Germ Tube Test is a screening test which is used to differentiate Candida albicans from other yeast. Germ tube (GT) formation was first reported by Reynolds and Braude in 1956. When Candida is grown in human or sheep serum at 37°C for 3 hours, they forms a germ tubes, which can be detected with a wet KOH films as filamentous outgrowth extending from yeast cells. It is positive for Candida albicans and Candida dubliniensis. Approximately 95 – 97% of Candida albicans isolated develop germ tubes when incubated in a proteinaceous media.
Principle of Germ Tube Test
Formation of germ tube is associated with increased synthesis of protein and ribonucleic acid. Germ Tube solutions contains tryptic soy broth and fetal bovine serum, essential nutrients for protein synthesis. It is lyophilized for stability. Germ tube is one of the virulence factors of Candida albicans. This is a rapid test for the presumptive identification of C. albicans.
Procedure of Germ Tube Test
- Put 0.5 ml of sheep or human serum into a small tube.
Note: Fetal bovine serum can also be used instead of human serum.
- Using a Pasteur pipette, touch a colony of yeast and gently emulsify it in the serum.
Note: Too large of an inoculum will inhibit germ tube formation.
- Incubated the tube at 37°C for 2 to 4 hours.
- Transfer a drop of the serum to a slide for examination.
- Coverslip and examine microscopically under low and high power objectives.
Results and Interpretation of Germ Tube Test
Positive Test: A short hyphal (filamentous) extension arising laterally from a yeast cell, with no constriction at the point of origin. Germ tube is half the width and 3 to 4 times the length of the yeast cell and there is no presence of nucleus. Examples: Candida albicans and Candida dubliniensis
Negative Test: No hyphal (filamentous) extension arising from a yeast cell or a short hyphal extension constricted at the point of origin. Examples: C. tropicalis, C. glabrata and other yeasts.
Quality Control in Germ Tube Test
Positive Control: C. albicans (ATCC 10231)
Negative Control: C. tropicalis (ATCC 13803), C. glabrata (ATCC 2001)
Limitations of Germ Tube Test
- C. tropicalis may form early pseudohyphae which may be falsely interpreted as germ tubes.
- The yeast formerly named Candida stellatoidea also produces germ tubes; however, it has been combined with C. albicans and no longer exists as separate species.
- This test is only part of the overall scheme for identification of yeasts. Further testing is required for definite identification.