Standardization of yeast inocula with an electronic impedance counter

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Abstract
The standardization of yeast inocula has been identified as an important variable in the performance of reproducible in vitro fungal susceptibility testing. We investigated the precision and accuracy of an electronic particle counter in preparing yeast inocula, with quantitative culture used as a "gold standard." Suspensions of Candida albicans and Torulopsis glabrata standardized with a particle counter at 10(6) counts per ml were highly reproducible when cultured quantitatively (coefficients of variation, 6.7 and 6.8%, respectively). Accuracies of particle counts, compared with those of quantitative culture, were -8.5 and +2.8% for the two species, respectively. Electronic cell counts were highly linear between 5 X 10(6) and 5 X 10(4) CFU/ml (R2 greater than 0.99). Multiple electronic counts of a single suspension of C. albicans had less variation than did multiple quantitative cultures of a suspension of the same organism (coefficients of variation, 2.4 versus 8.9%; P less than 0.01), suggesting that impedance counting is probably more precise than quantitative culture. Electronic particle counters can be used to prepare accurate, reproducible yeast inocula. The method may be more accurate and is more precise than other techniques commonly used to standardize yeast suspensions.