Lecture 7: Fungal Pathogens II

Damage-Response Framework

Defines microbial pathogenesis as the outcome of an interaction between a host and a microorganism

Disease severity is measured by

the degree of damage inflicted on the host- by the microbe or host’s own immune system

damage is driven by

fungal factors, host factors, or both

Candida species and flora

Some Candida species are part of the normal flora of most individuals

can candida be opportunistic

no

The immune system and microbiota act as gatekeepers to prevent

the transition from commensal to pathogen via controlling replication and functional state

C. albicans - Source

Natural part of human microbiota

• Gastrointestinal tract

• Mouth/ throat

• Vaginal Tract

• Skin

C. albicans - environmental source

Rarely isolated from the environment

• If found in soil, usually an indicator of human or animal waste contamination

C. albicans: Unlike Cryptococcusand Aspergillus

the source may be our own microbiota

C. albicans - commensal form

yeast

C. albicans - invasive/pathogenic form

hyphae

C. albicans - new studies surrounding hyphae growth

NEW studies show sometimes limited hyphae growth is important for commensal activity

C. albicans - antibiotic usage and hyphal growth

Antibiotic usage can lead to bacterial cell wall fragment “storms” that can signal yeast to hyphal shift

C. albicans - antibiotic usage and competition

Loss of microbial competition and signals they give off

C. albicans lives naturally as a member of

~60% of human’s microbiome

C. albicans: homeostatic conditions

acts as a commensal opportunistic pathogen

C. albicans: in dysbiosis

disruption of the balance of normal flora (dysbiosis), can lead to rapid multiplication of candida- overgrowth

C. albicans: person to person transmission

Mother-to-Infant

• During birth, as a child passes through the birth canal

• Many healthy newborns experience mild infections- oral thrush or yeast diaper rash

• Severe infections are rare, usually affect premature, hospitalized newborns

C. albicans: healthcare transmission

Less common, can be transmitted through contaminated healthcare worker’s hands or medical devices

Triggers for Candida albicans Overgrowth

Antibiotic usage- Killing protective and beneficial bacteria that keep Candida in balance

Weakened immune system- less yeast control

Corticosteroid usage

Diabetes that isn’t well managed

Hormonal shifts- raised estrogen levels

Pathogenicity of Candida albicans

is not intrinsic, it is the outcome of interactions between host and the fungi

Some of the same fungal factors are implicated in

pathogenicity and homeostasis

Candida albicans Dual- Function Traits

Morphological Switching (Yeast-to Hyphae)

Candidalysin Toxin Production

Adhesions and adhesion genes (ALS genes)

Candida albicans: morphological switching in virulence

Uncontrolled or overt filamentation, Hyphae penetrate tissues, cause inflammatory destruction

Candida albicans: morphological switching in commsensal

Low level or localized occur in gut microbiome, balances niche

Candida albicans: toxin production in virulence

Secreted by hyphae, damages epithelial cells, triggers inflammation

Candida albicans: toxin production in commsenal

POSSIBLE Low-levels help with fungal-microbe-host equilibrium and interactions during hyphal growth

Candida albicans: adhesions in virulence

For attachment to host surfaces, first step of tissue invasion and biofilm formation, attachment to medical devices

Candida albicans: adhesions in commensal

Non-destructive surface adhesion to stick to mucosal surfaces

Most infections are associated with

biofilm formation on/inside the host, or on indwelling medical devices

C. albicans biofilms contain a mixture of

yeast-form cells

Biofilms protect cells from

antifungals, host immune defenses, and environmental stressors

Cells released from the biofilm can

disperse and travel through blood stream

Quorum sensing

Molecule, Farnesol- accumulates in mature biofilms- it inhibits hyphal growth and promotes yeast formation, maybe an aid for dispersal

Candida albicans Infections: pathogen-mediated damage

tissue invasion, hyphal extension, expression of toxic Candidalysin

Candida albicans Infections: host-mediated damage

Excessive inflammatory immune responses

C. albicans- mucosal and surface infections

oral thrush, yeast infections, cutaneous candidasis

C. albicans- oral thrush

• Mouth, tongue/ inner cheeks

• Painful swallowing

C. albicans- vaginal yeast infection

Itching, burning, painful urination

C. albicans- cutaneous candidiasis

rash, red

C. albicans- Invasive Candidemia

Severe, life-threatening systemic bloodstream infection

Can affect deep organs

C. albicans Invasive Candidemia: nonspecific symptoms

fever, chills, nonresponsive to antibacterial drugs

C. albicans Invasive Candidemia: at risk populations

patients, premature infants, organ transplant recipients, cancer patients

Invasive Candidemia: progression

colonization (yeast form) → local infection (fungal overgrowth and translocation) → dissemination via the blood (systemic candidiasis)

Invasive Candidemia: risk factors

disruption of integrity, neurtopenia, dysbiosis

C. albicans Diagnosis: Oral Thrush

Visually looking for raised white lesions

Can swap for microscopic examination

C. albicans Diagnosis: Vagnial candidiasis

microscope sample

C. albicans Diagnosis: invasive candidiasis

Blood cultures

Biopsy/fluid culture via needle aspiration

Histopathology

C. albicans Treatment: Mild- to moderate mucosal infections

Treated with antifungal creams, gels, or single dose/short term pills

Clotrimazole, miconazole, nystatin, or fluconazole

C. albicans Treatment: invasive

IV antifungal medications- up to ~2 weeks after signs and symptoms resolve

C. albicans - Prevention

Managing risks

• Only taking antibiotics or corticosteroids as prescribed

• Practice good oral health

• Antifungals as prophylaxis - hospitalized patients

• Keeping skin dry and clean

• Managing stress

DRF: commensal/optimal immunity

Colonization of mucosal surfaces

Combination of host immunity and bacterial microbiota keep fungal growth low and controlled

No tissue damage or symptoms

DRF: low/absent immunity

Immunosuppression

Uncontrolled microbial proliferation

Pathogen-mediated damage (tissue invasion, candidalysin)

Often the case with oral thrush (disrupted immunity)

Invasive candidiasis (absent/low immunity)

DRF: strong/overreactive immunity

Host mediated damage

Vulvovaginal candidiasis

• Local inflammatory reactions against hyphae/yeast

Candida auris

MULTI-drug-resistant pathogenic yeast

Candida auris: severe infections in what setting

Severe hospital-acquired infections

Candida auris: geographical significance

Appeared simultaneously and independently across multiple continents in the late 2000s

Candida auris: in lab

Difficult to ID in lab setting

Candida auris: mortality

High mortality in invasive infections

Candida auris: fomite survival

Survives on environmental surfaces

Candida auris: transmission

Patient to patient transmission

Why Is Candida auris Different?

1. Multidrug resistance

• Resistant to multiple antifungal classes

2. Environmental persistence

• Survives on surfaces for weeks

3. Transmission

• Spreads between patients

C. auris - Morphology

Primarily a budding yeast

Produces pseudohyphae

C. auris - does it form true hyphae

no

C. auris - does it form biofilms

yes

C. auris - Transmission Settings

Hospitals and long-term care facilities

• Direct patient contact

• Healthcare workers

• Touching contaminated surfaces

Patients can have C. auris on their skin and not have

symptoms or colonization

Colonized people spread C. auris onto

surface/objects

C. auris: human microbiome

Not considered a normal part of human microbiome

C. auris: first identified

in 2009 from an ear canal of a patient in Tokyo, Japan

C. auris: genetic analysis of continents and spread

Genetic analysis showed independent emergence across continents around same time

C. auris: originally existed as

It is believed to originally existed as an environmental fungi, in wetlands and salt marshes, where is acquired thermotolerance and salinity tolerance

C. auris: exposure to what is a factor for resistance acquisition

antifungals in nature

C. auris: how did it cross into human populations

Crossed into human populations and colonized individuals without symptoms

C. auris: introduction to hospital settings

Introduced to hospital settings with vulnerable populations

• PERSISTENCE on surfaces

C. auris: mortality rate of bloodstream infection

30-60%

C. auris - Major Virulence Traits

Mutlidrug resistance (MDR)

Surface persistence

High thermotolerance and salt tolerance

C. auris - Mutlidrug resistance

Partially driven by high levels of alpha-1,3 glucan, and efflux mechanisms to remove antifungals from the cell

C. auris - surface persistence

On healthcare surfaces for up to ~14 days

Resistant to environmental stressors

Biofilm formation enhances survival

The major threat of C. auris is not dramatic tissue invasion but

its ability to persist, spread, and resist treatment

C. auris - 4 Infections

Ear infections

Wound infections

UTIs

Systemic infections (blood)

• Sepsis, shock, organ failure

C. auris - Standard yeast identification methods can lead to

misdiagnosis

C. auris - diagnosis

Collection clinical samples (blood, tissue sample)

• Molecular sequencing

• Antifungal susceptibility testing (best)

C. auris - resistance

Resistant to many antifungals

Pan-resistant strains have been reported

C. auris - Treatment

First line of defense is IV echinocandins

• Target fungal cell wall via inhibiting 1,3-β-D-glucan synthase

• Most strains are resistant to azoles, frequent resistant to polyenes; some emerging isolates are resistant to echinocandins

C. auris - Prevention

Handwashing

PPE

Disinfecting surfaces

Sterilization of medical equipment

Screening if facility has a suspected outbreak

Isolating patients with infection

environmental fungi temperature restriction

Most environmental fungi cannot grow at 37°C

Mammalian body temperature acts as

a natural defense against many fungal species

The ability to grow at mammalian body temperature is a

fundamental requirement for most invasive fungal pathogens

climate change induction of thermotolerance

increase in tempature/heat waves, increase agricultural fungicides, leads to increased geographic dispersal and thermotolerance

therefore increased exposure to fungi and increased trauma/wounds

environmental warming and fungi research question

Could environmental warming pre-adapt fungi to growth in humans?

We see graphical distribution of fungal pathogens changing with

climate change

Extreme weather events affect

spore dispersal

Extreme wet-weather events create conditions in which

fungal pathogens thrive

Pathogenic Fungi: TNF-alpha inhibitors

decrease cytokine release and response

have the side effect of allowing opportunistic infections

Why Are Emerging Fungal Pathogens a Growing Problem?

1. Limited antifungal drugs

• Humans and fungi are both eukaryotes; drug toxicity limits targets

2. Resistance and adaptation

• Agricultural fungicide exposure, biofilm formation

3. Growing susceptible populations

• Cancer therapy, Organ transplantation, Biologics (TNF inhibitors), ICU care

4. CLIMATE CHANGE

• Increasing selection for thermotolerance

• Expansion of geographical regions