A reemphasis-germ tubes diagnostic for Candida albicans have no constrictions

Abstract

Descriptive and illustrative material in several recent diagnostic manuals for medical mycology are unclear with respect to proper designation of germ tubes formed by Candida albicans. Because of the increasing significance of this and other yeast species in human disease, mycologists should be aware that germ tubes, unlike buds or pseudohyphae, do not have constrictions at the point of origin. Light and scanning electron micrographs are presented to emphasize this diagnostic characteristic.

The pathogenicity of the yeast Candida albicans for humans is well-known (9, 19). However, several other yeast species are opportunistic pathogens whose occurrence in humans has been noted with increasing frequency in recent years (2, 11, 17). Consequently, the clinical microbiologist has the task of accurately identifying yeast isolates as they assume added significance in medicine.

Perhaps the most used diagnostic procedure for separating C. albicans, the more common and serious yeast pathogen, from other species is the germ tube test. Most descriptions of the technique stress that germ tube walls exhibit no constriction at their points of origin (1, 4, 8, 20). A recent paper which reported germ tube-positive isolates of C. tropicalis (16) received further discussion (14). Comments underscored the need to distinguish clearly between pseudohyphae with basal constrictions in both C. tropicalis and C. albicans and true germ tubes in C. albicans which have no constrictions and which confers specificity to the test. This morphological distinction has been noted previously (5) with photomicrographs although the title of the article did not emphasize this aspect.

One of us (JDB) received a letter recently in response to inquiries regarding the eventual standardization of a selective C. albicans medium (6). The comment was made ‘... (the author) ... states that in C. albicans there is no constriction of the germ tube at its point of origin. This is not true. Although most C. albicans do not show constrictions, it certainly does sometimes occur as shown ... (ref. 10).’ In fact, the photo referred to shows only one yeast cell with a cellular extension which could possibly be interpreted as a germ tube although there is an obvious constriction at the base. Without conducting an extensive literature search, we did find another laboratory manual (12) which has a photo showing one cell having a structure identified as a germ tube. This too has a quite obvious constriction. In this publication, a germ tube is defined as an appendage one-half the width of and three to four times the length of the cell from which it arises. No mention is made of constriction or lack thereof. Another clinical manual (15) shows a photo of about ten C. albicans cells with cellular extensions, only one of which can clearly be discerned as a germ tube and it is poorly marked with an arrow since one other cell with a constricted pseudohypha is positioned over the germ tube. Again, germ tubes are defined only as ‘short lateral hyphal filaments.’

We are concerned that the differentiation of germ tubes from buds, pseudohyphae, and perhaps other cell extensions is becoming clearly less defined in the recent literature. We are disturbed that an eventual consequence may be that clinical workers now being trained will eventually produce inaccurate diagnoses of yeast infections which are increasing in medical importance.

To illustrate the matter of constrictions in C. albicans germ tubes, we used a culture recently isolated on mCA medium (6) from raw sewage collected at the University of Connecticut treatment facility. Cells were transferred from Sabouraud dextrose agar to commercially available calf serum and incubated at 37 C for 3 hours. Figs. 1A and 1B show, under phase microscopy, obviously constricted buds and/or pseudohyphae which closely resemble structures cited above (10, 12, 15) as being germ tubes. Further examination of the same culture revealed unconstricted cell extensions (Figs. 1C and 1D) which were in fact germ tubes.