Yeast and Carbohydrate Assimilation

• Yeast


  Appears when a patient is immunocompromised (usually diabetic patients), utilizes necessary nutrients for its growth and dissemination
• Infection or yeast are difficult to treat and requires aggressive treatment
• Subclasses of yeast

  • True yeast
  • Yeast-like
• True yeast


  Exhibit sexual reproduction whether formation of ascospore or basidiospores or sporulation
• Yeast-like


  Does not exhibit sexual reproduction
• Principal groups of yeast


  • True yeasts
  • False yeasts
• True yeasts


  Produce spores within the cell
• False yeasts


  Do not form spores under any condition
• Examples of true yeasts

  • Saccharomyces cerevisiae (common bread or baker's yeast)
  • Saccharomyces cerevisiae var. ellipsoideus (wine yeast)
• Examples of false yeasts

  • Cryptococcus (commonly found in cream and butter)
  • Mycoderma (causes grayish-white scum on pickle brines)
• Yeast appearance


  • Creamy, yellow, tan to salmon pink color
  • Some are phaeoid (dark pigmented) due to melanin content in cell wall
  • Other species show butter-like, velvety and wrinkled colonies
• Yeasts have a wide array of color and texture that varies from strain to strain
• Oral thrush


  Commonly caused by Candida albicans, affects immunocompromised patients, accompanied by bleeding, redness, painful lesions on the plate area and loss of taste
• Vaginal yeast infection


  Usually caused by change in vaginal environment, hormonal changes or weakened immune system, can also be transmitted sexually, exhibits redness, itchiness of vulva, burning sensation during urination, odorless smell, creamy discharge which looks like cottage cheese
• Cutaneous candidiasis


  Topical candida is mostly found in the crypts and folds of the skin that provide moisture (such as sweat) and aerobic environment, can cause redness, blisters, and pustules, often characterized as itchy bumps that thrive off moisture
• Systemic candidiasis/invasive candidiasis


  Candidemia is a condition where the Candida infection gets introduced in the bloodstream circulation, due to excessive immunosuppression, antibiotic use, nosocomial infection and central venous catheters, overuse of antibiotics can eliminate bacterial normal flora and allows the proliferation/overgrowth of yeasts
• Cryptococcus


  Most important causes of meningitis, pulmonary disease and septicemia, Cryptococcus neoformans being the most notable pathogen, found in soil contaminated with pigeon dropping and acquired by inhalation, demonstrates a capsule that produces a mucoid colony similar to Klebsiella, can be detected in spinal fluid with India ink or Nigrosin, latex agglutination is preferred over India ink due to its slow sensitivity
• Geotrichum


  Has been implicated in pulmonary disease in immunocompromised patients, microscopic evaluation reveals abundant arthroconidia formed from the vegetative hypha, colonies can exhibit white color to cream, yeastlike and can be confused with trichosporon, demonstrates arthroconidia
• Carbohydrate assimilation


  Uses sugar fermentation tests, labor intensive and time consuming, assimilation tests identify which carbohydrates a yeast can use aerobically as a sole carbon, assimilation patterns can be determined from automated identification system or various manual procedures
• API 20E system


  Used for carbohydrate assimilation pattern for determining Enterobacteriaceae and non-fastidious organisms, principle is color change
• API 20C system


  Used for yeast carbohydrate assimilation, most commonly used, freeze dried sugars are placed onto wells on a plastic strip, yeast isolates are pipetted and suspended into the wells, incubated for 72hrs at 30'C, as sugars assimilated, the wells become turbid with growth, results are matched with computer database, identifications are supported by percentage probability, utilizes glucose, glycerol, arabinose, xylose, xylitol, does not use inositol, lactose, trehalose, and O as sole carbon source
• Cornmeal agar for yeast identification


  Purpose is to aid in the identification of pathogenic yeasts by microscopic morphology, isolated yeasts are inoculated onto the surface of the agar plate for 3 days and examined microscopically for the presence of hyphae, pseudohyphae, arthroconidia, chlamydoconidia, and blastoconidia
• True yeast thatcan be founf in wine yeast

  Saccharomyces cerevisiae var. ellipsoideus