Week 24 - yeasts and moulds Flashcards
Fungi diversity - overview
Mycology: the study of fungi - (>100,000 species):
-Macroscopic and microscopic fungal organisms
-Microbiology includes ONLY microscopic fungi (yeasts and moulds)
Fungi characteristics
Chemical growth requirements - nutrition
Incapable of producing food, they are:
-Chemoheterotrophs AND saprophytes – use organic chemical substances as
sources of energy
-And specifically- obtaining nutrients from dead organic material
Oxygen requirements:
-Mostly obligate aerobes (need oxygen)
-Some yeasts, are facultatively anaerobic - obtaining energy by fermentation
-Saccharomyces cerevisiae responsible for wine alcoholic fermentation
Fungal chemical and physical requirements
Nutritional adaptation:
->Grow better at pH of 5
Selective fungal media:
-Sabouraud Media (5.5 pH) inhibiting bacterial growth
-Tolerant to hypertonic solution (high sugar/salt concentration) -> resistant to osmotic pressure
-Can grow in low moisture content
-Can metabolize complex carbohydrates
Microscopic fungi
Eukaryotic cells -> fungi
(yeasts/moulds)
Fungal microorganisms (eukaryotes)
YEASTS:
-Monocellular, occasionally
organised into pseudohyphae
-Asexual reproduction
-Form white, smooth, round,
uniform colonies
MOULDS:
-Pluricellular, organised into
hyphae
-Asexual or sexual reproduction
-Fuzzy and coloured colonies
Overview fungal cells
Main differences in the cellular composition of fungi and bacteria:
Cell Type: Eukaryotic
Cell Membrane: Sterol present
Cell Wall: Glucan and Chitin
(no peptidoglycan)
Nucleus: Present
Organelles: Present
Size: Bigger (2-10 μM)
Fungal vs mammalian cells
Differences to mammalian cells:
-1)Presence of a cell wall
-2)Plasma membrane possesses
different sterols -> containing ergosterol and not cholesterol
-3)Presence of vacuoles -> large compartments for storage of molecules, protein degradation
1&2 = Excellent selective targets
for antifungal drugs
Cell wall in fungi
Fungal cell wall:
-Chitin layer, a long-chain polymer of N-acetylglucosamine (NAG)
-Glucans (polysaccharides of D-glucose monomers)
network
-Mannoproteins (glycoproteins) on the external side
Target for selective antifungal drugs -> Echinocandin targets the synthesis of glucans
Plasma membrane in fungi
Fungal plasma membrane:
-Ergosterol is peculiar to fungal cells
Target for selective antifungal drugs:
-Polyenes targets ergosterol and causes pores in the cell membrane
-Azoles and allylamines prevent the synthesis of ergosterol (2 distinct steps)
Importance of fungi for humans
-Beneficial roles/activities of fungal microorganisms
-Degrading organic materials
-Making alcoholic beverages (wine, beer)
-Food preparation (bread, some cheeses, soy sauce)
-Commercial production of some organic acids (gallic, citric)
-Manufacturing of drugs (ciclosporins) and antibiotics (penicillin)
-Commensal microbes of normal microbiota (just some of them)
Harmful effects of fungal microorganisms:
->Food spoilage
->Contamination of pharmaceutical preparations
->Causing diseases -> MYCOSES
Mycoses – overview
Spread:
->Spread generally from the environment to people (by spores)
->By spores (moulds) or direct contacts (yeasts)
->Limited person-to-person spread
Entry sites:
->Skin - direct contact, cuts, splinters
->Lungs - inhaling spores
Virulence factors:
->Mycotoxins
->Enzymes
Types of fungal infections
Superficial -> Outer skin layer caused mostly by yeasts
Cutaneous -> Affects keratin-containing tissues (hair, nails, skin)
Subcutaneous -> Chronic infection of subdermal tissues may require surgical intervention
Systemic -> Infection deep within body, affects many
tissues and organs
Most of them can start as opportunistic infections - caused by normal microbiota fungal species that are not usually pathogenic
Mycoses - patients at risk
-Impaired immune system
-Surgery and long-term use of
corticosteroids
-Indwelling catheters
-Chemotherapy for malignant cancer
-Solid organ transplantation
Yeasts
-Unicellular and non-filamentous organisms with oval/spherical shape
Yeasts - reproduction
-Asexual reproduction (1 parent involved)
-Most by budding
-New organism develops from an outgrowth (bud)
-ASYMETRICAL cell division at one particular site
-Parent remains the same
->Others by binary fission (SYMMETRICAL division) 2 daughter cells
Clinically relevant yeasts
Genus Candida is the most clinically relevant yeast -> Multiple species that can cause diseases
Polymorphic yeast: Yeast
-Pseudohyphae may be formed
-> budding yeast cells that remain attached to each other
Normal microbiota /
opportunistic infections:
-Candida normally lives in the oral cavity, GI and genital tracts (normal microbiota)
-Most common cause of fungal opportunistic infections
->immunocompromised patients or when the microbiota is altered (overgrowth)
Candidasis
Candidasis – any infections caused by any species of Candida
->It can affect many areas and develop a wide range of diseases
ORAL candidiasis/thrush:
->Candida accumulates on the lining of your mouth (creamy white deposit on tongue or inner cheeks)
VAGINAL candidiasis/thrush
-Symptomatic inflammation of the vagina
INVASIVE candidiasis/thrush
->Serious infection that could affect many tissues and organs ->The severity usually depends on the state of the host’s immune system
Clinically relevant yeasts
Genus Cryptococcus -> 2 species that can cause diseases
-Spherical, single cell yeast
-Surrounded by a peculiar and unusual polysaccharide capsule
Pathogen with low virulence:
Cryptococcus is NOT part of normal microbiota
-However, it causes serious infections merely in severely immune compromised hosts
Moulds
Multicellular organisms consisting of threadlike tubular structures called hyphae
->The hyphae form together to produce a mat-like structure called a mycelium (fuzzy appearance)
Why two different areas of a mould colony?
-Vegetative mycelium elongates to obtain nutrients and anchor the mould on the Agar
-Aerial mycelium grow on top and have reproduction function (producing spores)
Mould reproduction
Reproduction can be sexual (2 parents) or asexual (from 1 parent)
Mould can reproduce asexually by fragmentation: Mycelium may fragment into segments, which can grow into new individuals
Spores:
Produce spores asexually or sexually
Asexual spore production (single parent): Organism produced by mitosis
Sexual spore production: Involves the fusion of haploid spores (after meiosis) from partner of opposite mating type
Aspergillosis
Genus aspergillus are ubiquitous moulds found in soil, on plants-> 2 species that can cause aspergillosis (A. fumigatus and A. flavus)
Clinical manifestations:
Different types of aspergillosis - people at risk (immunocompromised)
Pulmonary aspergillosis the most common -> interstitial pneumonitis or localized ball-
shaped infiltrates (Aspergilloma)
Invasive aspergillus –> infection can spread from lung to heart, brain, kidneys
Targets of different classes of antifungal drugs
Selective toxicity for antifungal drugs:
More challenging than antibiotics due to similarity with human cells - toxicity is observed - especially in chronic infection (long treatments)
Selective fungal targets – Cell wall & ergosterol (molecules and biosynthetic enzymes)
5 classes of antifungal drugs
Echinocandins:
Inhibition of biosynthesis pathway
Flucytosine:
Inhibition of nucleic acid synthesis
Polyenes:
Binding to ergosterol and distribution of membrane intensity
Azoles, allylamines, thiocarbamates:
Inhibition of ergosterol biosynthesis pathway
Griseofulvine
Inhibition of microtubule synthesis pathway
