Candida tropicalis

Lore & Background

The genus name Candida derives from the Latin term 'candidus' meaning 'glowing white', referring to smooth and glistening colonies. The genus originally referred to any asexual yeast lacking certain characteristics such as production of acetic acid, red/pink/orange pigments, arthroconidia, or ability to grow on inositol as sole carbon source. Although there are 200 species identified in this genus, the taxonomy remains undefined and incomplete due to changing representations, new species discoveries, and reclassification of old species. The defunct genera Oidium and Monilia were used to represent the genus Candida. Candida albicans is taxonomically close to C. tropicalis, sharing many pathogenic traits, while C. maltosa and C. sake are physiologically similar but can be differentiated by growth at 35 °C and assimilation of soluble starch.

C. tropicalis is easily identified using phenotypic and molecular methods. It is positive for the germ tube test, which distinguishes it from C. albicans and C. dubliniensis: C. tropicalis shows blastoconidial germination with constriction, whereas the others show a short hyphal extension with no constriction. Some isolates may produce pseudo-germ tubes after 3 hours but with constriction at the point of origin. The yeast is round to oval, 2–10 micrometers, and exhibits dimorphism, forming single-celled yeast (blastoconidia) that reproduce by simple budding. Colonies are white, smooth, and butyrous with a fringed border. Optimal growth temperature is 25–35 °C, and growth is enhanced with added sugar or fat.

C. tropicalis reproduces asexually by budding, forming blastoconidia that may elongate into pseudohyphae. Under reduced oxygen, submerged conditions, or 5-10% CO2, true septate hyphae may form. It is urease negative and diazonium blue B negative. It can mate in a parasexual cycle: diploid cells of opposite mating type form tetraploids, which may undergo chromosome loss to regenerate diploids. Opaque cells can form a complex sexual biofilm.

Reader's Guide

Candida tropicalis is notable as a common pathogen in neutropenic hosts, capable of spreading through the bloodstream to peripheral organs. In tropical countries, it is one of the most common colonizers and pathogens causing human disease, found on skin, in the gastrointestinal tract, and in the female genitourinary tract. It can be transmitted between health-care workers and patients, surviving up to 24 hours on hands, with a 69% probability of cross-transmission to a second hand and 38% to a third. It is responsible for approximately half of beyond-surface candida infections. The article notes that C. tropicalis is the second most common cause of such infections, though the sentence is cut off. Its significance lies in its role as an invasive pathogen in immunocompromised individuals, with treatments including amphotericin B, echinocandins, or extended-spectrum triazole antifungals. The yeast is also osmotolerant, able to survive in high salt concentrations, and a strain found in oil-contaminated soil can oxidize dodecane into dodecanedioic acid. Its identification relies on phenotypic features such as germ tube test with constriction, and molecular methods like DNA sequencing are increasingly used for accurate species determination.

Did You Know?

• C. tropicalis is positive for the germ tube test, showing blastoconidial germination with constriction, unlike C. albicans.
• It can survive up to 24 hours on hands and has a 69% probability of cross-transmission to a second hand.