Yeast: It’s Not Only Found in Food

Yeast is used to make bread rise and ferments sugar to make wine or beer. The most commonly studied yeast is Saccharomyces cerevisiae, which has historically been used for baking, winemaking, and brewing. S. cerevisiae is also a model organism widely used in molecular biology and genetics research. However, there are many other yeast species, some of which can spoil wine (Brettanomyces) or cause opportunistic fungal infections (Candida species).

A well-known yeast infection is vaginal candidiasis, most commonly caused by Candida albicans and occasionally by other Candida species. Ironically, C. albicans is a commensal yeast found naturally in the mouth, gut flora, and vaginal microbiome in small amounts. It becomes invasive under specific environmental cues, such as changes in vaginal pH. Normally, the vaginal pH ranges from 3.5–4.5, but it can rise due to menstruation, intercourse, douching, pregnancy, or menopause. These physiological changes may create conditions ideal for Candida overgrowth and lead to infection.

Oral yeast infection, or thrush, affects the mouth and esophagus and is frequently observed in newborns and immunocompromised individuals. In a healthy person, beneficial oral bacteria inhibit yeast overgrowth. In contrast, newborns have underdeveloped oral microbiota, and immunocompromised patients—especially those on antibiotics or corticosteroids—may lack the microbial defense that keeps Candida in check, resulting in infection.

Emery Pharma offers antifungal susceptibility testing  of a wide range of pathogenic yeast   using the Clinical and Laboratory Standards Institute (CLSI) broth microdilution method [1]. In this method, yeast isolates are grown and cultured from Sabouraud dextrose agar (SDA), then diluted into Roswell Park Memorial Institute (RPMI) 1640 media containing 3-(N-morpholino)propanesulfonic acid (MOPS). Antifungal compounds are prepared in DMSO or water and diluted starting at 8 µg/mL (or higher if requested). Serial two-fold dilutions are performed in buffer/media, and the inoculated 96-well plates are incubated at 35 °C and read at 48 hours (72 hours for Cryptococcus neoformans).

Control antifungal agents typically include Amphotericin B, Ketoconazole, Itraconazole, or Fluconazole, unless otherwise specified. Unlike bacterial assays, yeast broth microdilutions are scored by comparison to untreated wells:

This scoring method is especially important if the test compound is an azole or flucytosine analog. In those cases, the minimum inhibitory concentration (MIC) is defined as the lowest concentration with a score of 2. For all other antifungal classes, the MIC is defined as the first concentration with a score of 0.

After MIC testing, a minimum fungicidal concentration (MFC) assay can be conducted. In this assay, clear wells are plated onto SDA and incubated for 24–48 hours for colony enumeration. The MFC is defined as the lowest drug concentration that kills approximately 99% of yeast (≤ 1 colony)[2].

Emery Pharma provides MIC and MFC assays for evaluating candidate antifungal drugs. Additional services include biofilm testing and antifungal resistance profiling. For more information or to request a quote, please contact us at 510-899-8828 or through our Contact Us page.

References

1) “M27-A2 Reference Method for Broth Dilution Antifungal Susceptibility Testing of Yeasts; Approved Standard—Second Edition.” Clinical Laboratory Standards Institute. 22.15 (2002).

2) Canton, E., Pemán, J., Valentín, A., Espinel-Ingroff, A. and Gobernado, M. “In Vitro of Echinocandins against Candida krusei Determined by Three Methods: MIC and Minimal Fungicidal Concentration Measurements and Time-Kill Studies.” Antimicrobial Agents and Chemotherapy. 53.7 (2009).