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- •Foreword
- •Preface
- •Contents
- •Editors and Contributors
- •About the Editors
- •Contributors
- •1.1 Introduction
- •1.2 Conventional Methods
- •1.2.1 Microscopy
- •1.2.2 Culture
- •1.2.3 Germ Tube Test
- •1.2.5 Carbohydrate Assimilation Test
- •1.2.6 Nitrogen Assimilation Test
- •1.2.7 Carbohydrate Fermentation Test
- •1.2.8 Urease Test
- •1.2.9 Tween 80 Opacity Test
- •1.3 Nonculture-Based Conventional Methods
- •1.3.1 Serological Methods
- •1.3.1.2 ß-d-Glucan
- •1.3.1.3 C. albicans Germ Tube Antibody Assay (CAGTA)
- •1.4 Nucleic Acid-Based Detection
- •1.4.1 Polymerase Chain Reaction (PCR)
- •1.4.3 Peptide Nucleic Acid FISH (PNA-FISH)
- •1.4.4 PCR-Based Innovative Diagnosis
- •1.4.5 FilmArray System
- •1.4.6 Sepsis Flow Chip
- •1.4.7 ePlex System
- •1.4.8 The T2 Candida Assay
- •1.5 Rapid Identification Systems
- •1.5.1 Manual Rapid Identification System
- •1.5.1.1 The API System
- •1.5.1.2 The VITEK System
- •1.5.2 Automatic Rapid Identification System
- •1.5.2.1 MALDI-TOF MS
- •1.5.2.2 The MALDI Sepsityper IVD Kit
- •1.5.2.3 The BioFire FilmArray BCID2 Panel
- •1.5.2.4 The Accelerate Pheno BC Panel
- •1.6 Advanced Diagnostics
- •1.6.2 Biosensor-Based Tests
- •1.6.3 Next-Generation Sequencing (NGS)
- •1.7 Conclusion
- •References
- •2.1 Introduction
- •2.2.1.2 Echinocandins
- •First-Generation Echinocandin
- •Second-Generation Echinocandin
- •2.2.1.3 Other Cell Wall Inhibitors
- •2.2.2.1 Azoles
- •Imidazole
- •Triazole
- •Second-Generation Azole
- •Third-Generation Azole
- •2.2.2.2 Polyenes
- •Other Polyene Under Development
- •2.2.2.3 Allylamines
- •2.2.3 Flucytosine
- •2.3 Conclusion
- •References
- •3.1 Introduction
- •3.2.1 Control Diet
- •3.2.3 Toxification
- •3.2.4 Alternative Treatments
- •3.3.1 Prophylaxis
- •3.3.2 Preemptive Therapies
- •3.3.3 Empirical Therapies
- •3.4 Therapeutic Approach
- •3.4.1 Azoles
- •3.4.2 Echinocandins
- •3.4.3 Polyenes
- •References
- •4.1 Introduction
- •4.3 Eukarya Domain
- •4.4.1 Cryptococcus
- •4.4.2 Aspergillus
- •4.4.3 Mucorales
- •4.4.4 Candida
- •4.5.1 Candida albicans
- •4.5.2 Morphogenesis
- •4.5.3 Pathogenesis
- •4.5.4 Adherence
- •4.5.5 Morphological Switching
- •4.5.6 Invasion
- •4.6 Induced Endocytosis
- •4.7 Active Penetration
- •4.8.2 Biofilm Formation
- •4.8.4.1 Antifungals
- •4.8.4.2 Antifungal Resistance
- •References
- •5.1 Introduction
- •5.2.3.1 Serum
- •5.2.3.2 Low Nitrogen
- •5.2.3.5 Carbon Source
- •5.2.3.6 pH
- •5.2.3.7 N-acetylglucosamine (GlcNAc)
- •5.2.3.8 Quorum Sensing Molecule
- •5.5.5 Surface Colonization Factor1 (SCF1)
- •5.5.6 Other Putative Adhesins
- •5.6.1 Phospholipases
- •5.6.2 Proteinases
- •5.6.3 Hemolysins
- •5.6.4 Lipases
- •5.7 Secreted Cytolytic Peptide: Candidalysin
- •5.5.1 ALS Family
- •5.5.2 HWP Adhesin
- •5.5.3 HYR/IFF Family
- •5.5.4 EPA Family
- •5.9.2 Low Molecular Weight Hsp/Small Heat Shock Proteins
- •5.10.1 Amino Acid/Nitrogen Metabolism
- •5.10.1.1 Amino Acid Sensing Pathway
- •5.12.1.1 Glycolysis
- •5.12.1.2 Gluconeogenesis
- •5.12.1.3 Glyoxylate Cycle
- •5.12.1.4 Fatty Acid Oxidation
- •5.12.3.2 Iron Metabolism
- •5.12.3.3 Candida Iron Transport
- •5.12.3.4 Reductive System
- •5.12.3.5 Siderophore Uptake System
- •5.12.3.6 Haemoglobin-Iron Uptake System
- •5.13.2 Zinc Metabolism
- •References
- •6.1 Introduction
- •6.2 Morphological Switching
- •6.3 Phenotypic Switching
- •6.4 Biofilm Formation
- •6.5 Metabolic Flexibility
- •6.8.1 Hemolysin
- •6.8.2 Phospholipases
- •6.8.3 Proteinase
- •6.8.4 Candidalysin
- •6.12 Conclusion
- •References
- •7.1 Introduction
- •7.2.4 Polymorphism
- •7.2.5.1 Secreted Aspartyl Proteinases
- •7.2.5.2 Phospholipase
- •7.2.6 Calcineurin-Signalling Pathway
- •7.2.7 Ion Homeostasis
- •7.2.7.1 Iron
- •7.2.7.2 Copper
- •7.2.8.1 Capsule
- •7.2.8.2 Melanin
- •7.2.8.3 Heat Shock Proteins
- •7.3 Conclusions
- •References
- •8.1 Introduction
- •8.4.1 ATP-Binding Cassette (ABC) Transporters
- •8.4.2 Major Facilitator Superfamily (MFS) Transporter
- •8.5.1 Biofilm Architecture Among Candida Species
- •References
- •9.1 Introduction
- •References
- •10.1 Introduction
- •10.3 Biofilm
- •10.5 Adherence
- •10.6 Maturation
- •10.8 Dispersion
- •10.11 Animal Models
- •10.18 Photodynamic Therapy
- •References
- •11.1 Introduction
- •11.9 Concluding Remarks
- •References
- •12.1 Introduction
- •12.2 Epidemiology
- •12.3.1 Humoral Response
- •12.3.2 Cellular Immunity
- •12.4 Virulence Factors
- •12.6.1 Fluconazole
- •12.6.2 Polyenes
- •12.6.3 Echinocandins
- •12.7 Drug Resistance
- •12.8 Future Prospects
- •12.9 Conclusions
- •References
- •13.1 Introduction
- •13.4 Translation Research
- •13.4.1 Disease-Oriented Translational Research
- •13.4.2 Lab-Oriented Translational Research
- •13.4.3 Patient-Oriented Translational Research
- •13.5 Conclusion
- •References
- •14.1 Introduction
- •14.2.3 Cutaneous Aspergillosis
- •14.2.4 Ocular Aspergillosis
- •14.2.5 Aspergillus Endocarditis
- •14.2.6 Aspergillus Osteomyelitis
- •14.2.7 Sinus Aspergillosis
- •14.3.2 Histopathology
- •14.3.3 Serological
- •14.3.4 Breath Testing
- •14.3.5 Monoclonal Antibody (mAbs)-Mediated Methods
- •14.4.1 Conventional Therapeutics
- •14.4.1.1 Azoles
- •14.4.1.2 Polyenes
- •14.4.1.3 Echinocandins
- •14.4.1.4 Fluoropyrimidines
- •14.5 Nonconventional Therapeutics
- •14.5.1 Vaccine
- •14.5.2 Monoclonal Antibodies (mAbs)
- •14.5.3 Nanotechnology-Based Therapeutics
- •14.5.4 Immune Therapy
- •14.5.5 Combination Therapy
- •14.8 Conclusion
- •References
- •15: Aspergillus Therapeutics: Future Agents
- •15.1 Introduction
- •15.2.1 Fosmanogepix
- •15.2.2 Ibrexafungerp
- •15.2.3 Olorofim
- •15.2.4 Opelconazole
- •15.2.5 Rezafungin
- •15.2.6 MGCD290
- •15.2.7 Tetrazoles (VT-1129/VT-1161/VT-1598)
- •15.2.8 Nikkomycin Z
- •15.2.9 VL-2397
- •15.2.10 T-2307/ATI-2307
- •15.2.11 Encochleated Amphotericin-B
- •15.2.12 SUBA-Itraconazole
- •15.2.13 Immunotherapy
- •15.2.14 Drug Repurposing
- •References
- •16.1 Introduction
- •16.2 Antifungal Agents
- •16.2.1 Azoles
- •16.2.2 Posaconazole
- •16.2.3 Isavuconazole
- •16.2.4 SUBA—Itraconazole
- •16.2.5 Nanovoriconazole
- •16.2.6 Adverse Effects
- •16.3 Liposomal Amphotericin B (LAMB)
- •16.3.1 Echinocandins
- •16.4 Combination Antifungal Therapy
- •16.5 Therapeutic Drug Monitoring (TDM)
- •16.5.1 Azole-Resistant Aspergillus Spp.
- •16.6 Guideline Recommendations
- •16.10 Conclusion
- •References
- •17.1 Introduction
- •17.3 Potent Antifungal Molecules Under Investigations
- •References
- •19.2 Host–A. fumigatus Interactions
- •19.3.1 Hydrophobicity or Rodlet Layer
- •19.3.2 Conidiation
- •19.3.3 DHN Melanin
- •19.3.5 Siderophores
- •19.3.6 Biofilm Formation
- •19.4 Conclusion
- •References

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Surviving theStorm: How Candida
Species Master Adaptation
forPathogenesis
AishwaryaRana, NidhiGupta, ShumaizaAsif,
andAnilThakur
Abstract
The complex interplay between hosts and microbes in fungal pathogenesis, espe-
cially within Candida infections, presents a signicant healthcare concern.
Candida species, highlighted as high-priority pathogens by the WHO, pose a
global threat exacerbated by escalating drug resistance. This comprehensive
chapter delves into Candida’s multifaceted strategies in negotiating the host
environment, emphasizing regulatory mechanisms, phenotypic switching, and
metabolic adaptations crucial for virulence.
Phenotypic switching, a pivotal transformation, enables Candida to adjust
cellular forms in response to environmental cues, augmenting adhesion, tissue
penetration, and virulence. Metabolic exibility emerges as a pivotal virulence
factor, facilitating survival in diverse host niches marked by nutrient depletion
and stress, extending the focus beyond C. albicans to encompass metabolic pathways in other Candida species.
Adhesion, a critical infection step, relies on various adhesins—ALS family,
HWP adhesin, HYR/IFF family, and EPA family—facilitating host cell attachment and establishing footholds within the host. Enzymes like hemolysins, candidalysin, and lipases contribute to host surface interactions, inuencing
colonization and invasion.
As Candida species evolve resistance to existing drugs, the imperative to
comprehend molecular networks intensies to identify innovative therapeutic
targets. Here, we explore these intricate mechanisms, providing insights into
potential areas for disrupting fungal pathogenesis and addressing the growing
5
A. Rana · N. Gupta · S. Asif · A. Thakur (*)
Laboratory of Protein Translation and Fungal Pathogenesis, Regional Centre for
Biotechnology, Faridabad, Haryana, India
e-mail: anil.thakur@rcb.res.in
© The Author(s), under exclusive license to Springer Nature Singapore Pte
Ltd. 2024
S. Hameed, P. Vijayaraghavan (eds.), Recent Advances in Human Fungal
Diseases, https://doi.org/10.1007/978-981-97-4909-6_5
109

110
A. Rana et al.
challenge of drug resistance. These ndings guide the development of inventive
therapeutic strategies against Candida infections worldwide.
Keywords
Non-albicans Candida species · Pathogenesis · Drug resistance · Virulence factors · Biolm · Secreted aspartyl protease · Candidalysin · Amino acid permease ·
Oligopeptide transporter
5.1 Introduction
Host-pathogen interaction describes the intriguing relationship between a host and
its microbe, which is critical to fungal pathogenesis. Fungal infection by Candida
species is emerged as a pressing concern within the healthcare sector, reecting the
dynamic interplay between the human immune system and opportunistic pathogens.
Candida species (spp.) are placed in critical priority and high groups by the WHO
fungal pathogen priority list in 2022 (Fisher and Denning 2023). The emergence of
drug resistance lately has imparted further credence to the potential threat posed by
Candida globally. Candida infections pose a signicant threat to individuals with
weakened immune systems, as this typical benign microorganism can transform
into a formidable pathogen. According to the SENTRY Antifungal Surveillance
program in 2016, reports suggest a decline in Candida albicans infections, whereas
a sudden surge takes place in cases of non-albicans species such as Candida gla-
brata and Candida auris (Pfaller etal. 2019). Fungal pathogens, in general, are
armed with weapons ranging from adhesion and invasion to phenotype switching
and metabolic adaptations to navigate the challenges the host environment presents.
Survival in a dynamic host environment requires the fungi to alter their protein pool,
efciently respond to stressors and administer morphogenetic changes. Central to
these strategies are the regulatory mechanisms that control virulence
(Staniszewska 2020).
One key element of this transformation is phenotypic switching, where Candida
spp. adapt its cellular forms in response to environmental cues. This switching
involves a shift from a yeast form to a hyphal form, which enhances adhesion, tissue
penetration, and virulence. Environmental factors that trigger phenotypic switching
include temperature, pH, serum, nitrogen availability, glucose, and osmotic stress.
By modulating their cellular form based on these cues, Candida spp. can dynamically adapt to their surroundings and ensure their persistence in the host. The most
crucial step in Candida infections is adhesion to the host surface before pathogenicity ensues. Adhesins are the proteins involved in host cell attachment; several adhesion proteins include the ALS family, HWP adhesin, HYR/IFF family, and EPA
family. These adhesins facilitate the binding of Candida to host cells and tissues,
allowing the fungi to establish a foothold within the host. This adhesion is a critical
step, as it determines the success of subsequent colonization and invasion, which are
central to the pathogenicity of Candida infections. The persistence of a pathogen

5 Surviving the Storm: How Candida Species Master Adaptation…
111
and its ability to cause infection is further inuenced by a repertoire of enzymes
such as hemolysins (which regulate iron acquisition), candidalysin, lipases, and
many more, which interact with host surfaces and damage them for colonization
and invasion.
For survival and subsequent virulence, pathogens rely on metabolic exibility,
allowing the pathogens to thrive and infect diverse host niches characterized by
nutrient depletion and stress. This adaptability aids in assimilating host nutrients
and establishing infections a phenomenon not exclusive to C. albicans but extending to another Candida spp.
These metabolic pathways play a crucial role in surviving under nutrient deple-
tion. To efciently sense and utilize amino acids, Candida employs extracellular
sensing pathways, including nitrogen catabolite repression (NCR), the target of
rapamycin (TOR), and the SPS-mediated amino acid sensing pathway.
As we unravel the intricacies of these molecular networks, the quest to develop
novel therapeutic targets becomes crucial as all the Candida spp. have become
resistant to the drugs in use. With better insights into molecular mechanisms and
pathways, we can develop novel, innovative strategies to disrupt fungal pathogenesis with much better efciency and better control of the prevailing drug resistance
paradigm. This chapter meticulously explores how these mechanisms and pathways
that underscore the pathogenicity of Candida infections, thus highlighting the signicant potential target areas for therapeutic strategies to impede the onslaught of
infections globally.
5.2 Phenotypic Switching inCandida Species
Phenotypic switching is a strategy employed by organisms to adapt to their environment by altering their different cellular forms. In colonized or infected hosts, this
switching profoundly impacts the host-pathogen relationship. This phenomenon
has profound implications, as it plays a pivotal role in determining the virulence and
pathogenicity of these organisms. Among the fungal pathogens, Candida spp.,
responsible for a range of human infections, have drawn signicant attention due to
their ability to undergo phenotypic switching.
Different morphological forms of pathogens become indispensable at various
stages of infection, contributing signicantly to adhesion, tissue penetration, and
immune evasion. However, it is noteworthy that not all Candida spp. undergo morphogenesis, and in most cases, the propensity for morphological switching is correlated with their virulence.
Like other fungi, Candida spp. employ a range of regulatory mechanisms to cope
with environmental changes, such as pH, temperature, and exposure to host serum.
This adaptability bolsters their resilience and enhances genetic diversity, ensuring
their survival in the host environment. Candida spp., for instance, employ two distinct switching systems, morphological transitions (bud to hyphae) and white
opaque switching in response to changing environmental conditions (Jain
etal. 2008).

112
A. Rana et al.
5.2.1 Morphogenetic Switching (Yeast-Hyphal Transition)
inCandida Species
Due to its morphological alterations, Candida spp. can easily transmogrify from a
commensal to a pathogenic form. The yeast form prevails under typical conditions,
colonizing tissue surfaces. However, exposure to host-associated cues can trigger
the transition to the hyphal state. Hyphal forms of Candida are elongated, branched
laments with a larger surface area, enhancing their adhesion and tissue-penetration
capabilities.
The switch to hyphal formation is orchestrated by the hyphal-specic gene regu-
latory network, activated in response to environmental conditions (Witchley etal.
2019). Several hyphal-specic factors, including adhesins (Als), secreted proteases
(Sap), hyphal wall proteins (HWPs), and antioxidant proteins (Sod5), play a crucial
role in this transition (Noble etal. 2017). Yue and colleagues demonstrated that lamentation in C. auris can be induced by passing the fungal cells through a mammalian host’s body. The cells recovered from the liver and kidney showed lamentous
transition, whereas cells from the spleen, brain, and lung did not show any yeast-tolament growth. In Candida spp., morphogenesis is signicantly inuenced by a
multitude of environmental factors. Among these, several key determinants are outlined as follows:
5.2.2 Role ofTemperature inMorphogenetic Switching
inCandida Species
Temperature is a crucial element mainly involved in the yeast to hyphal transition.
C. albicans assumes a singular, rounded yeast-like form at standard ambient temperature. However, an increase in temperature triggers a notable morphogenetic
transition, resulting in the adoption of a lamentous structure. This transformation is meticulously regulated by the heat shock protein Hsp90, operating through
the cAMP/PKA pathway. Hsp90, in conjunction with the cochaperone Sgt1, modulates the pathway by interacting with Cyr1, subsequently impacting the cAMP/
PKA signalling pathway (Fig.5.1). In instances of thermal stress, the pivotal role
of this chaperone magnies signicantly, managing widespread protein
misfolding.
Consequently, it relinquishes its repressive effect on Cyr1, thereby depressing
the signalling pathway (Robbins and Cowen 2023). In C. auris, a nonheritable
switch from a lament-competent (FC) to a lamentous form is temperature dependent. FC cells are single-celled yeast forms originating from lamentous cells, and
their transition is favored under low temperatures (Fan etal. 2021). This transition
is regulated by Hsp90, with its inhibition or depletion leading to upregulated cell
surface genes similar to those found in C. albicans, emphasizing the role of Hsp90in
morphogenesis (Kim etal. 2019). Similar to C. auris, wherein lament formation is
favored at lower temperatures, it is also observed in Candida haemulonii (Deng
etal. 2021).

5 Surviving the Storm: How Candida Species Master Adaptation…
Fig. 5.1 Factors inuencing yeast hyphae morphogenetic transition: (a) environmental factors
such as temperature, CO2, N-acetyl glucosamine, nitrogen starvation, serum, carbon source, and
peptidoglycan induce lamentation, whereas farnesol inhibits the yeast hyphal transition. (b)
Transcription factors, namely, Efg1, Czf1, Cph1, and Cph2 promote lament growth and Tpk1,
Tpk2, and Nrg1 prevent lamentation
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5.2.3 Other Environmental Factors Influencing Morphogenesis
inCandida Species
Apart from temperature, several other environmental factors signicantly inuence
the yeast-hyphal transition in Candida spp.:
5.2.3.1 Serum
C. albicans typically coexist as a commensal in the gut environment, frequently
encountering bacterial peptidoglycans (PGN) within the serum. Candida recognizes PGNs and muramyl dipeptides through the pathogen recognition receptor
Cyr1, which possesses 14 leucine-rich repeats (LRR). These LRR domains enable
the detection of PGNs and muramyl dipeptides, subsequently initiating morphogenic responses by activating Ras1 within the cAMP/PKA signalling pathway (Xu
etal. 2008) (Fig.5.1).
5.2.3.2 Low Nitrogen
When the nitrogen content in the surrounding medium is signicantly depleted,
Candida spp. like C. albicans detect the presence of ammonium sulphate through
the ammonium permease Mep2. This detection event initiates the activation of
MAPK and cAMP signalling pathways. Consequently, these pathways trigger the
expression of hyphal-specic genes through the action of Cph1 and Efg1 (Biswas
and Morschhäuser 2005) (Fig.5.1). Similarly, in Candida glabrata, Ste12 protein is

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required for lamentation under nitrogen starved conditions. ste12∆ resulted in
very few pseudohyphae (Calcagno etal. 2003).
5.2.3.3 CO
2
Carbon dioxide (CO2), present in higher concentrations in the body, is another factor contributing to morphogenesis. In C. albicans, CO2 is detected by carbonic
anhydrase Nce3, which directly activates the adenylyl cyclase Cyr1 and cAMP/
PKA signalling (Klengel etal. 2005).
5.2.3.4 Osmotic Stress andCell Wall Damage
In response to osmotic stress or cell wall damage, the MAPK signalling pathway is
activated. The disturbance is detected by a protein complex consisting of three proteins: Sho1, Opy2, and Msb2. This complex interacts with Cdc42 and Cst20, ultimately triggering the phosphorylation of Cek1 which activates Cph1 inducing
lamentous growth. Mutants lacking these proteins exhibit sensitivity to cell walldamaging agents like Congo Red, zymolyase, and tunicamycin, underscoring their
involvement in cell wall formation. Interestingly, only sho1∆/∆ displayed sensitivity to osmotic stress among these mutants, emphasizing its specic role in osmotic
stress tolerance (Herrero de Dios etal. 2013).
5.2.3.5 Carbon Source
The yeast-to-hypha transition of C. albicans can be triggered by specic nutrient
sources. The glucose-sensing G-protein-coupled receptor Gpr1 is crucial in detecting glucose in the medium and inducing lamentation (Fig.5.1).
This glucose sensing via Gpr1 happens in the presence of methionine as wildtype cells induces lamentation as compared to ∆gpr1/∆gpr1. However, the cells
cease to transition into hyphae as the glucose concentration increases to 2%. This G
protein-coupled receptor (Gpcr) also senses other sugars such as fructose, maltose,
sucrose, and galactose, contributing to its regulatory role in morphogenesis (Maidan
et al. 2005) (Fig.5.1). Similarly, in Candida haemulonii, when cells from YPD
phloxine B plates were replated onto YPG (yeast peptone glycerol) agar, lamentation was induced due to using a different carbon source (Deng etal. 2021).
5.2.3.6 pH
The pH of the environment is a crucial factor inuencing the yeast hyphae morphogenetic transition in C. albicans. It notably contributes to the yeast-hyphae morphogenetic transition. C. albicans, functioning as a commensal, colonizes various sites
with varying pH levels, ranging from the stomach’s acidic environment to the gut’s
alkaline pH.This transition is governed by the Rim101 pathway, activated by its
upstream elements Rim8 and Rim21. Rim21 senses pH shifts and activates Rim8,
which, in turn, triggers the Rim101 pathway, initiating pH-dependent responses
(Bensen etal. 2004). However, the transcriptional regulator NRG1 opposes Rim101,
serving as a negative regulator of the yeast-hyphae transition (Murad etal. 2001).
Interestingly, contrast to above, the transition from yeast to hyphae occurs independently of the Rim101 pathway within macrophages, suggesting that pH changes do
not inuence morphogenesis in this particular context (Wilson and Lorenz 2023).
Mechanism of action is discussed in Sect. 5.8.

5 Surviving the Storm: How Candida Species Master Adaptation…
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5.2.3.7 N-acetylglucosamine (GlcNAc)
N-acetylglucosamine (GlcNAc) is a major structural component of the mucosa of
the GI tract, PGN, fungal chitin, and extracellular matrix (ECM) of human cells.
GlcNAc is intracellularly present and can also be uptaken externally from the environment via the Ngt1 receptor. In Candida albicans, GlcNAc can induce lamentation by a catabolic pathway or cAMP/PKA signalling (Konopka 2012). During the
catabolic pathway of GlcNAc, Nag1 deaminase converts glucosamine-6-PO4 to
fructose-6-PO4, which releases NH3, which raises the extracellular pH and promotes
hyphal development (Min etal. 2019). In the cAMP/PKA pathway, RAS activates
Cyr1, leading to increased cAMP levels. Elevated cAMP binds to PKA, releasing
Bcy1 from PKA catalytic subunits Tpk1 and Tpk2. Tpk1 and Tpk2 then phosphorylate target proteins, like Efg1, promoting lamentation (Cloutier et al. 2003)
(Fig.5.1). Interestingly, in Candida haemulonii, TEC1, a transcription factor, works
independently of the cAMP/PKA pathway and induces lamentation even in the
cyr/cyr and ras1/ras1 mutant. However, GlcNAc inhibits hyphal development in
Candida tropicalis (Xie et al. 2012), and interestingly, the catabolic genes for
GlcNAc are missing from S. cerevisiae and Candida glabrata.
5.2.3.8 Quorum Sensing Molecule
Farnesol, a quorum sensing molecule, uniquely regulates hyphal development in
C. albicans, a key virulence factor. It operates through two mechanisms: cAMP/
PKA dependent and cAMP/PKA independent. Endogenously produced in response
to cell density, farnesol acts as a negative regulator of morphogenesis by alleviating
Nrg1 repression, inhibiting yeast to hyphae transition. In the cAMP/PKA pathway,
farnesol inhibits Cyr1, adenylyl cyclase, downregulating Nrg1 expression, and
impeding cAMP/PKA signalling (Fig. 5.1). Additionally, farnesol triggers Cup9
degradation through Ubr1 regulation, releasing the inhibition of Sok1 and derepressing Nrg1 independently of the cAMP/PKA pathway, preventing yeast to
hyphae formation (Lu etal. 2014; Cao etal. 2006). Antifungal drugs, such as azoles,
echinocandins, and polyenes, have been found to impact the yeast-hyphal transition
in C. albicans. Azoles inhibit yeast hyphal transition in C. albicans as treatment
with azoles led to increased production of farnesol, an inhibitor of morphogenetic
transition. Echinocandins and polyenes change the cell wall structure or cause disruption, possibly preventing the transition (Hawser et al. 1996; Hornby and
Nickerson 2004).
5.2.4 White Opaque Switching inCandida albicans:
APhenomenon ofPhenotypic Transformation
In the realm of Candida albicans, white opaque switching emerged in 1985, courtesy of Slutsky etal. showcasing its inheritability across seven distinct phenotypes
(Slutsky etal. 1985).
Their pioneering work unveiled the inheritability of this intriguing phenomenon
across seven distinct phenotypes. Two years later, in 1987, they documented a pivotal event—the transition of the clinical strain WO-1 into two distinctive colonies:
one large, at, and opaque, and the other bulged, white, and hemispherical. This

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event marked the onset of white opaque switching, a remarkable occurrence characterized by its random, stable, and high-frequency nature (Slutsky etal. 1987). The
orchestration of white opaque switching is heavily inuenced by environmental factors that signicantly regulate the transcriptional feedback loop (Morrow et al.
1989; Kolotila and Diamond 1990). Notably, these two phenotypic forms exhibit
diverse characteristics depending on their niches within the host. For instance, white
cells exhibit higher virulence in intravenous infections, while opaque cells excel in
skin colonization (Kvaal etal. 1997; 1999). The opaque cells showed attenuated
virulence in oropharyngeal candidiasis due to the reduced expression of Als3, and
they were unable to activate the epithelial cells EGFR (Phan etal. 2007). These
forms signicantly differ in morphological appearance, virulence, and mating competence. The phenomenon of white opaque switching thus adds a captivating dimension to the multifaceted world of Candida albicans, shedding light on the intricate
mechanisms governing its phenotypic transformations.
5.2.5 MTL Locus Important forWhite Opaque Switching
In C. albicans, white opaque switching is regulated by the MTL (mating type like)
locus. MTL locus exists in two allelic forms: MTLa and MTLα. MTL locus codes
for four transcriptional regulators: a1, a2, α1, and α2. To facilitate mating, heterozygous a/α cells must undergo homozygosis (become a/a or α/α cells). Additionally,
they need to transition from the mating incompetent white phenotype to the mating
competent opaque phenotype, a switching process that adds complexity to the mating mechanism (Miller and Johnson 2002). Candida glabrata is reported to have
four distinct mating classes depending on the distribution of genes at three mating
type loci, MTL1, MTL2, and MTL3. MTL1 codes for two genes, a and α, whereas
MTL2 and MTL3 encodes for a and α gene, respectively (Srikantha etal. 2003).
MTL1 and MTL2 are transcriptionally active, and MTL3 undergoes telomeric silenc-
ing (Ramírez-Zavaleta etal. 2010).
The bistable switching mechanism in Candida albicans is orchestrated by the
master regulator WOR1, inuenced by MTL a1-α2, which acts as a repressor and
keeps cells in the white state. Deleting WOR1 impedes switching in homozygous
a/a or α/α cells, although ectopic expression in a/α cells allows switching, albeit
with reduced efciency (Huang etal. 2006). Transcriptional factors like Czf1 and
WOR2 promote the opaque state, while Efg1 favors the white phenotype. Czf1 is
crucial for switching, and WOR2 ensures stability in the opaque form. Notably,
Efg1 deletion prompts a signicant shift from a white to an opaque state, especially
in certain strains, underlining Efg1’s role as an upstream regulator of the a1-α2
repressor(Fig.5.2) (Zordan etal. 2007).
There are two switching systems in Candida glabrata: the core and irregular
wrinkled switching system. The core switching system in Candida glabrata showcases four colony variants, namely, dark brown (DB), light brown (LB), white (Wh),
and very dark brown (vDB). These colors become apparent when the fungus grow
on a CuSO4-containing agar medium. Additionally, Candida glabrata is known to
spontaneously and reversibly switch to an irregular wrinkled phenotype, adding
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