- Germ tube is a young hypha (short, non-nucleated and non-septate) growing out of a yeast cell or spore (in spore-releasing fungi) during their germination. They are ½ the width and 3 to 4 times the length of the cell from which they arise.
- It is a rapid method for identifying and differentiating albicans from other Candida spp.
- Buds and pseudo-hyphae show constrictions at the point of origin from cells, unlike germ tubes.
- Germ tube formation was first reported by Reynolds and Braude in 1956.
Principle of Germ Tube Test
When cells of Candida are incubated in protein rich medium like human, sheep or fetal bovine serum at 370C for 2-4 hours, Candida albicans produce short, slender, tube like structures called germ tubes. Germ tubes can be detected with a wet KOH film. Increased synthesis of protein and ribonucleic acid results in the formation of germ tubes.
Procedure:
- Put few drops of serum (human, sheep or fetal bovine) or media into a test tube.
- Take a sterile wooden applicator or loop and touch a colony or two of yeast and gently emulsify it in the serum.
- Incubate the tube for 2-3 hours at 37°C.
- Using a Pasteur pipette, place a drop of the suspension on a slide and cover with a coverslip.
- Examine the wet mount microscopically (at 40X) for production of germ tubes
Quality Control in Germ Tube Test
- Positive Control: albicans (ATCC 10231)
- Negative Control: tropicalis (ATCC 13803), C. glabrata (ATCC 2001)
Result and interpretation:
- Positive Result: A short hyphal (filamentous) extension arising laterally from a yeast cell with no constriction at the point of origin, no nucleus and the tube being half the width and 3 to 4 times the length of the yeast cell. Examples: Candida albicans and Candida dubliniensis.
- Negative Result: No hyphal extension arising from a yeast cell or a short hyphal extension with the constriction at the point of origin. Examples: Candida tropicalis, Candida glabrata and other yeasts.
Limitations:
- Candida tropicalis may produce hyphal extension with constriction at the point of origin after 3 hrs of incubation i.e. pseudo germ tubes, which may be falsely interpreted as germ tubes.
- Too heavy inoculum will inhibit the formation of germ tubes.
- Further testing is required for definite identification as this test is only a part of the overall scheme for identification of yeasts.