The mean (± SE) mycelium diameter was 53.6 ± 2.63 mm on the
plain patties, and mycelium filled the dish or plates (90 mm) in the
rest of the samples. Previous experience in 2015–2016 had indicated
that cheeseburger patties typically do support growth of R. stolonifer,
while patties with condiments generally do not. Bread from these
restaurants generally contains calcium propionate, added to inhibit
mold (Sancho-Madriz, 2003), while condiments may be acidic or
contain antimicrobial compounds (Silvério & Lopez, 2012).
Exercise 2: Screening Procedure &
During four semesters (2015–2018), our college biology students
selected and brought their own fast-food samples to inoculate for
the presence or absence of visible mold growth. We screened only
for presence or absence because the samples varied in size and
shape. Inoculations were made using R. stolonifer mycelium as
described above, and using a suspension of R. stolonifer spores in distilled water (spore density was not controlled). All plates were sealed
using Parafilm and kept sealed until they could be autoclaved.
In spring 2018, our students brought in 20 different items from
nine different sources, including a gas station, a grocery store bakery,
and five of the most internationally known fast-food franchises. Some
items were inoculated intact (e.g., chicken nuggets) while others were
cut (e.g., sandwich bread) to fit Petri dishes. Agar plugs were placed in
the center of the items to inoculate. Rhizopus stolonifer failed to
grow on all nine of the samples made of bread or tortillas, but it grew
on all the samples based on animal products or vegetables (including
potato products). Other opportunistic fungi grew in some samples,
suggesting that spores invaded during the inoculations or were
already present (Figure 3). In our experience with >60 items
Figure 1. Students inoculating fast-food samples with
Figure 2. Example setup of inoculated hamburger samples.
Rhizopus stolonifer mycelium is in the center of the samples
(red circles). Photographed samples are labeled only with type
of sample and date for anonymity, although samples used for
classes should include further identifying information such as
specific names of the food items and student names.
Figure 3. Opportunistic fungi growing on fast-food samples.
The presence of these fungi invites discussion about the
ubiquity of fungi in the environment. Plate photograph is
cropped to remove identifying information in label.