Determination of viability of Histoplasma capsulatum yeast cells grown in vitro: comparison between dye and colony count methods.

The viability of Histoplasma capsulatum yeast cells grown under different conditions was determined by dye tests with Eosin-Y and Janus Green B and by colony counts of cells plated on brain-heart infusion agar supplemented with histoplasma growth factor and bovine serum albumin (BHI-SAG). The test samples included cells grown on brain-heart infusion agar at 37 degrees C for 2-7 days, cells grown in glucose-cysteine broth medium for 1-31 days, and cells grown on brain-heart infusion agar for 3 days at 37 degrees C and then irradiated by ultraviolet light. The colony count indicated that the viability of the yeast cells grown on brain-heart infusion agar for 2 or 3 days varied between 68 and 100% depending on the isolates. The viability, however, dropped from 16 to 29% by day 7. The results of dye tests showed 78 to 99% dye-negative cells among the 2- and 3-day-old cultures while the number of dye-negative cells dropped to 32-36% on day 7. The colony count with the cells grown in the broth culture showed 100% viability until day 7 and dropped significantly by day 9. The results of dye tests showed no correlation with the colony count findings. The survival curve of ultraviolet-irradiated cells determined by colony count showed that irradiation at 180 erg mm-2 killed more than 50% of cells; fewer than 10% of cells survived 360 erg mm-2. The results of the dye test showed no difference between the irradiated and control populations.(ABSTRACT TRUNCATED AT 250 WORDS)