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Understand  –  Antibiotics

How do antibiotics work?

Antibiotics disrupt essential processes or structures in the bacterial cell. This either kills the bacterium or slows down bacterial growth. Depending on these effects an antibiotic is said to be bactericidal or bacteriostatic.

Bactericidal and bacteriostatic antibiotics

A bactericidal antibiotic kills the bacteria while the bacteriostatic antibiotics stops bacterial growth but does not kill the cells. The human immune system is then needed to clear the infection.

Antibiotic targets in bacteria

There are several different classes of antibiotics. These can have completely different bacterial targets or act on the same target but at a different place. In principal, there are three main antibiotic targets in bacteria:

  1. The cell wall or membranes that surrounds the bacterial cell
  2. The machineries that make the nucleic acids DNA and RNA
  3. The machinery that produce proteins (the ribosome and associated proteins)

These targets are absent or different in the cells of humans and other mammals, which means that the antibiotics usually do not harm our cells but are specific for bacteria. However, antibiotics can in some cases have unpleasant side effects. Read more under Why should I care? – Risks for the individual and society.

Narrow-spectrum and broad-spectrum antibiotics

Antibiotics can either have a narrow or broad spectrum of activity. Narrow-spectrum antibiotics are more specific and only active against certain groups or strains of bacteria. Broad-spectrum antibiotics instead inhibit a wider range of bacteria. Narrow-spectrum antibiotics are to prefer since the effect on other, non-disease causing bacteria are more limited. Unfortunately broad-spectrum antibiotics are often used since it can be difficult for doctors to diagnose the correct bacteria in time or when knowledge about how to correctly treat an infection is lacking. For more information, see How did we end up here – Use and inappropriate use – In human medicine.

See also these selected resources for more details on different antibiotics and their mechanisms of action.

Selected Resources

Resource Description
Eric’s Medical Lectures: Mechanisms and classification of antibiotics Video. Narrated lecture about antibiotics and their mechanism of action (24 min, YouTube).
How antibiotics work Video. Short video describing the difference between bacteriostatic and bactericidal antibiotics. Outlines why antibiotics are not effective against viruses (3 min, YouTube).
A brief overview of classes of antibiotics Fact sheet. Short description of different classes of antibiotics and their mode of action.
Antibiotics and bacterial resistance in the 21st century Journal article with short descriptions of clinically used antibiotic classes as well as examples of bacterial resistance mechanisms.
Antimicrobial Resistance Learning Site – Pharmacology Course website. Learn about concepts related to medical use of antimicrobials and of resistance. Focus on animal use.

More from "Antibiotics"

Public Health England. How antibiotics work [Internet]. 2014 [cited 2021 Mar 9]. Available from:
Compoundchem. A Brief Overview of Classes of Antibiotics [Internet]. Compound Interest. [cited 2015 Feb 13]. Available from:
Fair RJ, Tor Y. Antibiotics and bacterial resistance in the 21st century. Perspect Medicin Chem [Internet]. 2014;6:25–64. Available from:
Mechanisms and Classification of Antibiotics (Antibiotics - Lecture 3) [Internet]. 2013 [cited 2014 Dec 4]. Available from:
Michigan State University. Effect on bacteria [Internet]. Antimicrobial Resistance Learning Site. 2011 [cited 2014 Dec 2]. Available from: