2. Materials and methods
2.1. Microbial cultures
The recall-associated M. circinelloides strain isolated from spoiled yogurt and two clinical isolates were generously provided by the Joseph Heitman at Duke University (Durham, NC).
M. circinelloides strains NRRL 3615, 3614, and 3627 were obtained from USDA's Agricultural Research Service collection (Table 1).
All isolates were maintained as frozen stock cultures and plated on Potato Dextrose Agaracidified to pH 3.5 prior to use in experiments.
Inoculum was prepared by scraping plates which had been incubated at 25 °C for 48 h and re-suspending the hyphae / sporangiospores in peptone water or skim milk, depending on the experiment.
Yeast-like cells used to generate thermal destruction data were prepared by streaking M. circinelloides onto acidified PDA.
Plates were incubated at 25 °C for at least 48 h in a GasPack anaerobic jar.
Typical colony and cellular morphology was observed following incubation.
Following extraction from GasPack anaerobic jars, yeast-like cells were immediately used for thermal destruction experiments.