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MYCOLOGY
Mycology is the study of fungus
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Fungi are eukaryotic Protista that differ from bacteria and other
prokaryotes in many ways
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The simplest type of fungus is the unicellular budding yeast
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A tangled mass of hyphae constitutes the mycelium
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Hyphae may be septate or non-septate
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Yeasts are unicellular fungi which occur as spherical or ellipsoidal cells
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The elongated cells form a pseudomycelium
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Moulds are filamentous fungi
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They reproduce by the formation of different types of spores
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Dimorphic fungi can occur as filaments or as yeasts, depending on the conditions of growth
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The systematic classification of fungi, based on their sexual spore formation, recognizes four classes:
1
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They form endogenous
asexual spores, called sporangiospores, contained within swollen sac-like structures called
sporangia
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2
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They form sexual spores called ascospores
within a sac or ascus
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3
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4
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Most fungi of medical importance belong to this group
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MYCOSES
Infection caused by fungus is known as mycoses
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1
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Fungi causing superficial mycoses are
specialized saprophytes, with the capacity to digest keratin
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In surface infections the fungi live exclusively on the dead layers of the skin and its appendages
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The only changes produced
are cosmetic effects
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The most important cutaneous infection is dermatophytosis caused by a group of related fungi called
the Dermatophytes
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A variety of inflammatory and allergic
responses are induced in the host by the presence of the fungi and their metabolic products
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Though Candida infection is mostly
confined to the skin and mucosa, it can also cause systemic disease rarely, involving any organ
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1
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It is characterized by mild chronic infection of
the stratum corneum
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The areas involved are mainly the chest,
abdomen, neck, upper arms and back
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It occurs mainly in young
adults
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The fungus can be grown on Sabouraud’s agar, covered with a layer
of olive oil
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Tinea Nigra:
Tinea Nigra is characterized by black brown patches affecting the thickly keratinized sites such as palms
and soles
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Skin scrapings show brownish, branched septate
hyphae
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3
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The nodules are composed of fungus elements cemented together on the hair
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It is characterized by black hard nodules on hair shaft of
beard and scalp
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White piedra is characterized by white nodules on hair
shaft of axilla, moustache and beard
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Dermatophytoses:
Dermatophytes are a group of filamentous fungi that infect only superficial keratinized tissues such as
the skin, hair and nails
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Tinea Pedis (Athlete’s Foot) is the most prevalent
of all dermatophytoses
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Dermatophytes have been
Epidermophyton
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Tinea capitis - scalp
Tinea cruris - groin
Tinea pedis - foot
Tinea barbae - bearded area of the face and neck

2
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Subcutaneous
mycoses involve:
1
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The disease is quite common in Tamil Nadu, also called Madura
foot
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The causative agent is
believed to enter through minor trauma
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The abscess
bursts with discharging viscid, seropurulent fluid containing granules
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2
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The fungus enters the body through a
wound
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The lesions consist of
warty cutaneous nodules which resemble the florets of cauliflower
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The etiological agents are soil inhabiting fungi of the family Dematiaceae
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The disease is mainly tropical and is more common among barefoot agricultural workers and
woodcutters
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Sporotrichosis:
Sporotrichosis is a nodular, ulcerating disease of skin and subcutaneous tissue
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The fungus is a saprophyte found
widely on plants, thorns and timber
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The fungus spreads from the primary site through lymphatics upto the regional lymph nodes and rarely
beyond that
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Rhinosporidiosis:
Rhinosporidiosis is a chronic granulomatous disease characterized by the development of polyps, usually
confined to the nose, mouth or eye
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The causative agent is Rhinosporidium seeberi
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SYSTEMIC MYCOSES
Systemic mycoses are caused by fungi that are mostly soil saprophytes
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Systemic
mycoses occur in varying degrees of severity, ranging from asymptomatic infection to fatal disease
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Histoplasmosis:
Histoplasmosis is an intracellular infection of the reticuloendothelial system caused by the dimorphic
fungus Histoplasma capsulatum
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Infection is acquired by inhalation of spores
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The reticuloendothelial system is involved with
resultant lymphadenopathy, hepatosplenomegaly, fever, anemia and a high rate of fatality
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2
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It is a chronic
infection of the lungs which may spread to other tissues, particularly skin, bone and genitourinary tract
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Primary infection of the
lung may resemble tuberculosis or histoplasmosis
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The cutaneous disease is usually
on the skin of the face or other exposed parts of the body
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Coccidioidomycosis:
Coccidioidomycosis is an infection caused by the dimorphic fungus Coccidioides immitis
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The infection may be inapparent, benign, severe or even fatal
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Many persons develop
a self-limited influenza-like fever
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Cryptococcosis:
Cryptococcosis is a chronic infection caused by the yeast Cryptococcus neoformans
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Infection is usually
acquired by inhalation but may sometimes be through the skin or mucosa
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Pulmonary cryptococcosis may lead to a mild pneumonia
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Visceral forms simulate tuberculosis and cancer
clinically
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Cutaneous cryptococcosis varies from small ulcers to large
granulomas
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5
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The disease is caused by inhalation of spores from the environment
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OPPORTUNISTIC SYSTEMIC MYCOSES
Opportunistic infections occur in patients with debilitating disease such as cancer or diabetes, or in
whom the physiological state has been upset by immunosuppressive drugs, steroids, x-rays or
antibiotics
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They are called
opportunistic fungi
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Candidiasis:
Candidiasis (Candidosis) is an infection of the skin, mucosa, and internal organs, caused by a yeast-like
fungus Candida albicans
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Candidosis is
an opportunistic endogenous infection, the commonest predisposing factor being diabetes
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Intestinal candidiasis is a frequent sequel to oral antibiotic therapy and may present as diarrhea
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2
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Human disease caused by aspergillus is otomycosis
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It is very common disease and is usually
caused by species of aspergillus (A
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fumigatus) and penicillium
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Aspergillus asthma: occurs in atopic individuals following inhalation of aspergillus spores
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Aspergilloma: The fungus Colonizes in the pre-existing pulmonary cavities, such as in tuberculosis
or cystic disease
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Disseminated aspergillosis: Disseminated aspergillosis involving the brain, kidney and other
organs is a fatal complication sometimes seen in debilitated patients on prolonged treatment
with antibiotics, steroids and cytotoxic drugs
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Zygomycosis:
Zygomycosis is caused by the fungal class Zygomycetes which include Mucor, Rhizopus, and absidia
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The primary infection is usually in upper
respiratory tract or nose
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Gastrointestinal mucormycosis: occur in malnutrition and diarroheal diseases
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Penicillosis:
Penicillium species rarely cause opportunistic human infections
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marneffei has been reported to be an
important opportunist pathogen in the HIV infected patients
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MYCOTOXINS
Many fungi produce poisonous substances called mycotoxins that can cause acute or chronic intoxication
and damage
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A variety of mycotoxins are produced by mushrooms and their ingestion
results in disease called mycetismus
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Other fungi produce mutagenic and carcinogenic
compounds that can be extremely toxic to animals
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It can cause
hepatomas in animals
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Microscopy:
Specimen is placed in a drop of potassium hydroxide on a microscopic slide and covered with a coverslip
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Hyphal structure, presence or absence of septa, and
other special structures help in diagnosis
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Culture:
Fungi are usually cultured in Sabouraud’s Dextrose Agar (SDA)
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Microscopy is performed from fungal colony to
study the morphology of hyphae, spores and other structures
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Tissue Sections:
Fungal elements in tissue can be identified by methenamine silver stain and Periodic Acid Schiff (PAS)
stain
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These drugs form complexes
with fungal cell membrane resulting in membrane leakage
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It can be used in the treatment of candidiasis,
cryptococcosis, and other dermatiaceous fungal infections
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Azoles:

The antifungal imidazoles (ketoconazole) and the triazoles (fluconazole) are used to treat a wide range of
systemic and localized infections
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Echinocandins:

These drugs act by inhibiting the cell wall synthesis and are used for the treatment of aspergillosis and
systemic candidiasis
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These drugs act by inhibiting ergosterol
synthesis

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