- 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 #
- 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
Antibiotics #
- 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:
- 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;
- 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).
