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Modes of Action of Antibiotics

written by: J.Sace • edited by: Leigh A. Zaykoski • updated: 11/29/2008

Antibiotics are important drugs in combating bacterial infections. Learn the five types of antibiotics based on their modes of action against bacteria.

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    Almost a third of Europe’s population perished in the 14th century “bubonic plague" because of the infection caused by a rod-shaped bacterium called Yersinia pestis. The plague might not have happened if antibiotics were just discovered at that time. Unfortunately, the first antibiotic was discovered in the early 20th century or almost 600 years later. Just imagine the number of people who have died through the centuries because there are no antibiotics to cure their infections. Our generation is lucky because there are now antibiotics that save us from the fatality of microbial infection. Antibiotics are now highly commercialized and come with different brand names. Biotech and pharmaceutical companies are spending billions of dollars just to discover and evaluate new antibiotics (Dale 2008).

    Whatever brands of antibiotics we buy in the drugstore, we can classify them into five types based on their modes of action or the way they “treat" microbes. This article will discuss on these five modes of action.

    Antibiotics That Inhibit Cell Wall Synthesis

    The bacterium maintains its shape and protects itself from the external environment by having the cell wall. Without this wall, cytoplasmic contents would leak out and external materials would enter the bacterial cell. The bacteria could impossibly live if the cell wall is missing. Note that the major foundation of the cell wall is the peptidoglycan, just like bones for the human body. There are antibiotics that inhibit the synthesis of peptidoglycan in growing bacterial cells; this would weaken the cell wall eventually resulting to cell lysis. One example of this kind of drug is penicillin, which is first discovered by Alexander Fleming. Penicillin offers little toxicity to humans because our cells don’t have peptidoglycan (Tortora 2005).

    Antibiotics That Inhibit Protein Synthesis

    Protein synthesis is important for both the human cell and the bacteria. The drug should be selective enough by not interfering protein synthesis in the human cell but do so in bacterial cell. Humans have 80S ribosomes while a bacterium has 70S ribosomes. The bacterial 70S ribosomes are composed of 50S and 30S units. (Note that the letter “S" stands for Svedberg unit, which describes the relative sedimentation rate in a high-speed centrifuge). The antibiotic only affects 70S ribosomes in the bacteria and does nothing for the 80S ribosomes in human. Drugs of this kind include chloramphenicol, tetracycline, and erythromycin. These drugs stop the further elongation of polypeptides by blocking the active sites of the 50S units of 70S ribosomes (Tortora 2005).

    Antibiotics That Injure Plasma Membrane

    There are antibiotics that kill bacteria by blocking plasma membrane channels where important metabolites (food and minerals) enter towards the cytoplasm. When the bacteria do not get the needed nutrients for growth, they would eventually die. Example of this type of antibiotic is the polymyxin B.

    Antibiotics That Inhibit Nucleic Acid Synthesis

    There are antibiotics that interfere the replication and transcription of DNA in bacterial cells. Examples are rifampin and quinolones. However, these drugs are less preferred because they also interfere with human DNA replication and transcription.

    Antibiotics That Inhibit the Synthesis of Essential Metabolites

    An enzyme acts on a specific substrate to produce a certain metabolically important metabolite inside the cell. The substrate contains the raw elements for the formation of the metabolite, which is usually an important compound needed by the cell for normal growth. There are antibiotics that mimic the structure of the substrate in such a way that the normal substrate is replaced and don’t have contact with the enzyme (Dale 2008). In the process, needed metabolites were not created jeopardizing the well being of the bacteria. The compound sulfanilamide (a sulfa drug) replaces the normal substrate, para-aminobenzoic acid (PABA), of the enzyme that forms folic acid in bacteria. When the bacteria lack the needed amount of folic acid, purine and pyrimidine synthesis would then be impossible. The sulfa drug is selective because humans don’t produce their own supply of folic acid, but they get it from the foods they eat (Tortora 2005).

    Print Source: Tortora, Gerard. 2005. Microbiology: An Introduction. Singapore: Pearson Education, Inc.

    Dale, M. Maureen, and Mandelstam, Joel. "Antibiotics." Microsoft® Student 2008 [DVD]. Redmond, WA: Microsoft Corporation, 2007.