In this lesson students will use the Penicillium chrysogenum fungus, which
naturally produces the antibiotic penicillin, to investigate the effect of naturally
produced antibiotics on bacteria in laboratory cultures. Students co-culture
P. chrysogenum with three species of bacteria to observe differences between
penicillin-resistant and penicillin-sensitive bacteria. They will normalize fungal
spore suspension and bacterial culture concentrations before inoculating co-cultures. After bacteria have been exposed to the antibiotic, students will
quantify culture density to determine antibiotic effect in liquid culture and on
solid media. Students will learn about natural product antibiotics as well as
experimental design and application.
Key Words: antibiotics; bacteria; fungus; culture; spectrometry.
Antibiotics are powerful tools that greatly
reduce the risk of serious illness or death
from bacterial infections. Before the 1940s,
bacterial infections accounted for more
deaths than heart disease or cancer, and a
small cut could cause a lethal infection. Infections now rarely advance to a serious condition because of the use of antibiotics.
The clinical use of antibiotics began
with a discovery by Sir Alexander Fleming
(1881–1955), a microbiologist working to
understand Staphylococcus bacterial infec-
tions. Finding his culture dishes contami-
nated with mold, Dr. Fleming noticed an
interesting phenomenon: the bacteria on
the plate were unable to grow near the mold. Fleming termed the
observation the “zone of inhibition” and began investigating the
cause. He identified the mold as Penicillium, commonly known as
green bread mold, and named the antibiotic compound he isolated
By 1940, Fleming’s discovery caught the attention of medical Drs.
Howard Florey, Norman Heatly, and Ernst Chain, who began to
research penicillin in mice. One of Florey’s human patients, Albert
Alexander, developed a severe Staphylococcus infection that doctors
were certain was fatal. When Dr. Florey began treatment using penicillin, Alexander’s fever decreased and he appeared to be recovering.
Unfortunately, Alexander required more penicillin than the hospital
could produce, and his infection returned, ultimately causing his death.
Alexander did not die in vain. His treatment showed doctors
that penicillin could treat bacterial infections in human patients,
and researchers focused on finding a sufficient source of penicillin to serve the hospital. They found a candidate fungus on a
moldy cantaloupe from a market in Peoria, Illinois. The fungus was
identified as Penicillium chrysogenum, a close relative of Fleming’s Penicillium rubens. Upon characterization, P. chrysogenum produced several times
more penicillin than P. rubens, greatly improving
penicillin availability in hospitals by 1945 (ACS
& RCC, 1999).
To illustrate these early observations, we
designed a set of experiments to test the antibiotic
effect of P. chrysogenum. Students practice microbiology techniques, growing fungus both in liquid
culture and on agar plates. The fungus is tested
for penicillin antibiotic activity against three bacterial species with different levels of penicillin sensitivity: Staphylococcus epidermidis, Micrococcus
luteus, and Enterobacter aerogenes. To determine
bacterial growth, students measure optical density
using a spectrophotometer or colorimeter capable
of measuring light at 600 nm (OD600) (Brown,
1966; Morris, 1978). This activity will establish an understanding that
living organisms in nature produce antibiotics, and that antibiotic sensitivity differs among bacterial species.
This activity will
living organisms in
antibiotics, and that
The American Biology Teacher, Vol. 80, No. 7, pp. 530–535, ISSN 0002-7685, electronic ISSN 1938-4211. © 2018 National Association of Biology Teachers. All rights
reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Reprints and Permissions web page,
www.ucpress.edu/journals.php?p=reprints. DOI: https://doi.org/10.1525/abt.2018.80.7.530.
Penicillium Antibiotic Effect
• JESSE A. LEWIS, NADJA ANDERSON