General Characteristics

1
General Characteristics

• The isolation of Cryptococcus neoformans is an
  exception to these rules of thumb
• Although C. neoformans is considered to be an
  opportunistic pathogen, the recovery of it from a
  clinical specimen must always be considered
  clinically significant unless proven otherwise
  (?)
• C. neoformans causes serious pulmonary infections
  and is associated with a high mortality rate if
  it invades the central nervous system

2
Isolation from Clinical Specimens

• Yeast are not fastidious and grow well on simple
  mycology media such as Sabourauds
• Some are sensitive to cyclohexamide whereas
  others are resistant Mycosel is often used to
  select differentiate pathogenic yeast from
  mixed cultures in clinical samples
• They also grow very well on media designed to
  grow bacteria, especially blood agar

3
Isolation from Clinical Specimens

• Yeast usually grow well at 35-37oC though most
  grow optimally at 30oC
• Yeast frequently are isolated from clinical
  specimens submitted to the bacteriology lab.

4
Isolation from Clinical Specimens

• Even though some yeast can be induced to produce
  various structures in special media, many of them
  must be tested biochemically in a manner similar
  to the identification of some bacteria when it is
  determined that species designation is appropriate

5
Isolation from Clinical Specimens

• Colony morphology is of little use in the
  identification of yeast (beyond group
  designation) because many species look very
  similar. Exceptions include Rhodotorula sp. which
  usually has a red colony and some dematiacious
  yeasts which are not included in this discussion.
• Initial microscopic examination of material taken
  directly from a colony yields very few clues of
  its specific identity.

6
Preliminary Identification Procedures
7
Identification Procedures

• Germ Tubes
• Initial identification of a yeast should include
  the germ tube test
• It is very cost effective in terms of personnel
  time and laboratory materials
• Germ tubes are the beginning of hyphae production
• All hyphae-producing fungi produce germ tubes

8
Identification Procedures

• Germ Tubes
• However,C. albicans is the only yeast that
  produces this structure under the defined
  conditions of this test
• A germ tube test is completed on C. albicans (the
  most commonly isolated yeast) after 2 to 3 hours
  incubation. The test is inexpensive, has good
  specificity, and thus is a central procedure in
  the process of identifying yeast

9
Identification Procedures

• Germ Tubes refer to page 1041 Fig 19-68 on
  page 1042
• At least 95 of C. albicans isolates produce
  these structures making the germ tube test both
  sensitive and specific
• A potential source of error is to misidentify
  blastoconidia as germ tubes

10
Identification Procedures

• Germ Tube Procedure
• A small amount of a young (?3 days) pure culture
  is taken from primary isolation media (barely
  touch the colony with a wire or tip of a pipet)