Antibiotics are chemotherapeutic agents used to inhibit or kill bacteria (prokaryotic organisms). How does penicillin destroy bacteria without hurting our cells?

There are many different classes of antibiotics each exerting a different type of inhibitory effect that specifically impacts bacteria. Bacterial cells are prokaryotic; primitive cells that differ significantly from humans’ eukaryotic cells. Antibiotic penicillins work by inhibiting the synthesis of the bacteria cell wall.

Each article in this series covers a specific class of antibiotic and includes the following information:

• a list of the specific antibiotics that fall in the class (generic and brand names)
• mode of action
• type of infection the antibiotic class to against
• type of bacteria inhibited or killed
• pros and cons of using the class of antibiotics

The main classes of antibiotics include:

• Aminoglycosides
• Cephalosporins (a Beta-lactam)
• Macrolides
• Penicillin (a Beta-lactam)
• Quinolones
• Sulfonamides
• Tetracyclines

Penicillins

In 1929 Alexander Fleming, a British bacteriologist, inadvertently discovered penicillin. He had observed bacterial staphylococci colonies disappearing in cultures that were contaminated with mold.

Fleming eventually extracted the compound from the mold that had been responsible for destruction of the bacterial colonies. The product was named penicillin, after the Penicillium mold from which it was derived.

Penicillins include the following drugs (this is not a complete list):

• amoxicillin
• ampicillin
• bacampicillin
• oxacillin
• penicillin

Mode of Action

Penicillins and cephalosporins (beta-lactam antibiotics) work by interfering with interpeptide linking of peptidoglycan, the a strong, structural molecule found specifically bacterial cell walls. Cell walls without intact peptidoglycan cross-links are structurally weak, prone to collapse and disintegrate when the bacteria attempts to divide. Since the eukaryotic cells of humans do not have cell walls, our cells are not damaged by penicillins.

Antimicrobial Spectrum of Penicillins

Penicillins have a bacteriocidal effect on Gram-positive bacteria. In Gram-positive cells, peptidoglycan makes up as much as 90% of the thick, compact cell wall, and is the outermost layer.

Penicillins are not effective against Gram-negative bacteria, which have cell walls in which peptidoglycan is not the outermost layer, but that lies between the plasma membrane and a lipopolysaccharide (LPS) outer membrane. Penicillin cannot access the peptidoglycan of Gram-negative cells.

Type of Infections Penicillins Are Used For

Penicillins are used to treat Gram+ bacterial infections which, depending on the specific microbe involved, cause a wide range of illnesses including skin infections, dental infections, ear infections, respiratory tract infections, urinary tract infections, and gonorrhea.

Pros and Cons of Penicillins

Penicillin Allergy

Penicillins are typically very safe; the greatest risk being allergic reaction, which, when it occurs, can be severe. People who are allergic to penicillin are likely to also have an allergic reaction to cephalosporins, another type of beta-lactam antibiotic.

Gastric Breakdown and Absorption

Penicillin can be broken down in the stomach by gastric acids and is irregularly absorbed into the blood stream.

Bacterial Enzymes & Resistance

Many pathogenic Gram-positive bacteria produce enzymes called Penicillinases (or beta-lactamases) that can inactivate penicillin. The capability of bacteria to synthesize this enzyme is an adaptation that was selected for in the presence of substances that inhibit bacterial growth. This is an example of a type of antibiotic resistance. Penicillins are sometimes combined with beta-lactamase inhibitors, agents that protect the penicillin from these bacterial enzymes.

This article is for informational purposes only and is not mean to be used in the treatment of bacterial infections. If you are ill, see a medical professional.

More Antibiotic Resources

The information in this article was obtained from a variety of sources, including the text book Foundations in Microbiology by Kathleen Park Talero and Yuri Bayarski’s article Antibiotics and Their Types, Uses and Side Effects. To learn more about bacteria and prokaryotes in general, see the microbiology information on the science website Science Prof Online.