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How does penicillin work?

If you’ve ever gone to your doctor for strep throat, an ear infection, or any other type of bacterial infection, they likely prescribed you antibiotics like penicillin. How was penicillin discovered, how does it work, and what is its role in modern medicine?

Antibiotics, also known as anti-bacterial medications, are prescription drugs that help treat bacterial infections by eliminating the pathogenic bacteria.  There are many different types of antibiotics, and they all target specific strains of bacteria.  Penicillin was the first true antibiotic used in medicine, and its development revolutionized the practice of modern medicine.

How was penicillin discovered?

Penicillin was discovered in 1929 by bacteriologist Alexander Fleming at St. Mary’s hospital in London, U.K.; Fleming found a region on a cultured petri dish where bacteria did not grow.  This zone was accidentally contaminated with a fungus called Penicillum notatum.  Fleming was able to isolate an individual compound that was responsible for this antibacterial effect, and he named it penicillin.  Penicillin was finally isolated and first used as a medical treatment for bacterial infections in 1941.

How does penicillin work?

Penicillin works to eliminate bacteria by inhibiting the activity of an enzyme called transpeptidase.  This enzyme is responsible for keeping the bacterial cell wall together. When it is exposed to penicillin, the bacteria “lyses”, or bursts. 

Although penicillin affects a variety of different bacteria, it does not affect human or other animal cells because animal cells do not have cell walls whereas many bacteria do.

Penicillin is part of the beta-lactam class of antibiotics.  These antibiotics are effective against many different types of staphylococcus and streptococcus bacteria; these bacteria are sometimes the cause of upper respiratory infections, urinary tract infections, and skin infections, among others. 

Side effects of penicillin

Penicillin is only available with a prescription as it can have a variety of potential side effects, including nausea, vomiting, skin rash, and a hairy tongue.  Like other antibiotics, it can also impact the levels of good bacteria in the digestive tract; this could potentially contribute to an overgrowth of certain harmful bacteria. 

In addition, some people may experience allergic reactions to penicillin antibiotics.  Some of these allergies can be severe and life threatening, so it is important to tell your doctor about any known allergies to medication.  If you have an allergy to penicillin, this could prevent your doctor from prescribing a penicillin-derived antibiotic, such as penicillin-V-K, amoxicillin, ampicillin, and cloxacillin.

If you are prescribed penicillin or any other antibiotic, it is important to take your medication for the prescribed duration.  Stopping antibiotics early could potentially result in the development of antibiotic-resistant bacteria, and this contributes to the global problem of antibiotic resistance.  Antibiotic-resistant bacteria are harder to treat because there are fewer antibiotics that would be effective, so it is important to make sure that all the bacteria are eliminated by taking your full dose of prescribed antibiotics.

This article is not medical advice, and it is not intended to prescribe, diagnose, or promote specific treatment ideas for any condition.  If you think you have an infection or any pressing medical issue, it is important to visit a doctor or another qualified healthcare practitioner.


Gaynes, R. (2017).  The discovery of penicillin – new insights after more than 75 years of clinical use. Emer Infect Dis 23(5): 849-853. Doi: 10.3201/eid2305.161556

Kohanski, M.A., Dwyer, D.J., Collins, J.J. (2010). How antibiotics kill bacteria: from targets to networks. Nat Rev Microbiol 8(6): 423-435. Doi: 10.1038/nrmicro2333

NHS Inform. (2020 November 18). NHS. Retrieved 2021 February 3, from https://www.nhsinform.scot/tests-and-treatments/medicines-and-medical-aids/types-of-medicine/antibiotics

RxList: Side Effects. (2020 July 14). RxList Inc. Retrieved 2020 February 3, from https://www.rxlist.com/penicillin-vk-side-effects-drug-center.htm#overview

Tomasz, A., Waks, S. (2975). Mechanism of action of penicillin: triggering of the pneumococcal autolytic enzyme by inhibitors of cell wall synthesis.  Proceedings of the National Academy of Sciences of the United States of America 72(10): 4162-4166. Doi: 10.1073/pnas.72.10.4162

Image by jorono from Pixabay


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