA and NG). Although these clades have been described, little is known about their phenotypic characteristics, and not much is known about antifungal resistance with regard to each of these clades. The widespread use of fluconazole has led to its increased resistance reported world-wide. Resistance to fluconazole can be caused by point mutations in the ERG11 gene or overexpression of this gene, however, not much is known about the contribution of these mutations and over-expression to fluconazole resistance among different clades of C. albicans, and whether mutations or over-expression are clade-related. There is evidence to suggest that phenotypic switching may play a significant role in the ability of Candida strains to survive under adverse conditions and perhaps cause more severe forms of disease in the immunocompromised host (Vargas et al., 2004). Only limited studies on the relationship between phenotypic switching and fluconazole resistance of C. albicans have been done, and not much is known about this relationship among different clades of C. albicans. Objectives: This study undertook to investigate: (1) the induction of antifungal resistance among South African C. albicans isolates belonging to different clades, (2) the contributions of mutations in the ERG11 gene to fluconazole resistance among C. albicans isolates belonging to different clades, (3) the contributions of over-expression of ERG11 gene to fluconazole resistance among C. albicans isolates belonging to different clades, (4) and the relationship between fluconazole resistance and phenotypic switching among C. albicans isolates belonging to different clades. Study populations and Methods: To investigate the induction of antifungal resistance among South African C. albicans isolates belonging to different clades, a total of 100 C. albicans isolates (20 from each of clades I, II, III, SA and NG) were used. These yeast isolates were obtained from surveillance cultures on patients attending HIV/AIDS clinics in the Pretoria region. Resistance to fluconazole, miconazole, amphotericin B and nystatin was induced in all 100 isolates according to the modified National Committee of Clinical vi Laboratory Standards (NCCLS) broth microdilution method. Survival and retention of resistance among fluconazole resistant (n=100), miconazole resistant (n=100), amphotericin B resistant (n=100) and nystatin resistant (n==100) isolates after two years of storage at -80oC was determined in the presence of highest concentrations of each antifungal. To investigate the contributions of mutations in the ERG11 gene to fluconazole resistance among C. albicans isolates belonging to different clades, 30 isolates were used. These consisted of 3 isolates with induced fluconazole resistance and their 3 matching fluconazole susceptible isolates from each of clades I, II, III, SA, and NG. DNA was extracted, PCR performed with a high-fidelity Pwo DNA polymerase), and PCR products sequenced using BigDye® Terminator v3.1 Cycle Sequencing Kit on the GeneAmp® PCR System 9700. Obtained sequences were compared with the published ERG11 sequence from a wild-type, fluconazole-susceptible C. albicans strain (Lai and Kirsch, 1989). To investigate the contributions of over-expression of the ERG11 gene to fluconazole resistance among C. albicans isolates belonging to different clades, 30 isolates were used. These consisted of 3 isolates with induced fluconazole resistance and their 3 matching fluconazole susceptible isolates from each of clades I, II, III, SA, and NG. RNA was extracted, cDNA synthesized and Real time PCR performed on a Rotor-Gene 6000 instrument. Relative gene expression of ERG11 gene among resistant isolates, relative to susceptible isolates was quantified after normalization with the 18SrRNA house-keeping gene. To investigate the relationship between fluconazole resistance and phenotypic switching among C. albicans isolates belonging to different clades, 30 isolates were used. These consisted of 3 isolates with induced fluconazole resistance and their 3 matching fluconazole susceptible isolates from each of clades I, II, III, SA, and NG. Primary and secondary cultures were prepared on Lee’s medium agar supplemented with arginine and zinc, and containing phloxine B. The switched colonies were counted and colony morphologies viewed and photographed. Phenotypic switching behavior and different colony morphologies obtained between the resistant and susceptible isolates from different clades were compared. Switch phenotypes among fluconazole resistant isolates in different clades were compared. Switch phenotypes and MIC levels among fluconazole resistant isolates from different clades were compared. Results: Resistance to nystatin, AmB, fluconazole and miconazole was successfully induced in all of 20 (100%) C. albicans isolates from each of clades I, II, III, SA and NG. When survival and retention of resistance were determined, all 20 (100%) fluconazole resistant vii isolates from clades I, II, SA, NG, and 19 (95%) from clade III survived and retained their resistance. Of miconazole resistant isolates, all 20 (100%) isolates from clade I, II, and SA, and 19 (95%) from clade III and NG survived and retained their resistance. Of AmB resistant isolates, 12 (60%) from Clade NG survived and retained their resistance; 9 (45%) from Clade I; 8 (40%) from Clade III; 7 (35%) from Clade II and 6 (30%) from Clade SA survived and retained their resistance. Of the isolates resistant to nystatin, 12 (60%) from clade I survived and retained their resistance, 8 (40%) from clade II, 10 (50%) from clade III, 11 (55%) from clade SA, and 15 (75%) from clade NG survived and retained their resistance. No mutations associated with fluconazole resistance were observed in all isolates from clades I and II. Mutations associated with fluconazole resistance were observed in 33.3% of isolates from each of clades III, SA and NG , and some of the mutations observed in resistant isolates from clades III and NG were novel. A total of 50 novel mutations that have not been described previously were observed in both fluconazole resistant and susceptible isolates from this study. Previously described mutations, which were associated with fluconazole resistance, namely, D116E, K128T, V437I and E266D were also observed in this study. When relative ERG11 gene expression was quantified among fluconazole resistant and susceptible isolates from various clades, over-expression of ERG11 gene was observed in 66.6% of isolates from each of clades I, II and SA, and in 33.3% of isolates from each of clades III and NG. When the relationship between fluconazole resistance and phenotypic switching was investigated, phenotypic switching was related to resistance in 66.6% of the resistant isolates tested from each of clades I, II and III, and in 33.3% of the resistant isolates tested from each of clades SA and NG. When the switch phenotypes and MIC levels of resistant isolates from different clades were compared, stipple was the most common switch phenotype observed in all clades, and it was associated with the highest fluconazole MIC levels among isolates from all clades. Conclusions: The results of this study showed that resistance to polyenes and azoles could readily be induced in C. albicans isolates from all clades, and that induction was not claderelated. The ease with which azole and polyene resistance could be induced in this study can hold serious implications, especially in HIV/AIDS patients who are already immunocompromised, and in whom azoles/polyenes are mostly used for C. albicans infections. viii The study also showed that mutations contributed to fluconazole resistance in isolates from clades III, SA and NG, but not clades I and II, showing clade-relatedness. Novel mutations were observed, and their contribution to fluconazole resistance is at this stage not known. Genetic analysis of these mutations needs to be studied further to determine their significance to azole resistance, especially in C. albicans isolates from HIV/AIDS patients in South Africa. The results of the study showed that over-expression of ERG11 gene contributed to fluconazole resistance in isolates from all clades. However, over-expression was observed in more isolates from clades I, II and SA, and in less isolates from clades III and NG, showing clade-relatedness of ERG11 over-expression. The occurrence of over-expression of ERG11 gene in these clades is a cause for concern, especially in HIV/AIDS patients with OPC, as the increased expression of ERG11 allows for the cells to persist within the host, which in turn leads to the subsequent development of other more stable resistant isolates. In this study, phenotypic switching was found to be related to fluconazole resistance in isolates from all clades, with a high number of switch phenotypes occurring more in isolates from clade II as compared to others. This suggests that isolates belonging to this clade may survive better under adverse conditions than isolates from other clades. These results suggest that further study of differences between different C. albicans clades may be warranted, and that isolates from this clade need to be studied further. The stipple phenotype was found to be the most dominant in isolates from all clades, and was found to be associated with the highest fluconazole MICs levels. These findings suggest that the evaluation of colony phenotypes and their antifungal susceptibilities in C. albicans isolates may be useful in therapy. Recommendations: A continued analysis of clade-specific phenotypic characteristics of C. albicans isolates is recommended. Pathogens that can potentially infect HIV-infected individuals need to be studied to subspecies level in order to improve treatment of these patients. Continued antifungal surveillance is needed to predict the evolution of resistance in a particular population and to take timely measures. Evaluation of colony phenotypes and their antifungal susceptibilities in C. albicans isolates is recommended as this may be useful in therapy. Genetic analysis of the novel mutations observed is recommended to determine their significance to azole resistance, especially in C. albicans isolates from HIV/AIDS patients in South Africa.

El propósito del presente fue de identificar las especies de Candida implicadas en estomatitis subprotésica en pacientes del Departamento de Odontoestomatología del Centro Médico Naval “CMST” – 2007 Se analizaron los 30 primeros pacientes con diagnóstico de estomatitis subprotésica que acudían al Departamento, a los cuales se les realizó cuatro frotises, dos para el examen directo microscópico (con coloración Gram) para confirmar la presencia de levaduras, y dos para el cultivo en Agar Sabouraud+Cloranfenicol, del crecimiento en este agar, se hizo la prueba del tubo germinal para determinar la presencia de Candida albicans, de salir negativo esta prueba, se llevaba a cabo la identificación de la especie mediante el sistema Api Candida. Se obtuvieron entre otros resultados que Candida albicans fue la especie más implicada en la estomatitis subprotésica con un 96.66% seguido de Candida tropicalis con un 3.33%. / The purpose of this was to identify the species of Candida involved in stomatitis subprotésica in patients of the Department of Dental Naval Medical Center "CMST" - 2007 We analyzed the first 30 patients diagnosed with stomatitis subprotésica who went to the Department, to which were conducted four frotises, two for the direct microscopic examination (colouring Gram) to confirm the presence of yeast, and two to be planted in Agar Sabouraud + Chloramphenicol, growth in the agar, it was the germ tube test to determine the presence of Candida albicans, to leave this negative test was carried out to identify the species by the API system Candida. Among other results were obtained that Candida albicans was the most involved in stomatitis subprotésica with a 96.66% followed by Candida tropicalis with a 3.33%.

Thesis supervisor: David R. Soll. Includes bibliographic references (p. 280-316).

published_or_final_version / Medicine / Master / Master of Medical Sciences

Chitin biosynthesis in the fungal cell wall is essential for cell viability. Chitin play roles in maintaining cellular integrity and up regulation of chitin synthesis helps protect the cell from cell wall stresses or cell wall damaging antifungal agents such as caspofungin. In Candida albicans chitin is synthesised by four chitin synthases (Chs), Chs1, Chs2, Chs3, and Chs8. The biological function of Chs2 and Chs8 (class I chitin synthases) is less well understood. The major objective of this thesis was to understand the biological function of the class I chitin synthases. Chs2-YFP and Chs8-YFP showed a dynamic localisation at septation sites, which were first visualised as a bar which then contracted to a spot. This spot then separated into two spots, one on each sides of the septum. These two spots remained there until yeast cell separation, and remained at this location throughout several subsequent hyphal cell cycles. Chs2-YFP also localised to hyphal tips. The phenotype of a chs2Δchs8Δ double mutant was re-investigated using the propidium iodide. Intact dead germ tubes and hyphal tip lysis was observed in a chs2Δchs8Δ mutant cells. This suggested that Chs2 and Chs8 play a major role in the maintenance of hyphal tip integrity and polarised growth and perhaps a minor role in septum formation. Studies were also performed to assess whether phosphorylation regulated the localisation of Chs2-YFP. It was shown that the localisation of Chs2-YFP to septation sites was regulated by phosphorylation on S222. A version of Chs2-YFP that could not be phosphorylated (Chs2S222A-YFP) localised at the septa in lower amounts than the Chs2-YFP, and a version of Chs2-YFP that mimicked constitutive phosphorylation (Chs2S222E-YFP) localised normally. This suggested that phosphorylation of Chs2 on S222 facilitates the localisation of Chs2-YFP at septation sites, and that dephosphorylation is not required for this cellular localisation. In the presence of cell wall stresses (CaCl2/CFW) and caspofungin, more Chs2-YFP was observed and the average intensity of fluorescence of Chs2-YFP was higher in the presence of these stresses than in untreated cells.

Fungal nitrosative stress responses are understudied and poorly understood. Also, while the responses of the pathogen Candida albicans to individual oxidative and cationic stresses have been characterised relatively well, the responses of this pathogen to simultaneous combinations of these and nitrosative stresses have not been elaborated. Yet, C. albicans is exposed to these combinatorial stresses during disease progression. Therefore, the overall aim of this project was to characterise the "Nitrosative and Combinatorial Stress Responses in Candida albicans". The first objective was to elucidate nitrosative stress response mechanisms in C. albicans at a cellular, transcriptional and post-translational level. Genetic screens revealed that the Hog1 and Mkc1 signalling pathways contribute to nitrosative stress adaptation in addition to Cta4 signalling. The next objective was to characterise responses to combinatorial nitrosative, oxidative and/or cationic stresses with respect to the relevant signal transduction pathways. Unexpectedly, combinatorial stresses did not activate classical nitrosative (Cta4) and oxidative (Cap1) signalling mechanisms. Instead, Hog1 signalling was found to play a significant role in the combinatorial stress adaptation. Dramatic changes in the GSH redox potential were observed in response to both single and combinatorial stresses. Thus, GSH redox regulatory mechanisms were explored in more detail. BLAST searches of the C. albicans genome were performed, revealing genes encoding a putative NADPH-dependent glutathione reductase (Glr1) and a GSH-dependent S-nitrosoglutathione reductase (Fdh3). We confirmed that both conserved enzymes are involved in the maintenance of redox homeostasis and hence are crucial for C. albicans adaptation during host-pathogen interactions. This was achieved by showing that inactivation of Glr1 or Fdh3 disturbs GSH redox homeostasis, confers oxidative and formaldehyde stress sensitivity and attenuates virulence.

Candida albicans is the most common systemic fungal pathogen of humans. Virulence factors, fitness attributes and the host immunity ultimately determine the ability of C. albicans to reside as a commensal organism or an opportunistic pathogen in its human host. C. albicans displays a high level of flexibility in adapting to and infecting diverse niches in its human host. Generally occupying glucose-poor niches, C. albicans often depends upon alternative carbon sources to grow and establish infections. However, most cellular and molecular investigations of this pathogen have been performed on glucose-grown cells, and the relationship between physiological nutrients and drug resistance or virulence has not been studied. The main objective of this thesis was to study the effects of host carbon sources upon C. albicans cell biology and how this in turn affects the stress and drug resistance, immune recognition and virulence of this fungus. Growth on this physiologically relevant alternative carbon source was shown to influence the cell wall architecture of C. albicans and alter its resistance to a variety of stresses and antifungal drugs. Differences in stress resistance were less dependent on the key stress signalling pathways than on major architectural changes in the C. albicans cell wall that influenced its biochemical and biophysical properties. It was also shown that carbon source has a major impact upon C. albicans pathogenicity, altering cell adhesion and host virulence, in both systemic and vaginal infections. Considering the different niches that Candida can colonise in the body, these findings have the potential to change our views about the interactions that occur between the fungus and its human host. Taken together, the results of this project demonstrate that differential nutrient availability within diverse host niches influences the ability of pathogenic Candida species to counteract local chemical insults and pharmacological interventions.

No description available.

published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy