The medallion was created and inscribed by Scottish bacteriologist Alexander Fleming in the course of "one of the most pivotal discoveries in medial history," auction house Bonhams said in a statement published Wednesday.
The scientist "stumbled across the world's first antibiotic after accidentally leaving a stack of staph cultures next to an open window while on vacation in 1928," the statement reads.
"The cultures were contaminated by an airborne mold and before disposing of the cultures, he noticed that the mold had prevented normal growth by the staph," it adds.
Fleming created the medallion as a gift for his niece and inscribed it with the message: "The mold that made penicillin / Alexander Fleming."
The medallion contains a specimen of the original penicillin mold on blotting paper, mounted within a glass disc held in place by a black plastic rim 53 millimeters (two inches) in diameter, according to the auction listing.
It is expected to fetch $30,000-$50,000 in an online auction that runs from October 13 to 23.
Fleming gave a number of similar medallions to public figures such as Pope Pius XII, Queen Elizabeth II, Prince Philip, Marlene Dietrich, Winston Churchill and Theodore Roosevelt.
"These mold medallions, relics of Fleming's contribution to humanity, one of the most important advances of the 20th-century, are rare in private hands," the auction listing reads.
Fleming initially struggled to identify the exact strain of the fungus that created the bacteria-free circle in his petri dish, and over the years, several species of Penicillium have been identified as producing penicillin.
During the 1930s and 1940s, scientists in the United Kingdom and United States evaluated many different strains to see if any could be used to mass-produce penicillin, and the drug saved thousands of wounded soldiers and civilians during World War II.
Penicillin changed the course of modern medicine, with antibiotics key to the decline of many diseases over the course of the 20th century.
Now its effectiveness is threatened by antibiotic resistance.
In his lifetime, Fleming realized that his discovery could be endangered by antibiotic resistance and warned of the dangers in his Nobel lecture in 1945.
"The time may come when penicillin can be bought by anyone in the shops," he said. "Then there is the danger that the ignorant man may easily underdose himself and, by exposing his microbes to nonlethal quantities of the drug, make them resistant."
Drug resistance is expected to result in 10 million deaths a year by 2050 as bacteria outsmart our most sophisticated drugs.
CNN's Katie Hunt contributed to this report.
The-CNN-Wire & 2024 Cable News Network, Inc., a Time Warner Company. All rights reserved.