LAB FINALS

• Yeast


  • Can isolate From almost any specimen
  • Generally considered normal flora
  • Opportunistic pathogen
• Yeast


  • Unicellular
• Yeast reproduction


  • Asexual
  • Blastoconidia (budding yeast)
  • Pseudohyphae = elongated blastoconidia
  • Arthroconidia
  • Sexual
  • Ascospores (acid fast positive - Saccharomyces species)
• Direct examination of yeast


  • Observe reproductive structures
  • Gram stain: large, grampositive budding yeast and pseudohyphae
  • India ink prep: observe for capsules around cells (Cryptococcus)
• Yeast growth


  • Rate: 2-3 days on Sheep Blood Agar and most other nonselective primary isolation media
  • Temperature: 22-37°C (best at 30°C)
• Yeast colony morphology


  • Appearance: Similar to bacterial colony, Moist, smooth
  • Texture: Can be glabrous (waxy), mucoid, butter-like, wrinkled, or velvety, Can see filaments around perimeter of colony
  • Color: White, cream, tan (rarely pink or salmon)
  • May be confused with Staphylococcus species
• Yeast Identification Methods


  1. Germ Tube Production

  2. Microscopic morphology on Cornmeal-Tween 80 agar (CMT)

  3. Niger Seed Agar (Birdseed agar)

  4. Biochemical Tests
• Biochemical Tests


  • Urease production
  • Carbohydrate assimilation
  • Carbohydrate fermentation
  • Commercial identification systems
• Candida spp. identification


  • Candida albicans
  • Candida stellatoidea
  • Candida (Torulopsis) glabrata
  • Candida tropicalis
  • Candida krusei
• Dermatophyte genera


  • Microscporum
  • Epidermophyton
  • Trichophyton
• Dermatophyte culture
• Dermatophyte species


  • Microsporum species
  • Epidermophyton
  • Trichophyton species
• Specimen collection for dermatophytes


  1. Hair

  2. Skin

  3. Nails
• Direct examination of dermatophyte clinical material

  1. 10% KOH preparation

  2. Wood's Lamp
• Culture of dermatophyte clinical material


  1. Specimen processing

  2. Media

  3. Growth requirements
• Dermatophyte identification


  1. Growth rate

  2. Colony morphology

  3. Microscopic morphology
• Hair baiting for dermatophytes
• Procedure for collecting soil and hair


  1. Soil collected from any area

  2. Obtain sterile human hair

  3. Sterilize hair

  4. Obtain sterile water
• Procedure for hair baiting

  1. Add soil to petri dish

  2. Moisten soil with sterile water

  3. Sprinkle sterile hair onto soil

  4. Incubate at room temperature

  5. Examine hair microscopically
• It has been reported that one can recover more keratinophilic fungi from soils by adding antibiotics to the sterile water
• Antibiotics recommended


  • Penicillin (500 mg/ml)
  • Streptomycin (300 mg/ml)
  • Actidione (0.5 mg/ml)
• Many isolates of Trichophyton rubrum and Trichophyton mentagrophytes are difficult to distinguish between on the basis of colony morphology and microscopic appearance
• The hair penetration test is the best method to distinguish between these two dermatophytes
• Hair penetration test procedure

  1. Obtain sterile human hair

  2. Cut hair into 1 cm segments

  3. Autoclave hair

  4. Add sterile water and yeast extract to petri dish

  5. Inoculate fungus

  6. Incubate

  7. Examine hair microscopically for penetration
• If no penetration is observed after 4 weeks, the organism is Trichophyton rubrum. If penetration is observed, the organism is Trichophyton mentagrophytes
• This hair penetration test can only be used on T. mentagrophytes or T. rubrum, not on unknown dermatophytes
• Wood's light


  Ultraviolet light that can cause infected hair to fluoresce a bright yellow-green colour
• Procedure for using Wood's light


  1. Examine patient's scalp in a darkened room

  2. Remove fluorescing hair with forceps

  3. Use fluorescing hair for microscopic examination
• Organisms that cause positive fluorescence


  • Microsorum canis
  • Microsorum audouinii
• Organisms that cause negative fluorescence


  • Microsporum gypseum
  • Trichophyton species
• Rice medium


  Helpful to distinguish Microsporum audouinii from Microsporum canis
• Procedure for rice medium


  1. Add rice and water to flask

  2. Autoclave

  3. Inoculate with fungus

  4. Observe growth
• Microsporum audouinii on rice medium


  Either does not grow or grows very poorly
• Microsporum canis on rice medium

  Grows abundantly
• Preservation of fungi by freezing


  1. Grow fungus on agar slant

  2. Freeze slant

  3. To subculture, thaw slant and transfer to fresh medium
• Preservation of fungi under oil


  1. Grow fungus on agar slant

  2. Sterilize paraffin oil

  3. Pour oil over fungus to cover

  4. To subculture, remove piece of fungus from under oil and transfer to fresh medium