Yeasts

• Most common, clinically encountered yeasts:

  • Candida albicans/dubliniensis
  • Candida auris
  • Candida parapsilosis complex
  • Candida tropicalis
  • Candida glabrata complex
• Name changes - former Candida:

  • Pichia kudriavzevii (formerly Candida krusei)
  • Kluyveromyces marxianus (formerly Candida kefyr)
  • Clavispora lusitaniae (formerly Candida lusitaniae)
  • Meyerozyma guilliermondii (formerly Candida guilliermondii)
  • Yarrowia lipolytica (formerly Candida lipolytica)
• Yeasts:

  • Eukaryotic
  • Unicellular
  • Oval, elongate or irregular shaped
  • Multiply by producing blastoconidia (spores) through budding, pseudohyphae
  • Colonies are moist, glabrous (smooth, creamy)
  • May produce capsule (mucoid)
  • May be darkly pigmented (Melanin production)
  • Identified by macro and microscopic methods
• Clinical Significance:

  • Yeast normal microbiota & common contaminants
  • Interpretation of a significant isolate is difficult from sites where yeast is considered normal microbiota
  • Need to consider quantity of yeast vs. bacteria
  • Isolation from sterile body sites is always considered significant
• Specimen Collection & Transport:

  • Special transport not needed
  • Should be < 2 hours
  • Yeast can withstand refrigeration
  • Special processing considerations for Malassezia furfur
• Identification:

  • Microscopic identification includes Gram stain, wet prep, KOH, Calcofluor white stain, India Ink
  • Macroscopic identification includes growth characteristics, colony appearance, and biochemical tests like germ tube test, chromogenic agars
  • Biochemical identification includes rapid systems, molecular methods, and phenotypic characterization methods
• Candida spp. Virulence Factors:

  • Reversible Morphological Transitions
  • Adhesion
  • Invasion & Penetration
  • Biofilm Formation
• Candida albicans:

  • Habitat: Normal flora in skin, mouth, vagina, stool
  • Clinical Significance: Most common cause of mycoses worldwide
  • Growth at 42-45°C
  • Virulence mechanisms include adhesion, invasion, and biofilm formation
• Candida dubliniensis:

  • Habitat: Mainly found in oral cavities of HIV positive individuals
  • Clinical Significance: Opportunistic pathogen, rarely infects immunocompetent individuals
  • Germ-tube positive
• Candida auris:

  • Habitat: Hospital & nursing home acquired, found in various body sites and fluids
  • Clinical Significance: Serious blood stream infections, difficult to identify on automated systems, resistant to many antifungals
  • Treatment: Resistant to azoles, possible amphotericin B, 5FC, echinocandin resistance
• Candida auris:

  • Resistant to azoles, possible amphotericin B, 5FC, echinocandin resistance
• Candida glabrata (formerly Torulopsis):

  • Clinical significance: bloodstream, urogenital, lung and other infections
  • Difficult to treat, resistant to azole, reduced susceptibility to Amphotericin B
• Candida tropicalis:

  • Clinical significance: immunocompromised, problematic in leukemia patients, malignancies
  • Macroscopic: cream colonies, edges maybe wrinkled, CHROMagar = metallic blue
  • Microscopic: round to oval yeast cells, oval blastospores may appear singly or in clusters located along the long pseudohyphae
• Cryptococcus sp. Virulence Factors:

  • Capsule with glucuronoxylomannan (GXM), glucuronoxylomannogalactan (GXMGal), mannoproteins, hyaluronic and sialic acid
  • Melanin formation protects against phagocytosis, host ROS, RNS, and microbial peptides
  • Growth at mammalian body temperature
  • Degradation enzymes: Urease, phospholipase B
• Cryptococcus sp. - neoformans, gattii:

  • Habitat: C. neoformans - pigeon roosts, fruit, milk, plants; C. gattii - tropical and subtropical regions
  • Clinical significance: C. neoformans - primary pulmonary disease, CNS infections; C. gattii - lung infection, pneumonia
• Saccharomyces cerevisiae:

  • Virulence Factors: None
  • Clinical significance: non-pathogenic for immunocompetent, rare reports of fungemia
  • Can cause mucosal and disseminated infection in significantly immunocompromised hosts
• Malassezia sp. Virulence Factors:

  • Nutritional requirements lead to pathogenesis
  • Increased risk of infection in patients receiving central-line mediated total parenteral nutrition supplementation
• Malassezia sp. - furfur, globosa, sympodialis, obtusa, restricta:

  • Clinical significance: superficial skin infections, folliculitis, tinea versicolor
  • Habitat: lipophilic yeast-like cells found on skin and body surfaces of humans and animals
• Rhodotorula sp. - glutinis (rubra), minuta, mucilaginosa:

  • Habitat: air, soil, lakes, ocean water, dairy products
  • Clinical significance: opportunistic mycoses in AIDS or acute leukemia patients
• Geotrichum sp.:

  • Habitat: worldwide - soil, water, air, sewage, plants, cereals, dairy products
  • Clinical significance: opportunistic infections (geotrichosis), bronchial, pulmonary, disseminated infections
• Trichosporon sp.:

  • Habitat: soil, water samples, vegetables, mammals, birds
  • Clinical significance: white piedra, superficial and deep disseminated infections in humans