Candida

Introduction to Mycology (Candida)

  • Medical mycology – branch of medical science thatdeals with the study of medically important fungi
  • ‘fungus’ is derived from Greek ‘mykes’ meaning mushroom (a type of edible fungus)

Fungi differ from bacteria & other eukaryotes

  • Eukaryotic and possess eukaryotic cell organelles
  • Possess a rigid cell wall, composed of chitin, B-glucans and other polysaccharides
    • Cell membrane contains ergosterol instead of cholesterol
  • May be unicellular or multicellular
  • Lack chlorophyll and divide by asexual and/or sexual means by producing spores

Morphological Classification

  1. Yeast Round to oval cells that reproduce by budding
    • Cryptococcus neoformans (pathogenic)
    • Saccharomyces cerevisiae (non-pathogenic)
  2. Yeast-like: Yeasts forming pseudohyphae (e.g. Candida)
    • Differentiated from true hyphae as they have constrictjons at septa
  3. Molds: long branching filaments called hyphae
    • Hyphae – septate or nonseptate
  4. Dimorphic fungi: exist as molds (hyphal form) in the environment at ambient temperature (25°C) and as yeasts in human tissues at body temperature (37 degree C) Histoplasma capsulatum

Morphological forms of fungi

Taxonomical Classification

  • Based on the production of sexual spores
    1. Phylum Zygomycota: sexual spores – zygospores, and possess aseptate hyphae, e.g. Rhizopus and Mucor.
    2. Phylum Ascomycota: Sexual spores – ascospores and possess septate hyphae, e.g. Aspergillus.
    3. Phylum,Basidiomycota: Sexual spores – basidiospore e.g. Cryptococcus
    4. Phylum Deuteromycota (Fungi imperfecti): sexual state is either absent or unidentified yet

Culture Media

  • Sabouraud’s dextrose agar (SDA):
    • Most commonly used medium
    • Contains peptone (1%), dextrose (4%) and pH of 5.6
    • May not support some pathogenic fungi
  • Corn meal agar and rice starch agar:
    • Nutritionally deficient media used for stimulation of chlamydospore production
  • Brain heart infusion (BHI) agar and blood agar:
    • Enriched media, used for growing fastidious fungi like Cryptococcus and Histoplasma
  • Niger seed agar and bird seed agar: selective growth of Cryptococcus
  • CHROMagar Candida medium: selective as well as differential medium for speciation of Candida

Yeast

  • Yeasts are small round to oval single-celled budding organisms reproduce by budding
    • Cryptococcus
    • Trichosporcn
    • Rodotorula
    • Saccharomyces
    • Geotrichum

Yeast like organisms

  • Candida Species grow both as yeast and filamentous cells
  • Ecological Niche: Endogenous (mouth, gut, vagina)
  • GEOGRAPHIC OCCURENCE: WORLDWIDE

Infections with Candida usually occur when there is some alteration in:

  • Cellular immunity (Immunocompromised).
  • Normal Flora (Prolonged antibiotics)
  • Physiology (Cardiac surgery, catheter)
    Candida infections have increased due to transplant patients, prolonged ICU stay and invasive procedures

Virulence Factors

  • Adhesins
  • Enzymes – aspartyl proteinases and serine proteinases – tissue invasion
  • Toxins: Glycoprotein extracts of Candida cell wall are pyrogenic
  • Pseudohyphae: Presence of pseudohyphae indicates active infection
  • phenotypic switching – ¢ Ability to transform between three phenotypic forms in the tissue
    • yeast (blastospores), pseudohyphae, and true hyphae

Candida albicans versus non-albicans

Candida albicans Non-albicans candida
common candidemia more frequently in those with medical
devices or receiving sterolds
More virulent More resistant to antifungals
Less resistant to antifungals (azoles) – C.glabrata
– C. krusel
– C. haemulonil

CANDIDIASIS

  • The fourth most common nosocomial bloodstream infection.
  • Candida Spp are the most common organisms causing fungal UTI.
  • Candida albicans accounts for 74%
  • Glabrata 8%
  • Parapsolosis 7%
  • Tropicalid 3%

» Skin
» Vaginitis
» Urinary tract
» Mucous membranes
» Septicemia
» Endocarditis
» Pneumonia

Clinical Syndromes

  • Chronic mucocutaneous candidiasis-,
    • vulvovaginitis, Oropharyngeal candidiasis, Oesophagitis,Laryngitiis
  • Cutaneous candidiasis characterized by chronic, treatment resistant superficial Candida infections of the skin, nails.
  • Systemic candidiasis– Hematogenous dissemination (Nosocomial candidiasis- Fungemia especially in children admitted in pediatric ICU), UTI, endocarditis, endophthalmitis, pulmonary candidiasis (rare), meningitis (rare)
  • Allergic diseases– eczema, asthma, gastritis

Clinical Specimens of Candida

  • Sample depends on the presentation of the disease:
    • Sputum
    • Scrapings
    • material from cutaneous or mucocutaneous lesions
    • Blood smears
    • Vaginal discharge
    • Urine
    • Feces
    • Nail clippings

Laboratory Diagnosis

  • Microscopy
  • Culture
  • Serology
  • Molecular methods
  • Sugar assimilation test for speciation
  • Dalmau plate culture on corn meal agar for speciation.

Microscopy

  • Wet mount(10-20% KOH)– Budding yeast cells, pseudohyphae 10-12 microns in diameter pseudohyphae
  • Gram staining- gram positive budding yeasts with pseudohyphae
  • Mycelial forms – more significant (colonisation and tissue invasion)

Culture

  • grows overnight on most bacterial and fungal media.
  • Sabouraud dextrose agar
  • Blood agar
  • Colonies: creamy, white, smooth and have a yeasty odour
    • Candida albicans, culture on Sabouraud dextrose agar.

Colonies of Candida on SDA

Germ Tube Test

  • Also called Reynolds Braude phenomenon
  • Principle:
    • Approximately 95 – 97% of Candida albicans Isolated develop germ tubes when incubated in a proteinaceous media.
    • Rapid identification of C.albicans- germ tube test
    • Candida non albicans- germ tube test negative (C. tropicalis, C. krusei({R to fluconazole), C. glabrata (R to fluconazole), C. parasilosis, C. viswanathii, C. lusitanae etc.)
  • Germ tubes are short non-septate germinating hyphae. They are 1/2 the width and 3 – 4 times the length of the cell from which they arise. The junction of the germ tube and cell is not constricted. Buds and pseudo- hyphae can be distinguished from germ tubes by the constricted attachment.

Morphology of Corn meal agar

Tests for Species Identification

  • Growth at 45°C:
    • It differentiates C. albicans (grows) from C.ubliniensis (does not grow at 45°C)
  • Carbohydrate fermentation test
  • Carbohydrate assimilation
  • Molecular methods

Immunodiagnosis

  • Antibody detection: ELISA, latex agglutination tests – antibodies against cell wall mannan antigen
  • Antigen detection: cell wall mannan and cytoplasmic antigens – ELISA
  • Enzyme detection: enolase, aspartate proteinase
  • Test for metabolites: mannitol, arabinitol
  • G test is done for detection of b-1-3 -D-glucan
  • PCR

Therapy

  • Amphotericin B
  • Nystatin
    • Vaginitis
    • Cutaneous
  • Triazoles like Fluconazole, Itraconazole ,Voriconazoles
    • Cutaneous candidiasis : topical azole
    • C. glabrata and C. kruse/ exhibit intrinsic resistance to azoles
  • Echinocandins like caspfungin, micafungin

Candida auris

  • In 2009, a novel Candida species, Candida auris, in the C. haemulonii complex, was first described from a patient in Japan after its isolation from the external ear canal.
  • In the same year, 15 isolates of C. auris were reported from otitis media patients in South Korea.
  • Genotyping of these isolates revealed a clonal origin.
  • In addition, in 3 patients, persistent fungemia caused by C. auris was resistant fluconazole: 2 of these patients died.

Why is Candida auris is considered a worldwide health threat ?

  • It causes serious infections: like bloodstream infections . More than 1 in 3 patients with invasive C.auris infection die.
  • It’s often resistant to Antifyingals: Some C.auris infections are resistant to all types of antifungals.
  • It’s becoming more common. Discovered in 2009, it has spread quickly and caused infections in more than a dozen countries.
  • It’s difficult to identify: it can be misidentified.
  • It can spread in hospitals and nursing can spread through contact with affected patients and contaminated surfaces

How to stop the superbug?

  • Isolation of patients and their contacts,
  • Wearing of personal protective clothing by healthcare workers.
  • Screening of patients on affected wards.
  • Skin decontamination with chlorhexidine.
  • Environmental cleaning with chlorine-based reagents, and terminal room decontamination with hydrogen peroxide vapor.

Other Microbiology Notes :-

Ancylostoma duodenale (Hook worm)

Schistosoma: Blood Flukes (Bilharzia)

. Stool Examination

Subcutaneous Fungal Infection

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