Mode of action of antibiotics

Antibiotics work by interfering with some aspect of the metabolism of a microorganism, such as cell wall formation, DNA synthesis or protein synthesis. Since viruses lack cellular structure and metabolic processes, antibiotics are ineffective against them. This is a vital point to stress to students.

Some antibiotics are bactericidal (kill bacteria), others are bacteriostatic (arrest bacterial growth).

Penicillin interferes with bacterial cell wall synthesis. The wall which is formed in the presence of penicillin is mechanically weak so that tissue fluid, which has a high water potential, brings about endosmosis and the susceptible pathogen undergoes lysis.

Thus penicillin is clearly a bactericidal antibiotic, not a bacteriostatic one.

Chloramphenicol is an example of an antibiotic with bacteriostatic activity.

Some antibiotics have a broad spectrum of activity, being active against a wide range of pathogens; others have a narrow spectrum of action.

Streptomycin was the first major antibiotic to be discovered after penicillin. It is made by the bacterium Streptomyces and is a broad spectrum bactericidal substance. The original penicillin affects only Gram positive bacteria and, in that sense, has a narrow spectrum of activity. Many of the newer penicillins, such as ampicillin, possess a much broader spectrum of activity, some of them being active against Gram negative bacteria. Thus many types of bacterial infection are commonly treated with one or other of these newer substances. Penbritin, for example, is commonly prescribed to treat a wide range of bacterial infections.

Background

Examples of action sites for antimicrobial chemicals

Generally, antibacterial chemicals exploit differences in cellular chemistry which exist between prokaryotic and eukaryotic cells so that the least harm is done to patients.

• cephalosporins, like penicillin, inhibit bacterial cell wall synthesis.

  
  

• polymyxins increase cell membrane permeability

  
  

• tetracyclines inhibit protein synthesis by damaging ribosomes

  
  

• chloramphenicol also inhibits protein synthesis, preventing the linking together of amino acids, interfering with cell division in many bacteria. It is a thus a broad spectrum bacteriostatic substance.

  
  

• rifampicin inhibits nucleic acid synthesis by preventing RNA transcription

  
  

• trimethoprim, imidazoles and quinolones interfere with bacterial DNA

  
  

• sulphonamides mimic p-amino benzoic acid, an essential growth factor required by certain bacteria for folic acid synthesis. The drug is taken up by, but blocks, the 'folic acid pathway'. Thus almost deprived of folic acid, the bacteria cannot divide. This is a good example of a bacteriostatic, rather than bactericidal, action. However it is worth pointing out that sulphonamides, being chemically synthesised substances (sulpha drugs) are not strictly antibiotics.

Fungi have eukaryotic organisation and so selective action against them, without undue toxicity to the patient, is much more difficult to achieve. This is one of the reasons why fungal infections tend not to be completely eradicated by treatment, and thus to recur.