• the term antibiotic strictly refers to substances that are of biological origin whereas the term chemotherapeutic agent refers to a synthetic chemical;
  • the distinction between these terms has been blurred because many of our newer “antibiotics” are actually chemically modified biological products or even chemically synthesized biological products;
  • the generic terms to refer to either antibiotics or chemotherapeutic agents are antimicrobic or antimicrobial agent.;
  • however, the term antibiotic is often used to refer to all types of antimicrobial agents.



  • chemotherapy is the use of drugs to treat or cure infections;
  • effective drugs show selective toxicity, killing the pathogen but having no effect on host cells;
  • there is a wide range of chemotherapeutic agents for bacterial and fungal infections, but few for viral infections;
  • some drugs are derived from natural compounds but others, such as isoniazid used for treatment of tuberculosis, are synthetic



  • are natural chemotherapeutic agents made by microorganisms;
  • in dilute solutions they inhibit the growth of, or kill, other microorganisms;
  • many antibiotics are modified chemically to increase their effectiveness;


Mode of action of antibiotics

although there are a number of different types of antibiotic they all work in one of two ways:

  1. a bactericidal antibiotic kills the bacteria by either interfering with the formation of the bacterium’s cell wall or its cell contents; e.g. penicillin is a bactericidal;
  2.  a bacteriostatic stops bacteria from multiplying.
  • antibiotics interfere with some aspect of growth or metabolism of the target organism such as
    • synthesis of bacterial walls;
    • protein synthesis (transcription and translation);
    • cell membrane function;
    • enzyme action;
  • penicillin functions by preventing the synthesis of the cross links between the peptidoglycan polymers in the cell wall;
  • this means that they are only active against bacteria which are growing;
  • most types of bacteria have enzymes for destroying penicillin (penicillinases) and are therefore resistant to the antibiotic;
  • the main sites of action are shown in the diagram below:
  • different diseases are treated with different antibiotics;
  • all strains of some bacteria are resistant to several antibiotic; e.g. tuberculosis is resistant to penicillin;


Broad spectrum and narrow spectrum antibiotics

  • antibiotics are classed as bactericidal or bacteriostatic according whether they kill bacterial cells directly or indirectly;
  • they are also divided into classes such as cephalosporins or macrolides depending in their chemical structure and action;
  • all antibiotics can also be described as either narrow spectrum or broad spectrum.
  • broad spectrum are effective against a wide range of bacteria
  • narrow spectrum are effective only against a few;


Narrow spectrum antibiotics

  • those with a narrow spectrum of action can kill only a small number of species of bacteria, maybe even just one;
  • narrow spectrum antibiotics tend to be very specific and act on a molecule in the metabolism of one particular type of bacteria that is special to that species;


Advantages of narrow spectrum antibiotics

  • the narrow-spectrum antibiotic will not kill as many of the normal microorganisms in the body as the broad spectrum antibiotics. So, tt has less ability to cause superinfection.
  • the narrow spectrum antibiotic will cause less resistance of the bacteria as it will deal with only specific bacteria.


Disadvantages of narrow spectrum antibiotics:

  • narrow spectrum antibiotics can be used only if the causative organism is identified.
  • if you don’t choose the drug very carefully, the drug may not actually kill the microorganism causing the infection.


Broad spectrum antibiotics

  • broad spectrum antibiotics are active against a wide range of bacterial species;
  • broad spectrum antibiotics act on structures or processes that are common to many different bacteria, such as the components of the cell wall.


Selection of antibiotics

  • antibiotics should be chosen carefully;
  • screening antibiotics against the strain of the bacterium or fungus isolated from sufferers ensures that the most effective antibiotic can be chosen;
  • the diagram below shows results of antibiotic sensitivity test carried out on a strain of the human gut bacterium coli that causes epidemics of food borne and water borne diseases;
  • bacteria are collected from feaces, or from food or water and grown on an alga medium;
  • different antibiotics are absorbed onto discs of filter paper placed on the agar plate;
  • the plate is incubated and the diameters of the inhibition zones where no bacteria are growing are measured;
  • the diameters are compared with standard diameters as in the tables below and the most appropriate antibiotics are chosen;
  • increasingly, bacteria which were once susceptible to antibiotics are now resistant;
  • this has a great impact on disease control as it prolongs epidemics, lengthening the period of time when people when people are ill and increasing the risk of higher mortality rates;
  • the inappropriate and widespread use of antibiotics should therefore be discouraged;
  • some drugs should be kept for use as a last resort when everything else has failed, and drug companies must continue to invest in research for new drugs to replace those which quickly become redundant;


Advantages and limitations of using antibiotics

  • kill pathogen after infection;
  • interfere with growth/metabolism of pathogen;
  • g. protein synthesis/synthesis of cell wall;
  • do not harm host;
  • not effective against viruses;
  • mainly used against bacteria and fungi;
  • some narrow spectrum/effective against a limited range of micro-organisms; – bacteria can become resistant


Role of antibiotics in treatment of infectious diseases

  • mainly used against fungi and bacteria;
  • cell wall synthesis inhibited/stops growth;
  • protein synthesis inhibited;
  • disrupts translation/DNA replication;
  • disrupts cell wall/membrane function;
  • acts as an enzyme inhibitor;
  • causes lysis;
  • bactericidal;


Side-effects of antibiotics

Below is a list of the most common side-effects of antibiotics:

  • diarrhoea – researchers from Stanford University School of Medicine found that rise in sugars in the gut following antibiotic treatment allows harmful bacteria to get a foothold and cause infection. Harmful bacteria thrive on sugar;
  • feeling and being sick;
  • infections of the mouth, digestive tract and vagina.


Below is a list of rare side-effects of antibiotics: 

  • formation of kidney stones (when taking sulphonamides).
  • abnormal blood clotting (when taking some cephalosporins).
  • sensitivity to sun (when taking tetracyclines).
  • blood disorders (when taking trimethoprim).
  • deafness (when taking erythromycin and the aminoglycosides).

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