Last updated on September 18th, 2021
Candida albicans is an endogenous organism, found in 40 to 80% of normal human beings as commensal in gastrointestinal tract, oropharynx. It is a commonest cause of candidiasis (moniliasis). Candida albicans is a gram-positive yeast with a single bud. Other most common isolates of Candida species are Candida tropicalis, Candida parapsilosis, and Candida glabrata.
Some of the multiple choice questions test your knowledge regarding this fact; e.g., MCQs like, ‘all of the following organisms are dimorphic fungi except’; or ‘which of the following fungal pathogen is Gram positive in nature’
In direct stained smear (of the pathogenic sample), the yeasts can often be seen attached to pseudohyphae. Both the yeasts and pseudohyphae are Gram-positive.
Most Candida infections are opportunistic, occurring when a patient has some alteration in cellular immunity, normal flora, or normal physiology. Infection may be caused by endogenous yeasts or may be nosocomial. Candidiasis is the most common fungal infection in HIV-infected individuals.
- The more debilitated the host, the more invasive the disease.
- Associated with prolonged broad-specturm antibiotic or steroid therapy
- Invasive procedures e.g. surgery / indwelling catheters predispose to Candida
- Fibronectin receptor on Candida albicans facilitates its adherence to the (fibronectin, a component of the host extracellular matrix) epithelium of the gastrointestinal or urinary tract.
- Hydrophobic molecules on the surface of Candida also helps in adhesion.
- Aspartyl proteases found in C. albicans has shown increased ability to cause disease in animal models.
- Phenotypic switching and presence of phospholipase also play a role in pathogenesis.
C.albicans is responsible for several different types of infections in healthy and immunocompromised patients. Main diseases include;
- Oropharyngeal candidiasis (oral thrush): Common in those with HIV/AIDS.
- Vulvovaginal candidiasis (vaginal thrush): Common infection during pregnancy
- Candidemia/disseminated infections
Other diseases caused by Candida are paronychia, onychomycosis, endocarditis, eye infection, intertriginous candidiasis, etc. Disseminated infection of Candida and meningitis is seen mostly in immunocompromised and/or seriously ill patients.
Specimen depends on disease presentation. The common submitted sample includes; urine (in case of UTI), vaginal discharge (suspected cases of vaginal thrush) or CSF (when meningitis is suspected), sputum (when pneumonia* is suspected), blood, or other exudates from the mucosal surface.
*Note: Candida is not a cause of pneumonia, except possibly in leukemia or lung transplant patients or in neonates. Even in those cases, the growth of Candida in lower respiratory specimens, regardless of species, does not correlate with disease. Yeasts are normal inhabitants of the mouth.
Microscopy and Staining
Candida yeast cells can be detected in unstained wet preparations or Gram stained preparations of sample. In Gram-stained smears, Candida appears as gram-positive budding yeast cells (blastoconidia) and/or pseudohyphae showing regular points of constriction.
Routine bacterial culture is sufficient for the detection of Candida species whether aerobic blood culture bottle or agar media are used.
Candida albicans grows well on Sabouraud dextrose agar and most routinely used bacteriological media. Cream-colored pasty colonies usually appear after 24-48 hours of incubation at 25-37°C. The colonies have a distinctive yeast smell and the budding cells can be easily seen by direct microscopy in stained or unstained preparations.
Candida albicans can be recognized by the formation of hyphal elements radiating from colonies on blood-containing media within 48 h of initial incubation. These colonies with “feet,” which can also resemble stars.
In Blood Agar, Candida albicans gives white, creamy colored colonies which can be mistaken for Staphylococcus spp. Whenever you are analyzing the culture report of ‘high vaginal swab’, take extra care as the colony you are observing can be of Candida albicans instead of Staphylococcus aureus or vice versa (quick solution for this is to perform wet mount or gram staining and observing under a microscope).
Germ tube test
Candida albicans can be identified presumptively by a simple germ tube test.
Yeasts can be presumptively identified as Candida albicans when there is the appearance of “feet” on blood agar in 48 h and/or the germ tube is positive.
Molecular methods may be used to identify Candida species within 60
min from positive blood cultures when the Gram stain is suspicious for
Candida spp., oval yeasts with or without pseudohyphae. A FISH method
using PNA probes, called the Yeast Traffic Light PNA FISH assay
(AdvanDx, Woburn, MA), can differentiate the most commonly encountered Candida spp. causing fungemia.
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References and further readings
References and further readings
- Amy L Leber. Clinical Microbiology Procedures Handbook, 4th Edition (2016). ASM Press, Washington, DC.
- Procop, G. W., & Koneman, E. W. (2016). Koneman’s Color Atlas and Textbook of Diagnostic Microbiology (Seventh, International edition). Lippincott Williams and Wilkins.
- Tille, P. (2017). Bailey & Scott’s Diagnostic Microbiology (14 edition). Mosby.