The Use of the Prompt Inoculation System® in Preparing a Standardized Yeast Inoculum

Abstract

One of the most important steps in performing broth dilution susceptibility testing of yeast isolates is the preparation of the starting inoculum. Although not specifically developed for yeast inoculum preparation, the Prompt Inoculation System® (3M) provides a novel alternative approach that may provide a more standardized yeast inoculum than previously employed methods. The authors examined the relationship between the number of colonies picked with the Prompt Inoculation Wand® and the hemacytometer and viable colony counts for each of six test organisms, including Candida albicans, Candida tropicalis, Candida krusei, Candida parapsilosis, Candida glabrata, and Cryptococcus neoformans. They found that by picking five colonies 1-2 mm in diameter, inoculum densities of 1 to 5 × 10⁶ CFU/mL were obtained with most of the test organisms. A considerably higher inoculum density was observed with C. glabrata (1 to 2 × 10⁷ CFU/mL) because of the small size of this organism. No overfilling of the Inoculation Wand was observed when more colonies were touched. This study indicates that the Prompt Inoculation System offers a convenient and simple method for yeast inoculum preparation.