An Overview of Medical Mycology: Fungi and Their Characteristics

MEDICAL MYCOLOGY

MeLs-311

Chrs:3

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Chapter one

  Introduction to  Medical

Mycology

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Addis Ababa University

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Jimma University

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Hawassa University

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Haramaya University

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University of Gondar

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American Society for Clinical Pathology

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Center of Disease control and Prevention in Ethiopia.

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Objectives

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At the end of this chapter, students will be able to:



Define fungi



List the differentiating characteristics of fungi from

bacteria.



Explain the morphology of fungi.



Describe the classification of fungi.



Discuss the disease of fungi.

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Mycology

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The study of Fungi

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Medical Mycology

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The study of Fungi that infect humans

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Fungi are :

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Plant structures

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Lack the definite root, stem, and leaves of highly

organized plants.

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 All fungi are eukaryotic(with true nucleus).

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They show less differentiation than plants, but a

higher degree of organization than the prokaryotes

bacteria

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Macroscopic and microscopic forms.



Macroscopic fungi



Often called fleshy fungi



Are filamentous organisms



Microscopic fungi exist as either Molds that grow on

bread, fruits and cheese or Yeasts used in baking

industry

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Cell walls are composed of chitin

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Chitin is a (β1-4)-linked polymer of 

N-acetyl-D-

glucosamine (GlcNAc).

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Cell membrane contain Ergosterol rather than

cholesterol.

Fig 1.cell wall and

membrane of

fungi

Adopted from Sherris

Medical Microbiology

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Do not contain chlorophyll



Are unable to manufacture their own nutrients,



Depend on an external source of organic

compounds for growth and survival.



Nutrients are usually obtained from dead

organisms. But Some fungi use living  tissue as a

food source e.g. Candida

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Fungi are non motile

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Medically important fungi are microscopic and exist into

two morphological forms:



Mold



Yeast

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Filamentous , multi-cellular, forms mostly found outside

the body and at room temperature.

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Is the basic microscopic unit of the molds

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Long tube like structure

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responsible for the fluffy or cotton ball appearance of

the macroscopic mold colony.

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Fig 2. morphology of

fungi

Adopted from Kyser General Mycology

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Mold are classified and identified partially on the basis of

whether the hyphae are septate or aseptate.

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Septate hyphae

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Filaments with cross-walls or septa that

partition the hyphae in to individual cellular

compartments.



 Have pores that allow the migration of

cytoplasm and many organelles.

•

 Aseptate hyphae - no physical boundaries to distinguish

individual cells in the hyphae.

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Fig 3. septate and

aseptate hyphae

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

When mold grow on substrate it can form :-



Vegetative mycelium (Thallus)



Nutrient absorbing and water exchanging

portion of the fungi



Grows in or on a substrate.



Are actively growing and form the main body of

the colony.



Arial hyphae (mycelium)



Fungal filaments or hyphae that protrude above

the surface



Produce asexual spore

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 are single cells, usually spherical to ellipsoid in shape

and occurs  singly, in pair or form a chain(bud)

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Structure mostly found inside tissue or at body

temperature



Form psedohyphae not a true hyphae

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Yeast cont’d

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Fig 4. yeast and its pseudohphae Adopted

from Sherris Medical Microbiology 2004.

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

Some fungi grow only in the yeast form; others grow

only as molds. but there are fungus that exist as mold

and yeast depending on certain growth conditions;

such phenomenon is dimorphism



For example, many fungi grow as molds at room

temperature (

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body temperature (37

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

Most of the more dangerous human pathogenic fungi

are dimorphic

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

Fungal spores are metabolically dormant, protected

cells.



They can be borne by air or water to new sites, where

they germinate and establish colonies.



Mold reproduce by Asexual spores and Yeast reproduce

with budding and sexual spore

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

Spores can be generated either asexually or sexually



Many fungi can produce both sexual and asexual

spores.



Fungi that exhibit  sexual phase are known as the

perfect fungi



Fungi imperfecti  or deuteromycetes are the designation

for a type of fungi in which the sexual cycle are either

unknown or missing entirely.



Sexual spores are rarely agents of human disease.

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

 In some fungi, vegetative cells may transform into

conidia (eg, arthroconidia, chlamydospores).



In others, conidia are produced by a conidiogenous

cell, such as a phialide, which itself may be attached

to a specialized hypha called a conidiophore.

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

In the zygomycetes, sporangiospores result from

mitotic replication and spore production within a sac-

like structure called a sporangium, which is

supported by a sporangiophore.



 The color of a typical fungal colony seen on bread,

fruit, or culture plate is caused by the conidia.

Conidia can become airborne and, therefore, are a

major source of fungal infection 

.

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Adopted from Jawitz Medical Microbiology

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

The products of meiosis of this transient diploid become

sexual spores .



Structures that contain sexual spores in fungi are called

fruiting bodies.

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

Compared to asexual sporulation, sexual sporulation is

relatively rare among human fungal pathogens.



 Spores, especially sexual spores, often have a

characteristic shape and surface ornamentation pattern

that may serve as the primary or only means of species

identification.

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Mushroom is a common fungal fruiting body that

produces basidiospores,

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 Spores formed at the tip of a club like structure called a

basiduim (plural, basidia).

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Ascospore is found in a sac-like structure called an

ascus (plural asci)

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several asci are usually contained in a single large

fruiting body.

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Fig 5 the yellow  circle indicate sexual spore and the red one asexual spore

(blastoconidia) Adopted from Jawitz Medical Microbiology

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are not associated with well-developed fruiting bodies.

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are encased in a thick, darkly pigmented wall - makes

resistant to extremes in temperature and desiccation.

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are also not associated with well-developed fruiting

bodies.

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are large flagellated spores produced by certain aquatic

fungi.

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Fungus  classified in to five categories based on



spore types



morphology of hyphae,



sexual cycles.

•

These groups are:



 Chytridiomycetes/oomycetes



zygomycetes



Ascomycetes



Basidiomycetes



Deuteromycetes

.

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Except for the deuteromycetes, all fungi produce sexual

spores

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 Most fungi reproduce by sexual sporulation.

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asexual spores that are generally responsible for

initiating human  infection.

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characterized by the production of flagellated sexual

oospores.

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chemtactic and can direct themselves toward suitable

nutrients within their local environment.

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Following germination, the flagella are lost and non-

motile vegetative cells develop.

•

Many are unicellular; others are composed of aseptate

hyphae.

•

Eg. Phytophthora infestans- it is the etiologic agent of

potato blight.

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are characterized by aseptate hyphae, asexual

sporangiospores, and sexual zygospores.

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For example, Rhizopus nigricans (black bread mold) and

mucor - common soil fungi.

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are usually harmless to humans

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occasionally they cause serious disease in diabetic

individuals and other people with lowered resistance to

infection.

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Used in the industrial production of



Tempeh (an Asian food)



The enzyme rennet, which is used in cheese

production



 Beta-carotene (nutritional supplement coloring

agent for margarine)



steroids.

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Includes yeasts and many of the common molds that

can produce ascospores.

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include many fungi beneficial to man, such as the

yeasts of the genus saccharomyces that leaven bread

and ferment alcoholic beverages.

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They include fungi as:

•

Pencillum notatum

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mold that used to produces the antibiotic penicillin.

•

A mold Aspergillus

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common microbial contaminants.

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causes a fatal pulmonary disease (Aspergillosis) in

people with  lowered resistance.

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consists of higher fungi that produce exogenous

sexual basidiospores on a basiduim or base and

septate hyphae.

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This class includes mushrooms, toad stools, and

puffballs are considered fleshy fungi;

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Don’t known sexual cycle of reproduction

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 These organisms are all grouped with the

deuteronmycetes.

•

Since sex is either non existent or undiscovered in these

organisms, they are commonly called the imperfect

fungi.

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All fungi are carbon heterotrophs, which means they are

dependent on exogenous nutrient substrates as sources

of organic carbon

•

 with a few exceptions, fungi are obligate aerobes.

•

Many species are capable of maintaining metabolic

activity in the most basic of nutrient mediums.

•

The known metabolic types of fungi include thermophilic,

psychrophilic, acidophilic, and halophilic  species.

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The metabolic capabilities of fungi are exploited in the

food industry (e.g., in the production of bread, wine,

beer, cheese, or single-cell proteins) and in the

pharmaceutical industry (e.g., in the production of

antibiotic substances, enzymes, citric acid, etc.).

•

The metabolic activity of fungi can also be a damaging

factor. Fungal infestation can destroy foods, wooden

structures, textiles, etc.

•

Fungi also cause numerous plant diseases, in particular

diseases of crops.

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In general the metabolism of fungi

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Most fungi:

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Saprophytic, free living organisms

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Obtain nutrients from dead organic matter

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Important decomposers in biogeochemical

cycles.

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 Many fungi have evolved successful relationships with

plants or microbes in which both partners benefit

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Example Lichens

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Lichen is a composite of a fungus and a photosynthetic

microbe, either an algae or cyanobacterium.

•

The fungus functions efficiently as absorption machinery,

extracting water and minerals for the photosynthetic

process.

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At least half of the organic food products of

photosynthesis are provided to the fungus by the

cyanobacterium or alga. 

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

Some fungi acquire nutrients by attacking live animals

or plants.



Of the 150,000 known species of fungi, about 50 have

been identified as primary human pathogens

.

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Little is known about the pathogenic  mechanisms and

virulence factors involved in fungal infections.

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Some fungus like yeasts, are able to colonize the

mucosal surfaces of the GI and female genital tracts.

•

It has been shown experimentally that the ability to

adhere to buccal or vaginal epithelial cells is associated

with colonization and virulence.

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Adherence usually requires a surface adhesin on the

microbe and a receptor on the epithelial cell.

•

In the case of 

C. albicans, mannoprotein components

extending from the cell wall  

have been implicated as the

adhesin

•

 Fibronectin and other components of the extracellular

matrix act as the receptor(s).

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

Passing an initial surface barrier, whether skin,

mucous membrane, or respiratory epithelium,  is an

important step for most successful pathogens.



Some fungi are introduced through mechanical

breaks.



 Fungi that initially infect  the lung must produce

conidia small enough to be inhaled past the upper

airway defenses.( arthroconidia of 

Coccidioides

immitis )

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Triggered by temperature and possibly other cues,

dimorphic fungi  change their morphology and growth to

a more invasive form.

•

Extracellular enzymes (eg,proteases, elastases) are

associated with the hyphal form of 

Candida and with the

invasive 

forms of many of the dimorphic and other

pathogenic fungi.

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None of the extracellular products of opportunistic fungi

or dimorphic pathogens have been shown to injure the

host directly during infection in a manner analogous to

bacterial toxins.

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o

x

i

n

s

,

c

a

l

l

e

d

m

y

c

o

t

o

x

i

n

s

,

i

n

t

h

e

e

n

v

i

r

o

n

m

e

n

t

b

u

t

n

o

t

i

n

v

i

v

o

.

49

Disease cont’d

•

The structural components of the cell do not cause

effects unlike gram negative lipid A component though

mannan is known to circulate widely in the body.

•

The injury caused by fungal infections seems to be due

primarily to

•

 The inflammatory immune responses that are

stimulated by the prolonged presence of the

fungus.

50

1

.

6

.

O

v

e

r

v

i

e

w

o

f

F

u

n

g

a

l

d

i

s

e

a

s

e

F

u

n

g

a

l

A

l

l

e

r

g

i

e

s



Fungal allergies are common



Molds grow on any damp organic surface, and spores

are constantly in the air.



Generally occurs in individuals with other allergies.

51

F

u

n

g

a

l

d

i

s

e

a

s

e

c

o

n

t

’

d

•

M

y

c

o

t

o

x

i

c

o

s

i

s

•

Some fungi produce toxic substances that poison a

person who ingest them.

•

These poisonous substance are collectively called

mycotoxins (myco= fungus, toxin= poison).

•

Mycotoxcosis may result from ingestion of fungal

contaminated foods. Example – Poisonous mushroom

•

Most fungal toxins are produced when the fungus grows

in moist environment at relatively high temperature.

52

F

u

n

g

a

l

d

i

s

e

a

s

e

C

o

n

t

’

d

•

F

u

n

g

a

l

i

n

f

e

c

t

i

o

n



Diseases caused by fungi are collectively called

mycoses (singular, mycosis).



There are four general categories on the basis of the

primary tissue affinity of the pathogen.

53

S

u

p

e

r

f

i

c

i

a

l

m

y

c

o

s

e

s



Are fungal infections confined to the outer most dead

layers of skin, hair and nails



Symptoms - discoloration, scaling, or de-pigmentation

of the skin.



Do not elicit a cellular response from the host.



Infections are generally painless.

54

C

u

t

a

n

e

o

u

s

M

y

c

o

s

e

s

•

Restricted to keratinized layer of the skin (epidermis),

hair, or nails.

•

Elicit an inflammatory response, resulting in more

serious skin diseases with uncomfortable painful and

sometimes even crippling symptoms.

•

Symptoms - include

•

Itching, Scaling or ring like patches of the skin;

•

Brittle or broken hairs;

•

Thick discolored nails.

55

S

u

b

c

u

t

a

n

e

o

u

s

m

y

c

o

s

e

s



Are infections confined to :



The subcutaneous tissue (dermis or fatty tissues)



The deeper layer of the skin



Symptoms include :



Ulcers, that progress and do not heal and the

presence of draining sinus tracts.

56

S

y

s

t

e

m

i

c

m

y

c

o

s

e

s

•

Are caused by pathogenic and/or opportunistic fungi

affect internal organs or tissues of the body.

•

Symptoms

•

very general as fever, and fatigue;

•

a chronic cough and chest pain.

•

Are infections caused by a group of fungi which cause

infections in patients who are immunocompromised,

57

1

.

7

.

P

r

e

d

i

s

p

o

s

i

n

g

F

a

c

t

o

r

s

f

o

r

f

u

n

g

a

l

i

n

f

e

c

t

i

o

n

Increased moisture 



It creates a more hospitable environment for

proliferation of pathogens.

O

b

e

s

i

t

y



Obesity promotes accumulation of surface moisture in

body folds.

58

P

r

e

d

i

s

p

o

s

i

n

g

c

o

n

t

’

d

I

n

v

a

s

i

v

e

m

e

d

i

c

a

t

i

o

n

p

r

o

c

e

d

u

r

e

s



I

t

d

i

s

r

u

p

t

s

t

h

e

p

h

y

s

i

c

a

l

i

n

t

e

g

r

i

t

y

o

f

t

h

e

s

k

i

n

.

W

o

u

n

d

s

•

Exposes the underlying tissue to environment that

facilitates their development of skin infection.

•

Disease can be prevented by  practicing good hygiene

and sanitary habit to prevent wound colonization.

59

P

r

e

d

i

s

p

o

s

i

n

g

c

o

n

t

’

d

T

i

g

h

t

c

l

o

t

h

i

n

g

•

It also disrupts the physical integrity of skin through

continual abrasion.

U

n

d

e

r

l

y

i

n

g

d

e

b

i

l

i

t

a

t

i

n

g

d

i

s

e

a

s

e



Cause malfunction of the immune system

60

P

r

e

d

i

s

p

o

s

i

n

g

c

o

n

t

’

d



D

e

c

r

e

a

s

e

p

h

a

g

o

c

y

t

i

c

f

u

n

c

t

i

o

n

.



D

i

a

b

e

t

i

c

s



Impairs granulocyte function



M

a

y

p

r

e

v

e

n

t

p

r

o

t

e

c

t

i

v

e

c

e

l

l

s

i

n

c

i

r

c

u

l

a

t

i

o

n

f

r

o

m

r

e

a

c

h

i

n

g

l

o

c

a

l

t

i

s

s

u

e

s



H

I

V

i

n

f

e

c

t

i

o

n



D

e

f

e

c

t

i

n

c

e

l

l

s

o

f

t

h

e

i

m

m

u

n

e

s

y

s

t

e

m

l

e

a

d

t

o

p

r

a

c

t

i

c

a

l

o

r

c

o

m

p

l

e

t

e

l

o

s

s

o

f

i

m

m

u

n

i

t

y

61

P

r

e

d

i

s

p

o

s

i

n

g

c

o

n

t

’

d

P

r

o

l

o

n

g

e

d

a

d

m

i

n

i

s

t

r

a

t

i

o

n

o

f

c

o

r

t

i

c

o

s

t

e

r

o

i

d

s

o

r

a

n

t

i

b

i

o

t

i

c

o

r

c

h

e

m

o

t

h

e

r

a

p

y

•

It decrease rate of surface shedding.

•

D

i

s

c

o

n

t

i

n

u

e

t

h

e

m

e

d

i

c

a

t

i

o

n

t

o

a

l

l

o

w

n

o

r

m

a

l

e

p

i

t

h

e

l

i

a

l

s

h

e

d

d

i

n

g

t

o

r

e

s

u

m

e

.

M

a

l

n

u

t

r

i

t

i

o

n

I

t

i

m

p

a

i

r

s

i

m

m

u

n

o

l

o

g

i

c

s

y

s

t

e

m

.

B

u

r

n

s

Expose the underlying tissue, diminish immunologic

capacity.

62

1

.

8

.

F

u

n

g

a

l

I

m

m

u

n

i

t

y

P

h

a

g

o

c

y

t

e

I

n

t

e

r

a

c

t

i

o

n

s

•

There is considerable evidence that normal persons

have a high level of natural resistance to most fungal

infections.

•

 This is particularly true of opportunistic molds.

•

An important component of this resistance is the ability

of healthy neutrophils to kill hyphae of most fungi if they

reach the tissues.

63

F

u

n

g

a

l

i

m

m

u

n

i

t

y

c

o

n

t

’

d

•

A small number of species, all of which are dimorphic,

are able to produce mild to severe disease in otherwise

healthy individuals.

•

C. immitis, one of the best-studied species, has been

shown to contain a component in 

the wall of its conidial

(infective) phase that is antiphagocytic.

64

F

u

n

g

a

l

i

m

m

u

n

i

t

y

c

o

n

t

’

d

A

d

a

p

t

i

v

e

I

m

m

u

n

e

R

e

s

p

o

n

s

e

•

the course of any fungal disease is worse in

immunocompromised than previously healthy persons

.



H

u

m

o

r

a

l

I

m

m

u

n

i

t

y

•

Antibodies can be detected at some time during the

course of almost all fungal infections, but for most there

is little evidence that they contribute to immunity.

•

The only encapsulated fungus, 

Cryptococcus

neoformans, is one example of a fungus against which

antibody plays 

a role in controlling infection

65

F

u

n

g

a

l

i

m

m

u

n

i

t

y

c

o

n

t

’

d



C

e

l

l

u

l

a

r

I

m

m

u

n

i

t

y

•

Considerable clinical and experimental evidence points

toward the importance of cellular immunity in fungal

infections.

•

 Most patients with severe systemic disease have

neutropenia or depressed T lymphocyte–mediated

immune reactions.

66

S

u

m

m

a

r

y

•

Define medical Mycology.

•

 Define fungi.

•

List and explain the general characteristics of fungi.

•

Discuss mold and yeast form of the fungi.

•

Discuss fungal disease of the fungi.

•

Mention the predisposing factors.

•

Explain immunity to fungal infection.

R

e

f

e

r

e

n

c

e

s

•

C.j.Alexopoulos and Mims. Introductory mycology third

edition,1979

•

Sherris, Medical Microbiology, An introduction to

infectious disease

•

Jawetz, Melnick, & Adelberg's Medical Microbiology, 24th

Edition

68