fungal infections

What are fungi?

• Fungi are eukaryotic, and obtain their nutrition from their environment. They occur worldwide, and may be environmental, commensal or pathogenic.

• Non-photosynthetic (cannot generate their own food) not autotrophs

• Obtain nutrients by absorption (secretion of digestive enzymes)

Structural features of fungi -celll membranes and cell walls

1. Fungi contain chitin in their cell walls (polymer of N-acetylglucosamine)

2. Fungi contain ergosterol in their plasma membrane

hyphae structural features - mycelium

Sometimes comprised of a mycelium: a large mass of hyphae (singular: hypha) Hyphae: structures produced by apical extension which have parallel sides

Features of human fungal pathogens 4

1. Growth at 37 degrees - human core temperature but Majority of fungi cannot survive, prefer 18-34 C

2. Ability to avoid/resist immune clearance • Required for persistence in the host

3. Lysis of human tissue and absorption of products • Acquisition of nutrients is essential

4. Locomotion through or around host barriers • Dissemination through the host

Factors which predispose to fungal infection

1. Age (infancy and elderly)

2. endocrine disorders - diabetes

3. Defects in cell-mediated immunity

4. Cancer treatment (chemotherapy)

5. Drug addiction

6. Use of antibiotics, corticosteroids, immunosuppression

7. Use of medical plastics (especially intravenous catheters: biofilms)

Superficial fungal infections - hair shafts and dead skin

Superficial infections are extremely common: occur in otherwise healthy individuals

1. Malassezia

2. Pityriasis versicolor (Malassezia)

3. tinea

Superficial fungal infections - nails, epidermis, subcutaneous

Superficial infections are extremely common: occur in otherwise healthy individuals

1. Ringworm (Tinea cruris)

2. Onychomycosis (Trichophyton rubrum)

3. Sporotrichosis (Sporothrixspp)

Fungal diseases of the immunocompromised

Most fungal pathogens cannot cause serious disease in immunocompetent people

immunocompromised examples

1. Use of corticosteroid inhalers

2. Chemotherapy

3. Solid organ transplants

4. Low CD4+ cell count (HIV/AIDS)

5. Neutropenia - low neutrophil count

Systemic diseases in the immunocompromised

candida albicans

• Yeast: approx 60% carriage rate in the human population

• Can be commensal (typically GI tract) or pathogenic

• 200,000 deaths and >150 million mucosal infections annually

• 50% of HIV and 90% of AIDS patients: oral infections

• 75% of all healthy women: vaginal candidiasis - can cause disaese in healthy people

• Systemic infections can occur in immunocompromised individuals and are often fatal

Candida infections target all tissues and organs

Candidiasis (i.e., albicans, glabrata, auris)

• Caused by Candida species (i.e., albicans, glabrata, auris)

• Synonymous with mucosal infections (e.g., oral thrush)

• C. auris is highly resistant to antifungal therapy

candidiasis

- Pseudomembranous candidiasis

• Morphological switching facilitates C. albicans infection C. albicans secretes a range of virulence factors

• Forms biofilms readily

• Immunosuppression is a strong predisposing factor to candidiasis

candida infections target all tissues and organs

C. albicans: Commensal colonisation

Commensal colonisation of mucosal surfaces often occurs at birth (natural microbiota)

1. Adhesion Passive: electrostatic attraction

2. Active: through expression of adhesins i.e., Als5p (yeast specific adhesin)

3. Nutrient acquisition

C. albicans morphological switching

under certain conditions - it can change morphology - eg immune suppression

Defective immunity:

• Fungal overgrowth

• Yeast-to-hypha transition (morphological switching)

• Tissue invasion

how do C. albicans hyphae create tissue damage?

1. Create an “invasion pocket”

2. Damage cells and tissues by secreting virulence factors

3. Breach mucosal barriers and invade underlying tissues

C. albicans hyphal invasion pocket formation 2 options

1. Active penetration of hyphae (fungus driven) - tugor pressure pushes through the tissue

2. Receptor-induced endocytosis (host driven)

C. albicans virulence factors

• C. albicans hyphae secrete virulence factors to facilitate pathogenicity

• Lipases/Phospholipases (lipids)

• Secreted Aspartyl Proteinases (proteins)

• Candidalysin: Cytolytic peptide toxin (damages tissues and activates immune responses)

Oral manifestations of candidiasis

Erythematous candidiasis

pseudomembranous candidiasis

chronic hyperplasic candidiasis

angular cheilitis - candidiasis

Chronic mucocutaneous candidiasis CMC

Genetic predisposition (mutations which cause a defective T-helper 17 (Th17) immune response) Infection of the nails is also common with CMC

Invasive Aspergillosis (IA) - BACKGROUND

• Caused by Aspergillus species (i.e., fumigatus, flavus)

• Soil-dwelling saprophytic mould (EXOGENOUS )

• Releases conidia/spores (spores; 2–3 μm) into the air

• Conidia are inhaled deep into the lung - respiratory epithelium

• Conidial swelling, germination, and hyphal growth

• Respiratory infections in patients with compromised immunity

Invasive Aspergillosis (IA) - CLINICAL FEATURES

• Growth of Aspergillus in the lung - big as a tangerine

• Physical obstruction

• Secretion of hydrolases (proteases, phospholipases)

• Production of mycotoxins: secondary metabolites - bad impacts on the human body

• Gliotoxin (detectable in serum of IA patients) → Inhibits neutrophil/macrophage function

• Severe infections can become systemic

Gliotoxin

• (detectable in serum of IA patients) → Inhibits neutrophil/macrophage function

Allergic Bronchopulmonary Aspergillosis (ABPA)

• immune-mediated reaction against A. fumigatus antigens

• Hypersensitivity following colonisation of lung - immune mediated

• ABPA most commonly complicates the course of patients with asthma and cystic fibrosis

• Cutaneous hypersensitivity to A. fumigatus antigen, elevated IgE

• Blood and sputum eosinophilia (overproduction)

• lower infection →Bronchiectasis (widening of airways, persistent cough, phlegm)

Cryptococcosis

• Caused by Cryptococcus species (i.e., neoformans, gattii)

• Found in soil, pigeon droppings (yeasts are inhaled)

• Thick polysaccharide capsule (reduces phagocytosis)

• Cryptococcus can persist within phagocytes

• Diverse morphologies: titan cells- big (arrows) – but no hyphae

• Infects the lungs and central nervous system (brain/spinal cord)

Cryptococcosis - lung infections

• Respiratory distress

• Pneumonia-like symptoms

• Dissemination to the brain

Cryptococcosis - brain infection

• Brain infection (Cryptococcal meningitis):

• Acquired from the lung

• Fungus crosses the blood-brain barrier

• Headache, sensitivity to light, nausea, confusion/behavioural changes

Pneumocystis pneumonia

• Caused by Pneumocystis jirovecii

• Causes lung infections (fever, shortness of breath, hypoxemia, non-productive cough)

• Associated with severe immunodeficiency AIDS-defining disease in USA (CDC, 1981)

• Incidence of Pneumocystis pneumonia is reduced with HAART therapy for HIV/AIDS (not always available)

• Also occurs with solid organ transplantation/use of corticosteroids)

• Several aspects of pathogenesis are poorly understood

Mucormycosis

Caused by fungi in the phylogenetic order Mucorales

Common examples include Rhizopus delemar, R. oryzae

Predisposing factors include:

• Diabetic ketoacidosis (elevated ketones in blood)

• Neutropenia (low neutrophil counts)

• Mucormycosis can be:

• Cutaneous

• Rhino-orbital/Cerebral

• Pulmonary/systemic

Rhino-cerebral mucormycosis

• Often has life-changing impact

• Systemic mucormycosis has a mortality rate of ~90%

• Requires surgical intervention/reconstruction

• Extensive antifungal therapy

Covid-19-Associated Mucormycosis

in patients who have had covid - 2021

Mucoricin

Extremely stable protein toxin

Associated with, and shed from R. delemar hyphae

Mucoricin causes:

1. Organ damage

2. Necrosis (ribosome inactivation: depurination of ribosomal RNA)

3. Haemorrhage and vascular leakage

4. Immune cell infiltration

all subsequent cards are from last years lecture

the main characteristics of yeast are 3

1. unicellular

2. reproduce by budding

3. some may produce hyphae and pseudohyphae

what are the main characteristics of moulds ? 3

1. multicellular - linked

2. reproduce using specialized spore structures

3. produce hyphae

what are the structures produced by yeasts?

1. buds

2. pseudo hyphae

3. true hyphae

Pseudohyphae

yeasts - Pseudohyphae - produced by bud elongation, can resemble a string of sausages

true hyphae

yeasts - True hyphae - produced by apical extension, have even and parallel sides

how can you describe the colonies yeast form?

what do moulds produce?

true hyphae

Mould hyphae form a colony on solid media that is round, sub-surface growth occurs and special spore structures may be on the surface (agar) - spores - distributed by air or water

It is possible to distinguish mould and yeast infections in tissue by microscopy (example using fluorescence stain)

• in moulds - hyphae are true and parallel and regularly septate

• in candida infected skin - budding yeasts, pseudohyphae, true hyphae

mould infection

yeast infection

are there any commensal yeasts?

yes

• Candida albicans - GI tract, oral

• Other Candida species may be found in the GI

• Malassezia - Skin

are there any commensal moulds?

no

Factors predisposing to candidiasis 8

1. Age - infancy, elderly

2. Endocrine disorders

3. Defects in cell mediated immunity

4. Cancer

5. Drug addiction

6. Drug therapy -antibiotics (removes competition), corticosteroids, immunosuppression

7. Surgery

8. Intravenous catheters - biofilm formation

Candidiasis

• Candida species are commensal to GI tract, most infections arise from endogenous flora

• Commonest causes: Candida albicans C. glabrata, C. tropicalis, C. parapsilosis, C. krusei

• Occasionally exogenous sources implicated - eg. hospital outbreaks, and the commonest cause of these is C. albicans.

• Latest nosocomial problem is Candida auris. Colonises readily, persists in environment, highly resistant to antifungals

candidiasis - Acute pseudomembranous, detachable plaques

candidiasis - Chronic pseudomembranous, AIDS persistent

candidiasis - Chronic mucocutaneous candidiasis

Oral candidiasis in children

• Angular chelitis - corner infection may be present

• Oral candidiasis occurs in 4-8% infants, but if HIV+ 72-99%.

Superficial infections caused by Candida albicans

predisposing factors for intertrigo candidiasis

obesity and diabetes

predisposing factors for Fingerweb / finger nail candidiasis

occlusion and wetness, carriage of candida.

Candidiasis - nappy dermatitis

• Buttocks, perianal, groin.

• Erythema, scaling, satellite lesions

• 1o or 2o invader?

• Association with faecal carriage of C. albicans - abrasion, urine and faeces → candida can get in

Chronic mucocutaneous candidiasis

• Presents in childhood or infancy, recurrent oral, skin, nail infection.

• Skin lesions crusted on face and scalp - “candida granuloma”

• Immunological abnormalities involved, not characterised CMC may be inherited, associated with hypoparathyroidism or hypoadrenalism, hypothyroidism, or idiopathic

Systemic candidiasis - predisposing factors

1. age

2. antibiotic treatment

3. abdominal surgery

4. immune suppression

5. catherisation

6. prolonged hospitalization

systemic candidiasis - sites affected

blood, lungs and internal organs, skin

systemic candidiasis - endogenous and exogenous sources

• C.albicans is an opportunist

• Endogenous source– resulting from predisposing factors

• Exogenous source – outbreaks can occur in wards with severely debilitated hosts, uncommon

How does Candida albicans cause infection? 7

1. ability to adapt to changes in the environment

2. ability to adhere to different surfaces

3. production of destructive enzymes

4. Changes in cellular morphology

5. Production of biofilms

6. Evasion of host defence mechanisms

7. Toxin production

candida - ability to adapt to different environments

eg. pH. Sites include mouth, GI, vagina, skin

candida - Ability to adhere to different surfaces

Surface molecules bind to: iC3b, fibrinogen, fibronectin, laminin, epithelial cells , bacterial coaggregation with Streptococci and Fusobacteriumspecies

candida - Production of destructive enzymes

Secreted aspartyl proteinases, phospholipases, hyaluronidase, degrade extracellular matrix proteins, etc enabling invasion

candida pathogenicity - Secretory aspartyl proteases SAP

Secretory aspartyl proteses (SAPs) have roles in adherence, invasion and development of disease.

can turn on different proteases depending on what theyre doing

candida - changes in cellular morphology

Yeast, hyphae, pseudohyphae

candida - production of biofilms

Offers protection from environment, phagocytosis, and antifungals

candida - evasion of host defence mechanisms

Block oxygen radical production and degranulation of PMN, kill monocytes. Cell wall components may have immunomodulatory effects, may stimulate cytokine release, may activate complement cascade

candida - toxin production

Candidalysin is a cytolytic peptide toxin secreted by C. albicans during hyphal invasion. Damages tissue and activates the immune response

Cryptococcosis

A chronic, subacute to acute pulmonary infection resulting from inhalation of cryptococcus yeasts.

On dissemination, the yeast shows a predilection for the CNS and cryptococcal meningitis occurs. Systemic spread may result in skin lesions, and infection of bone and internal organs may occur

Cryptococcosis - distribution, risk factors and causative organism

• Distribution - worldwide especially associated with bird droppings (eg. pidgeon)

• Causative organism Cryptococcus neoformans

• Risk factors - HIV/AIDS

Cryptococcus gattii

and related species can also cause infection of mostly immune competent hosts. This species is associated with trees and soil and is endemic to certain regions. eg. Vancouver Island, Canada

Cryptococcus infection - yeast or mould?

Cryptococcus exists only in the yeast form, no hyphae produced.

cryptococcosis - major virulence factor

Major virulence factor: capsule is protective, prevents phagocytosis. melanin is immunomodulatory

capsuled microbes likely cause meningitis

Mould infections

• No commensal mould to the human body.

• Infections always exogenously acquired.

• examples showing different pathogenicity mechanisms and relationships with the host

Dermatophytes - what is it and the source

• mould infection

• Common

• superficial infection,

• sources include human, animal and soil, healthy host

Aspergillus - what is it and what’s the source?

• mould infection

• Uncommon,

• systemic infection

• environmental source

• immune compromised host or predisposing factors underlie disease

What factors predispose to mould infection? 5

1. History of trauma to site of infection (cutaneous and subcutaneous)

2. Host immune status - can determine the extent of disease, duration, outcome

3. Underlying disease – may influence susceptibility to certain types of infection

4. Exposure to a source

5. Portal of entry

Dermatophyte infections - what is there substrate? ring worms

• Dermatophytes are a family of mould/fungi that use keratin as a substrate

• Species can infect skin, nail and hair

Clinically dermatophyte infection is referred to as

tinea

Dermatophyte infections- facts

• Infections result from contact with a source Affect healthy and immune compromise people

• Commonest cause of human mould infection

• Commonest cause of skin and nail infection is Trichophyton rubrum.

• Species are linked to primary hosts – humans, animals or soil

• Dermatophyte infections - Tinea capitis

• In UK cities the commonest form is spread anthropophilically - person to person . Clinical presentations may be sub-clinical to kerion. Often fine scaling, mild erythema, patchy alopecia. Hairs break at skin level -black-dot alopecia.

• Highly contagious among pre-pubescent children.

• Practice good hygiene and disinfect chairs and headrests.

Tinea capitis - risk factors

• age – most infections before puberty (mid teens). Uncommon in adults. Sebum production is inhibitory to fungal growth.

• contact with infection - relevant history of exposure to source (human, animal, soil)

• minor trauma to inoculate - scratching, hair dressing, barbers, sharing hats / towels etc

• affects healthy hosts. Not more common in immune compromised hosts, but infections may be more florid

Pathogenicity mechanisms for tinea 6

1. Adherence: adhesins, enzymes (subtilisin in M. canis), fibrillar projections on fungal cell surface

2. Invasion: phospholipases, subtilisins, metalloproteinases, carboxyproteinases, serine proteinases etc. Complex process regulated by protein content and pH.

3. Utilise keratin: proteins in the cornified layers of skin are rich in disulphide bridges. Dermatophytes use a sulphite pump to reduce the disulphide bond in proteins which are then cleaved. Subtilisins and metalloproteasess can digest these proteins, and aminopeptidases and carboxypeptidases digest further to form peptides and amino acids

4. Manipulation of the immune response - cell wall mannans suppress lymphoproliferative activity → from plants → worse symptoms

5. Host adaptation - zoophillic species produce more protease than anthropophilic species - less IL-1α was induced by T. rubrum than T. mentagrophytes andT. tonsurans elicits a lower cytokine release from keratinocytes than animal species

6. Asymptomatic or minor infection that is ignored → increases spread

Aspergillus infections - Aspergillosis

• environmental moulds - leaves

• Causes systemic disease following inhalation of spores.

• Type of disease is determined by host status. Commonest causes: Aspergillus fumigatus, A. flavus, A. nidulans, A. niger, A.terreus.

• All are referred to as species complex as each comprises closely related organisms that are only distinguished genetically

Aspergillus infections

• Allergic aspergillosis - temporary presence of aspergillus in respiratory tract, healthy host. Agricultural link, or exposure to large nos. spores

• Aspergilloma - colonisation of pre-existing cavities (TB), fungal ball in lung, predisposing factors for lung cavitation but may otherwise be healthy

• Invasive aspergillosis - pulmonary focus, dissemination possible, immune compromised host

• Systemic aspergillosis - lungs, brain, other organs, immune compromised host

Cutaneous aspergillosis

• Cutaneous aspergillosis – primary infection due to skin damage (healthy or immune compromised). - secondary manifestation due to disseminated disease.

• Risk factors – trauma, neonate, immune compromised host. A. fumigatus, A flavus commonest

• A. flavus causes aflatoxicosis due to ingestion of toxin produced in contaminated foods (peanuts), healthy host

Invasive aspergillosis

zygomycosis