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Opportunistic mycoses


1. Candidiasis

  1. Commonest mycosis involving skin and its appendages, mucosa & internal organs
  2. Caused by yeast like fungus
  • Candida albicans (common species 80-90% of cases, germ tube positive)
  • stellatoidea (germ tube positive)
  • dubliniensis (germ tube positive)
  • C. tropicalis, C. guilliermondii, C. krusei, C. glabrata, C. lusitaniae

iii. They occur as normal flora of skin, mucous membranes, gastro intestinal tract. Therefore, it is an endogenous infection



iv. Predisposing factors: AIDS, Diabetes, IV catheters, neutropenia, malignancy etc.



v. Clinical types:

  • Mucocutaneous: oral (thrush), oesophageal, vulvovaginitis, balanitis, conjunctivitis, keratitis
  • Skin & nail infections: intertrigenous areas (axilla, groin, perineum, submammary folds)
  • Systemic: UTI, pulmonary, endocarditis, meningitis, arthritis, osteomyelitis 

C. albicans- produces pseudohyphae both in culture & in tissues.
Secr. Asp. Proteinase (SAP)
Presence of pseudohyphae indicate colonization
Grow well on SDA & on blood agar also at 25 - 37°C


a. Laboratory diagnosis:

  1. Specimens: exudates from lesions, skin & nail scrapings, blood culture
  2. Direct microscopic examination: Gram staining, KOH mount
  3. Oval or spherical gram positive yeast cells with pseudohyphae (buds continues to grow but fails to detach, elongated cells are constricted at septations)
  4. Culture: Growth on SDA at 37°C shows cream colored smooth pasty colonies

b. Identification

  1. Germ tube test: culture inoculated in sheep or human serum and incubated at 37°C for 2-4 hours. A drop of suspension examined under microscope. Long tube like projections extending from yeast cell (true hypha). There is no constriction at the point of attachment. Called as Reynolds-Braude phenomenon.
  2. Growth on corn-meal agar (C. albicans produces terminal chlamydospores)
  3. Physiological tests for identification- Sugar fermentation tests and Sugar assimilation tests

C. C dubliniensis-
Seen in HIV patients
Resistance to fluconazole
Dark green on chrom -agar
Germ tube +


2. Cryptococcosis

  1. Caused by encapsulated true yeast, Cryptococcus neoformans
  2. Five serotypes based on polysaccharide capsular antigen (A,B,C,D,AD)

C. neoformans var. neoformans

C. neoformans var. gattii

Filobasidiella neoformans

Filobasidiella bacillispora

Serotype A,D,AD

Serotype B,C

Prevalent throughout the world

Parts of California, Australia

Isolated form pigeon excreta

Associated with eucalyptus trees (river reed gum)

More common

Less common

Spherical yeast cell

Oval yeast cell

Susceptible to canavanine

Resistant to canavanine

Cannot utilize glycine & d-proline as sole source of nitrogen

Can utilize glycine & d-proline as sole source of nitrogen

Urease not sensitive to EDTA

Urease sensitive to EDTA


iii. Infection occurs after inhalation of yeast cells


iv. From lungs yeast can metastasize to any organ of the body


a. Clinical forms:

  1. Pulmonary (commonest type)
  2. CNS (meningitis)
  3. Cutaneous/Osseous/Visceral 

b. Laboratory diagnosis:

  1. Specimens: CSF, tissue sections
  2. Direct microscopic examination: Negative staining (nigrosin/ India ink) to demonstrate capsule,  PAS, methenamine silver, H&E, Mayer’s mucicarmine staining
    Spherical budding yeast (5-20μm in diameter) cells surrounded by wide capsule that may be twice as thick as diameter of the cell
  3. Culture: Growth on SDA at 37°C shows cream colored smooth pasty colonies

C. Identification

Produces enzyme phenoloxidase (brown colored colonies on bird seed agar/ niger seed agar)

Produces enzyme urease

Inositol & nitrate assimilation test positive

Mice pathogenicity test positive

Antigen detection: in serum and CSF by Latex agglutination; diagnostic and prognostic importance


3. Aspergillosis

  1. Important species: A. fumigatus, A. niger, A. flavus, A. terreus, A. nidulans
  2. Inhalation of conidia or mycelial fragments
  3. Predisposing factors:
  • Immunosuppressive therapy,
  • neutropenia,
  • malignancy,
  • bone marrow transplantation,
  • AIDS 

a. Clinical forms:

  1. Allergic bronchpulmonary aspergillosis
  2. Aspergilloma (fungus ball)
  3. Invasive aspergillosis (angioinvasive)
  4. Endocarditis,
  5. Paranasal granuloma 

b. Laboratory diagnosis:

  1. Specimens: sputum, BAL, transbronchial biopsy
  2. Direct microscopic examination:10% KOH preparation, H&E, methenamine silver staining; special fungal stains better than H&E staining: Hyaline, septate hyphae, 3-5μm in diameter, dichotomus branching at acute angle
  3. Culture: SDA at 25°C, isolate identified by growth characteristics and microscopic morphology 

4. Zygomycosis/ mucormycosis

  1. Caused by fungi of the class Zygomycetes
  2. Rhizopus spp., Mucor spp., Absidia spp. Commonest species Rhizopus arrhizus
  3. Predisposing factors:
  • Diabetes mellitus,
  • metabolic acidosis,
  • leukaemia,
  • lymphoma etc.


a. Clinical forms:

  1. Rhinocerebral: serious fulminating infection, angioinvasive, thrombosis and ischaemic necrosis of the tissue
  2. Pulmonary/ Gastrointestinal/ Disseminated/Dermal/Subcutaneous 

Laboratory diagnosis:

  1. Specimens: tissue biopsy
  2. Direct microscopic examination: H&E staining best, special fungal stains not useful
    Hyaline, aseptate hyphae, ribbon like, 7-15μm in diameter, irregular branching at the obtuse angle
  3. Culture: SDA at 37°C, isolate identified by growth characteristics (dense, hairy colony) and microscopic morphology (aspetate hyphae, sporangiophore, sporangiospores)

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