The value of cellular fatty acid analysis in the identification of oral yeasts

Cellular fatty acids (CFAs) of oral yeast isolates were determined by gas-liquid chromatography using rigidly standardised methods. A total of 67 isolates, previously identified according to conventional biochemical methods, were tested including 21 Candida albicans, 5 Candida glabrata, 1 Candida guillier-mondii, 7 Candida krusei, 1 Candida parapsilosis, 2 Candida tropicalis, 2 Rhodotorula spp, 15 Saccharomyces cerevisiae as well as less frequently isolated species like 1 Candida boidinii, 1 Candida catenulata, 3 Candida fabianii, 2 Candida holmii, 1 Candida lusitaniae, 1 Cryptococcus albidus, a black yeast Exophiala jeanselmei, a fungus Lecytophora mutabilis, 1 Pichia anomala and 1 Trichosporon beigelii. Cells were grown to stationary phase at 30 degrees C on a rotary shaker, washed, lyophilized and methylated. The methyl esters were extracted with hexane and gas chromatography performed on a Hewlett Packard model 5830 A gas chromatograph. The relative percentages of palmitic acid (16:0), palmitoleic acid (16:1), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2) and linolenic acid (18:3) were determined and verified by mass spectrometry. A similarity matrix was calculated using the FAMEDATA RELATION PROGRAM. Cluster analysis was subsequently performed and a dendrogram constructed which placed 84% of the isolates in two major groups with group I having a 96% and group II with a 98% similarity. This method clearly distinguished Candida albicans from Candida glabrata, Candida holmii, Candida parapsilosis, Cryptococcus albidus, Exophiala jeanselmei, Lecytophora mutabilis, Rhodotorula glutinis, Rhodotorula rubra and Saccharomyces cerevisaea. In addition this phenotypic characteristic can distinguish Candida holmii from Saccharomyces cerevisiae and Candida glabrata while Exophiala jeanselmei showed a unique fatty acid profile due to a high percentage 18:2. The presence or absence of poly-unsaturated fatty acids like linoleic (18:2) and linolenic (18:3) are major contributing factors in the distinction between the groups. Strains or: the same species clustered together, confirming the value of this phenotypic characteristic in the identification of yeasts associated with the oral cavity. Despite the wide variety of oral yeast isolates investigated in this study, CFA analysis did nor prove satisfactory as sole identification method in distinguishing between the individual yeast isolates. CFA analysis is a rapid, easily applied and reliable method which can complement other identification methods in yeast characterization. Consideration should be given to the adoption of standardised preparation procedures in order to expand existing information and pool information generated by various researchers for the establishment of a yeast CFA data bank.