Pellicle associated adherence film above incubation broth surface - an inexpensive adjunct to recognizing Candida krusei in the laboratory

BACKGROUND: Candida species including Candida krusei have become common pathogens, especially in immune-compromised patients. Pellicle on the surface of incubating nutrient broth extending with an adherent film above the broth has been described as a feature of this organism. We investigated whether this easily observable adherent film could be useful in the identification of this yeast. We also wanted to see if this process involved any morphological changes from the yeast form on the part of C. krusei. FINDINGS: Common and less frequently isolated species of Candida were inoculated into YPD broth and observed for pellicle formation. For C. krusei different inoculum sizes and time periods were studied to establish earliest period and the smallest number of organisms needed for this process. A cover-slip assay was established to observe the architecture of the film formed by this organism. Among the clinically common Candida species, only C. krusei formed a visible film, requiring 10(6 )organisms to produce it at 24 hours post inoculation. Film formation also differentiated C. krusei from C. inconspicua usually reported as a complex by carbohydrate assimilation assays. Rarely isolated C. famata and C. norvegensis also formed pellicles and film but less robustly. Microscopic observations of the film showed only yeast forms, no hypha or pseudohypha were seen. CONCLUSIONS: Pellicle formation following inoculation of a clinical specimen into liquid culture, is a useful alert to the probable presence of C. krusei and likely fluconazole resistance, while awaiting the results of more definitive identification assays. Pellicle and adherence film formation are not a result of polymorphic changes on the part of C. krusei as only yeast forms were present.