Candida

Candida albicans is a commensal of mucous membranes and the gastrointestinal tract. Environmental isolations have been made from sources contaminated by human or animal excreta, such as polluted water, soil, air and plants.

RG-2 organism.

Culture: 
Colonies (SDA) white to cream-coloured smooth, glabrous, yeast-like.

Microscopy: 
Spherical to subspherical budding blastoconidia, 2-7 x 3-8 µm in size.

India ink preparation: 
Negative - no capsules present.

Dalmau plate culture:
Branched pseudohyphae with dense verticils of blastoconidia. Spherical chlamydospores, mostly terminal, often on a slightly swollen subtending cell, are formed near the edge of the cover slip.

Key features: 
Germ tube positive, production of chlamydospores on Dalmau plate culture, fermentation of glucose, sugar assimilation profile and a distinctive green colour on CHROMagar. Note: Germ tube negative variants (previously known as C. claussenii), and sucrose-negative variants (previously described as C. stellatoidea) may occur.

Candida auris is an emerging multidrug resistant yeast that causes invasive infections, that was first described in 2009 in Japan and has since been reported from numerous countries (Lockhart et al., 2017a,b; Jeffery-Smith et al., 2018; Spivak and Hanson, 2018; Heath et al., 2019; Zhu et al., 2020). Infections and outbreaks caused by C. auris in hospital settings have been rising over the past several years. Difficulty in its identification, multidrug resistance properties, associated high mortality rates, and long-term survival on surfaces in the environment make C. auris particularly problematic in clinical settings (Du et al., 2020). Candida auris is now a notifiable infection under public health regulations in many countries including Australia.

Candida auris may be misidentified as C. haemuloni or other yeast species using conventional phenotypic and biochemical methods (Kathuria et al., 2015; Jeffery-Smith et al., 2018). However, unlike these other Candida species, C. auris grows at 42^oC, and this has become a useful differential characteristic (Casadevall et al., 2019). Candida auris is best identified by MALDI-ToF MS or by molecular methods (Du et al., 2020; Zhu et al., 2020).

MALDI-ToF MS: Can accurately differentiate Candida auris from other fungal species; however, accurate identification is dependent on the reference database being used (Kathuria et al., 2015; Jeffery-Smith et al., 2018; Zhu et al., 2020).

Molecular identification: ITS and D1/D2 sequencing provide accurate species identification (Kathuria et al., 2015; Kordalewska et al., 2017; Leach et al., 2018; Du et al., 2020; Zhu et al., 2020).

RG-2 organism.

Culture:
Colonies (SDA) white to cream-coloured smooth, glabrous, yeast-like.

Microscopy:
Spherical to subspherical budding blastoconidia, 2-3 x 2-5 µm in size.

India ink preparation:
Negative - no capsules present.

Dalmau plate culture:
Ovoid budding yeast cells only. No pseudohyphae produced.

Physiological tests: (+ Positive, - Negative, v Variable, w Weak, s Slow).

Fermentation:

Growth reactions:

Key features:

Candida auris is difficult to identify using conventional phenotypic and biochemical methods and has often been misidentified as Candida haemuloni or Saccharomyces cerevisiae. Growth at 42^oC on CHROMagar™ Candida produces white, pink, or dark purple colonies (Kumar et al., 2017), however on CHROMagar™ Candida Plus, C. auris colonies display a differential light blue colour with a blue halo (Mulet Bayona et al., 2022). Reliable identification methods are MALDI-ToF MS or ITS and D1/D2 sequencing.

Candida dubliniensis is an occasional cause of candidaemia and mucosal infection, especially in HIV patients.

RG-2 organism.

Culture:
Colonies (SDA) white to cream-coloured smooth, glabrous, yeast-like.

Microscopy:
Spherical to subspherical budding blastoconidia, 3-8 x 2-7 µm in size.

India ink preparation:
Negative - no capsules present.

Dalmau plate culture:
Branched pseudohyphae with dense verticils of blastoconidia and spherical, mostly terminal chlamydospores.

Key features:
Germ tube positive, similar to C. albicans, except for absence of growth at 42C; glycerol (mostly +), methyl-a-D-glucoside (-), trehalose (-), and D-xylose (-). Initial colonies dark green on CHROMagar and producing rough colonies on bird seed agar. ITS sequencing and MALDI-TOF can reliably distinguish C. dubliniensis from C. albicans.

Candida haemuloni has recently been reclassified as a complex of three phenotypically identical but genotypically distinct entities: C. haemuloni, C. duobushaemulonis and C. haemulonii var. vulneris, based on ITS and D1/D2 sequencing (Cendejas-Bueno et al. 2012, Ramos et al. 2015).

Note: Candida haemuloni and C. haemulonis are orthographic variants, but C. haemuloni is considered the correct name

Candida haemuloni

RG-1 organism.

Culture: 
Colonies (SDA) white to cream-coloured smooth, glabrous, yeast-like.

Microscopy: 
Ovoid to globose, budding yeast-like cells or blastoconidia, 2-7 x 2-7 µm. No pseudohyphae produced.

India ink preparation: 
Negative - no capsules present.

Dalmau plate culture: 
No pseudohyphae produced.

Key features: Germ tube negative yeast and sugar assimilation pattern. Molecular identification may be required. C. haemulonii has been reported from a few cases of fungaemia but clinical isolations remain rare.

Candida inconspicua is a rare cause of candidaemia.

RG-2 organism.

Culture: 
Colonies (SDA) white to cream-coloured smooth, glabrous, yeast-like.

Microscopy: 
Ellipsoidal budding blastoconidia, 3-5 x 1.8-3 µm in size. No pseudohyphae or chlamydospores produced.

India ink preparation: 
Negative - no capsules present.

Dalmau plate culture: 
No pseudohyphae produced.

Key features:
Germ tube negative yeast and sugar assimilation pattern and colonies are white on Candida CHROMagar.

Recently Candida parapsilosis has been recognised as a complex of four species: C. parapsilosis, C. orthopsilosis, C. metapsilosis and Lodderomyces elongisporus (Tavanti et al. 2005).

These four species are phenotypically indistinguishable and are best identified by ITS sequencing or MALDI-TOF MS analysis.

Candida parapsilosis Candida metapsilosis Candida orthopsilosis

Candida tropicalis is a major cause of septicaemia and disseminated candidiasis. It is also found as part of the normal human mucocutaneous flora and environmental isolations have been made from faeces, shrimp, kefir and soil.

RG-2 organism.

Culture: 
Colonies (SDA) white to cream-coloured smooth, glabrous, yeast-like.

Microscopy: 
Spherical to subspherical budding yeast-like cells or blastoconidia, 3.5-7 x 5.5-10 µm.

India ink preparation: 
Negative - no capsules present.

Dalmau plate culture:
Abundant, long, wavy, branched pseudohyphae with numerous ovoid blastoconidia, budding off. Terminal vesicles (chlamydospores) are not produced.

Key features:
Germ tube negative yeast and sugar assimilation pattern. Colonies are dark blue on Candida CHROMagar.