Antibiotics - AQA AS Biology Unit 2

For AQA AS Biology unit 2 - the variety of life. 

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  • Created by: Helen
  • Created on: 01-05-12 19:18

What are antibiotics?

  • Substances produced by living organisms that can inhibit the growth of or destroy microorganisms.
  • Used to treat bacterial infections and dieseases. They have not effect on viruses.
  • Can be made synthetically or semi-synthetically.
  • First discovered by accident by Alexander Fleming in 1928. He discovered penicillin which was the first antibiotic to be used. 
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How do antibiotics work?

  • Prevent bacteria from making normal cell walls. 
  • When bacteria are actively dividing the antibiotic prevents cross links from forming between the cellulose chains that give the cell wall its strength.
  • The cell wall stops the bacterial cell from bursting when water enters it.
  • As the peptide cross links cannot form when water enters the cell the cell wall cannot resist the pressure and the cell bursts, killing it. This is known as osmotic lysis
  • Only works if bacteria are actively dividing as it prevents the cross links from forming.
  • Do not work on viruses as they have a different coating to bacteria.
  • Penicillin works in this way. 
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Antibiotic resistance

  • Resistance is due to mutations is the DNA of the bacteria, producing a protein that provides resistance, either by destroying the antibiotic or some other method.
  • It is not the presence of the antibiotic that causes the bacteria to mutate. They happen by chance.
  • The mutant gene is then passed on to the next generation of bacteria by vertical gene transmission.
  • The mutant gene is passed on to other species of bacteria by horizontal gene transmission.
  • Bacteria that posess the mutant gene will not be killed by the antibiotic and can survive to multiply. 
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  • Antibiotics must be taken for a long period of time.
  • Initially they kill the least resistant strains of bacteria, which makes the patient feel better so sometimes they stop taking the antibiotics.
  • The most resistant strains are then left and can divide without the antibiotic being there to kill them. These are then spread to other people.
  • This leads to the development of strains that are resistant to the antibiotic.
    Other strains can recieve the mutant gene by horizontal gene transmission.
  • So strains that are resistant to multiple antibiotics are produced.
  • Then a cocktail of antibiotics have to be taken to ensure all the bacteria are killed as so many are now resistant.
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  • MRSA stands for methicillin resistant Staphylococcus aureus.
  • MRSA is especially prevelent in hospitals and is dangerous because the people tend to be old and are sicker and weaker than normal so they are more vulnerable to infection. 
  • Doctors and nurses come into contact with many patients which spreads infection. 
  • Many antibiotics are used in hospitals so mutant strains have an advantage over non-mutant strains. As many different antibiotics are used strains easily develop multiple resistance. 
  • MRSA is difficult to treat because it has developed resistance to almost every known antibiotic. 
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