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LF223: Microbial Pathogens > Lecture 12. Fungal Pathogens 1 > Flashcards

Lecture 12. Fungal Pathogens 1 Flashcards

1
Q

What is mycology?

A

The study of fungi - fungi includes yeast, moulds, and fleshy fungi

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2
Q

What are characteristics of fungi?

A

Fungi are eukaryotic
Have a rigid cell wall, consisting of layers of polysaccharides, which forms a rigid matrix
Are chemoheterotrophs (require organic compounds for both carbon and energy sources)
Obtain nutrients as saprophytes (live off of decaying matter) or as parasites (live off living matter), hence are recyclers
Some of them stimulate the plant roots to proliferate

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3
Q

What are lichens composed of?

A

Fungi and a photosynthetic component, either a eukaryotic alga or a cyanobacterium

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4
Q

What are disease-causing fungi?

A

Infect mostly skin, hair, and nails. They are able to hydrolyse keratin, which is a tough protein found in dead skin cells and in nails
Several fungal pathogens can cause lung infections
Asperigillus species produce a toxic compound called aflatoxin which causes liver cancer
Ergot, the active ingredient in the hallucinogenic drug LSD, is also produced by fungi

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5
Q

Of the 100,000 species of fungi, how many are pathogenic for animals?

A

Only about 100

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6
Q

What do fungi play a major role in?

A

The recycling of nutrients by their ability to cause decay and are used by industry to produce a variety of useful products

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7
Q

What undesirable economic effects do fungi cause?

A

Spoilage of fruits, grains, and vegetables, as well as the destruction of unpreserved wood and leather products

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8
Q

What two kingdoms are prokaryotes?

A

Archaebacteria and Eubacteria

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9
Q

What five kingdoms are eukaryotes?

A

Protista (now often called Protozoa), Chromista, Plantae, Animalia and Eumycota

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10
Q

What two kingdoms do fungi fit into?

A

Chromista and Eumycota

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11
Q

What does the definition of a fungus describe?

A

Not a single phylogenetic line, but rather a way of life shared by organisms of different evolutionary backgrounds

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12
Q

What are Chromistan fungi?

A

Pseudofungi with cellulosic hyphal walls - Phyla Oomycota and Hyphochytriomycota

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13
Q

What are Eumycotan fungi?

A

True fungi with chitinous hyphal walls - Phyla Chytridiomycota, Zygomycota, Glomeromycota, Dikaryomycota

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14
Q

How many species of fungi cause human disease?

A

Only about 50, most fungi are harmless to humans

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15
Q

What are the three major mechanisms that allow fungi to cause disease?

A

By causing immune responses that result in allergic (hypersensitivity) reactions following exposure to specific fungal antigens
By producing toxins (e.g. mycotoxins – a large diverse group of fungal exotoxins)
By infection, the growth of a fungus on or in the body is a mycosis

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16
Q

What fungus commonly grows on improperly stored food such as grain and what does this fungus produce?

A

Aspergillus flavus produces aflatoxins - which induce tumours in birds feeding on contaminated grain

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17
Q

What do dermatophytes attack?

A

Dermatophytes have evolved a rather specific ability to attack the outer surface of human beings

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18
Q

What are thermal dimorphc saprobes?

A

Normally soil organisms, but have also adapted to life in the unusual and rather hostile environment of the human body, often responding to this environment by developing a different morphology

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19
Q

When do opportunistic saprobes attack?

A

Attacks humans only when our defences are down - when our immune systems themselves are diseased or deficient, or when we artificially suppress them e.g to prevent the rejection of transplanted organs

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20
Q

What are the three main types of human fungal infections (mycoses)?

A

Cutaneous (superficial) mycoses
Subcutaneous mycoses
Systemic mycoses

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21
Q

What do cutaneous (superficial) mycoses involve and what do they cause?

A

The outer layers of the skin and cause an allergic or inflammatory response

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22
Q

What do subcutaneous mycoses involve and what do they cause?

A

Usually involving fungi of low inherent virulence which have been introduced to the tissues through a wound of some kind, and which remain localised or spread only by direct mycelial growth

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23
Q

How are systemic mycoses caused and where can they spread?

A

Caused, either by true pathogenic fungi which can establish themselves in normal hosts, or by opportunistic saprobic fungi which could not infect a healthy host, but can attack individuals whose immune system is not working or is compromised. Both kinds of fungi sometimes become widely disseminated through the body of the host

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24
Q

What are examples of cutaneous mycoses?

A

Ringworm
Athlete’s Foot
Jock itch
Candidiasis

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25
Q

What are examples of subcutaneous mycoses?

A

Sporotrichosis
Chromoblastomycosis

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26
Q

What are examples of systemic mycoses?

A

Cryptococcosis
Coccidioidomycosis
Histoplasmosis
Blastomycosis
[Candidiasis opportunistic]

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27
Q

What fungus causes Tinea capitis (ringworm)?

A

Trichophyton tonsurans

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28
Q

What fungus causes Tinea cruris (jock itch or crotch rot)?

A

Epidermophton floccosum

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29
Q

What fungus causes Tinea pedis (athlete’s foot)?

A

Trichophyton rubrum

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30
Q

What does Tinea mean?

A

The medical name for a group of related skin infections, including athlete’s foot, jock itch, & ringworm

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31
Q

What fungus causes histoplasmosis?

A

Histoplasma capsulatum

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32
Q

What fungus causes blastomycosis?

A

Blastomyces dermatitidis

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33
Q

What fungus causes coccidioidomycosis?

A

Coccidioides immitis

34
Q

What do cutaneous mycoses include?

A

Superficial cosmetic fungal infections of the skin or hair shaft where no living tissue is invaded and there is no cellular response from the host.Essentially no pathological changes are elicited e.g. dandruff caused by Malassezia furfur

35
Q

What do cutaneous mycoses commonly refer to?

A

Superficial fungal infections of the skin, hair or nails caused by a group of closely related mould fungi dermatophytes which can colonise and digest keratin; a variety of pathological changes occur in the host because of the presence of the infectious agent and its metabolic products

36
Q

What are the causative organisms of dermatophysis (ringworm of the scalp, skin and nails)?

A

Dermatophytes (Microsporum, Trichophyton, Epidermophyton) - Common

37
Q

What are the causative organisms of candidiasis of skin, mucous membranes and nails?

A

Candida albicans andrelated species - Common

38
Q

What are the causative organisms of dermatomycosis?

A

Non-dermatophyte moulds(Hendersonula toruloidea Scytalidium hyaliumScopulariopsis brevicaulis) - Rare

39
Q

How many species of fungi can cause ringworm?

A

20 species of dermatophyte fungi grouped into 3 genera, Trichophyton, Microsporum & Epidermophyton

40
Q

How are ringworm infections spread?

A

By direct or indirect contact with an infected individual or animal e.g. a fragment of keratin containing viable fungus (direct); the floor of a swimming pool or shower or on combs, towels etc (indirect)

41
Q

What two reasons make the disease process in ringworm unique?

A

No living tissue is invaded; the keratinised stratum corneum is simply colonised. However, the presence of the fungus and its metabolic products usually induces an allergic and inflammatory response in the host. The type and severity of the host response is often related to the species and strain of dermatophyte causing the infection.
The dermatophytes are the only fungi that have evolved a dependency on human or animal infection for the survival and dissemination of their species

42
Q

How do dermatophytes utilise keratin as a nutrient source?

A

Invading keratin via enzymic digestion [keratinase] & mechanical pressure

43
Q

How are Tinea pedis infection ususally caused?

A

Usually caused by the shedding of skin scales containing viable infectious hyphal elements [arthroconidia] of the fungus
Scales may remain infectious in the environment for months or years. Therefore transmission may take place by indirect contact long after the infective debris has been shed
Substrates like carpet and matting that hold skin scales make excellent vectors

44
Q

How are Tinea infections treated?

A

Topical therapy (i.e. local therapy e.g. creams applied directly to the skin) is usually fine for skin infections but oral antifungals e.g. fluconazole, are required for extensive skin infections or those of the nail or scalp

45
Q

How do swimmers reduce the spread of Tinea infection between each other?

A

Prophylactic use of antifungal foot powder after bathing (though antiseptic foot baths in swimming pools are commonly of no value)

46
Q

What is fluconazole?

A

A widely used bis-triazole antifungal agent. As with other triazoles, it has five-membered ring structures containing three nitrogen atoms

47
Q

What is the mechanism(s) of action of fluconazole?

A

Inhibits cytochrome P450 14a-demethylase an enzyme in the sterol biosynthesis pathway that leads from lanosterol to ergosterol (an essential component of the fungal cytoplasmic membrane)

48
Q

What infections can yeasts cause?

A

Superficial and systemic infections

48
Q

What are the morphological features of yeast?

A

Are unicellular fungi which usually appear as oval cells 1-5 µm wide by 5-30 µm long
They have typical eukaryotic structures
Are facultative anaerobes: get their energy through aerobic respiration as well as fermentation
They have a thick polysaccharide cell wall

49
Q

How does the body initially detect the presence of microorganisms?

A

The body does this by recognising molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns

50
Q

What do components of yeast cells trigger within the host?

A

Components of the yeast cell wall bind to pattern-recognition receptors on a variety of defence cells of the body and triggers innate immune defences such as inflammation, fever, and phagocytosis.
Yeast cell wall components also activate the alternative complement pathway and the lectin pathway, defence pathways that play a variety of roles in body defence

51
Q

What can the yeast Candida albicans produce (in addition to it’s usual oval budding form)?

A

Pseudohyphae

52
Q

What are pseudohyphae?

A

Branching filaments of attached, elongated yeast cells resembling the hyphae of moulds

53
Q

What are blastospores?

A

Asexual reproductive spores

54
Q

What are chlamydospores?

A

Thick-walled survival spores

55
Q

How do pseudohyphae form?

A

Buds elongate forming a tube-like structure called a germ tube. The elongated buds remain attached to one another and eventually produce a filament called a pseudohypha because it resembles the hypha (a long filament of cells) of a mould

56
Q

How does the pseudohyphae help the yeast?

A

Helps the yeast to invade deeper tissues after it colonises the epithelium

57
Q

Since Candida is able to grow as a yeast as well as in a pseudohyphal, what do we call it?

A

It is said to be dimorphic i.e. it has two growth forms yeast-like and mould-like

58
Q

How do yeasts reproduce asexually?

A

Through a process called budding. A bud is formed on the outer surface of the parent cell as the nucleus divides. One nucleus migrates into the elongating bud. Cell wall material forms between the bud and the parent cell and the bud breaks away.

59
Q

How do yeasts reproduce sexually?

A

By means of sexual spores called ascospores which result from the fusion of the nuclei from two cells followed by meiosis
Sexual reproduction is much less common than asexual reproduction but does allow for genetic recombination

60
Q

What is Candida albicans?

A

Found as normal flora on the mucous membranes and in the gastrointestinal tract; usually held in check by: normal flora bacteria; and normal body defences

61
Q

Who is more at risk of being infected by opportunistic infections via Candida?

A

People who are debilitated, immunosuppressed, or have received prolonged antibacterial therapy
Women who are diabetic, pregnant, taking oral contraceptives, or having menopause are also more prone to vaginitis because these conditions alter the sugar concentration and pH of the vagina making it more favourable for the growth of Candida

62
Q

What do people who are immunosuppressed frequently develop when infected with Candida?

A

Thrush, vaginitis, and sometimes disseminated infections

63
Q

What are all Candida infections called?

A

Candidiasis

64
Q

What does Candida most commonly cause?

A

Vaginitis, thrush, balantitis and cutaneous infections

65
Q

What is vaginitis?

A

Inflammation of the mucous membranes of the vagina

66
Q

What is thrush?

A

Infection of the mucous membranes of the mouth

67
Q

What is balanitis?

A

Candida lesions on the penis from a female with vaginitis

68
Q

What does Candida less commonly infect?

A

Candida can infect the lungs, blood, heart, and meninges (the membranes covering the brain and spinal cord), especially in the compromised or immunosuppressed host.

69
Q

How many of all septicaemia cases (where microorganisms enter the bloodstream) are caused by Candida?

A

~10%

70
Q

What is oropharyngeal candidiasis?

A

Clinically, white plaques that resemble milk curd form on the buccal mucosa and less commonly on the tongue, gums, the palate or the pharynx. Symptoms may be absent or include burning or dryness of the mouth, loss of taste, and pain on swallowing.
Thrush is included in this group

71
Q

What is cutaneous candidiasis?

A

Can occur between the fingers or toes, or in the groin
“Nappy rash” candidiasis is common in infants under unhygienic conditions of chronic moisture and local skin maceration associated with irritation due to irregularly changed unclean nappies

72
Q

What is vaginal candidiasis?

A

A common condition in women
Often associated with the use of broad-spectrum antibiotics, the third trimester of pregnancy, low vaginal pH and diabetes mellitus
Sexual activity and oral contraception may also be contributing factors and infections may extend to include the vulva and the cervix
Symptoms include intense itching, burning or soreness and production of a creamy white, curd-like discharge.
Chronic refractory vaginal candidiasis, associated with oral candidiasis, may also be a presentation of HIV infection or AIDS

73
Q

In healthy individuals, what are Candida infections usually due to?

A

Impaired epithelial barrier functions which occurs in all age groups, but are most common in the newborn and the elderly

74
Q

When is systemic candidiasis seen?

A

In patients with cell-mediated immune deficiency, and those receiving aggressive cancer treatment, immunosuppression, or transplantation therapy

75
Q

How is candidiasis diagnosed?

A

Through collecting samples
Direct microscopy

76
Q

Where should samples be collected to help diagnose Candida infections?

A

From skin (using a blunt scalpel) from the edges of the lesions (where most viable fungus is likely to be)
From mouth or vagina from areas of white plaques

77
Q

How does direct microscopy allow us to diagnose Candida infections?

A

Examine specimens for the presence of small, round to oval, thin-walled clusters of budding yeast cells (blastoconidia) and branching pseudohyphae (the finding of just budding yeast cells in such material is of little diagnostic importance – it may have occurred between sample collection and processing) provided the clinical manifestations support the diagnosis

78
Q

What do Candida colonies typically look like?

A

White to cream coloured with a smooth, waxy surface

79
Q

How can Candida infections be treated and prevented?

A

Correct the underlying conditions that allow Candida to colonise the skin or mucosa i.e. to restore the normal epithelial barrier function
For cutaneous candidiasis control of excessive moisture, heat and friction which cause local skin maceration and treatment with a topical imidazole compound is usually effective
For oral candidiasis nystatin, amphotericin B or miconazole are effective
Azoles prevent the synthesis of ergosterol
Polyenes (e.g. amphotericin B and nystatin) interfere with the integrity of the fungal cell membrane by binding to membrane sterols
Most cases of vaginal candidosis can be treated with a topical imidazole or the oral fluconazole

80
Q

What is oral treatment essential for in terms of Candida infections?

A

Oral treatment is essential for the treatment of intractable chronic Candida infections. Prolonged therapy may be required and development of resistance e.g. to fluconazole can occur

LF223: Microbial Pathogens (15 decks)

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