How Do Antibiotics Work to Kill Bacteria?

Antibiotics are medications used specifically to kill or disable bacteria. All bacteria are prokaryotic organisms, meaning that their cells are different than cells of the human body. These differences can be exploited, so that antibiotic drugs can kill bacteria without harming us.

How Does Penicillin Work?

When introduced during World War II, penicillin was a miracle drug immediately put to work battling one of the biggest wartime killers-infected wounds. The problem is that penicillin only works against certain types of bacteria; those that are considered Gram-positive.

Most of the bacterial world falls into one of two categories, Gram+ and Gram-, based on differences in the cell wall structure of bacterial cells. Penicillin works only against Gram-positive bacteria by interfering with the structure of a strong cell wall molecule called peptidoglycan. When the structural integrity of the bacterial cell wall is compromised, the cell loses its protection and ultimately dies.

Why Do We Need Antibiotics Other Than Penicillin?

Gram+ cells are only part of the bacterial world. There are many types of harmful bacteria that are considered Gram-negative. Penicillin does not work against Gram-negative bacteria, since the Gram- cell wall has an extra layer of protection that interferes with the drug reaching the peptidoglydan in Gram-negative bacterial cell walls. Therefore other antibiotics have been developed to work against Gram- cells. Some drugs, called broad-spectrum antibiotics, even work against both types of cells.

Antibiotic resistance is another problem that has led to the need for different types of antibacterial drugs. Once a certain antibiotic becomes widely used, bacteria quickly develop resistance to being harmed by it, so new antibiotics have to be found to kill these tougher bacteria.

How Do Other Antibiotics Work?

Since the introduction of penicillin, a variety of other classes of antibiotics have been discovered and put to use in fighting bacterial infections. Each class of antibiotics has its own mode of action or MOA. The mode of action is specifically how the drug works to disable or kill bacteria.

Penicillins: belongs to a class of antibiotics called B-lactams, the class of drugs interferes with peptidoglycan cell wall structure. Human cells don't have cells walls or peptidoglycan, so are not harmed by Beta-lactam drugs.

Tetracyclines: Broad spectrum antibiotics that disable bacteria by interfering with the cell's ability to make proteins. Although human cells also make proteins, the protein-making cellular machinery is different in eukaryotic cells, so it is not harmed by Tetracyclines.

Quinolones: This class of antibiotics interferes with the bacterial cell's ability to copy its DNA (genetic material); something that must be done before a cell can divide and multiply. The eukaryotic DNA of human cells is packaged differently, so its replication is not disabled by Quinolones.

These are just three examples. There are many classes of antibiotics that work in different ways to specifically disable prokaryotic cells without harming the cells of the host (infected person).Sources

Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.

Park Talaro, K (2008) Foundations in Microbiology. McGraw Hill

Article originally published in Suite101 online magazine.