Antibiotics

• Antibiotics


  Chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue
• Antibiotics


  • Many are derived from naturally occurring substances that are harmful to prokaryotic cells (structurally or physiologically) but usually do not affect eukaryotic cells
  • The aim is to aid the body's immune system with fighting a bacterial infection
• Antibiotics


  • Penicillin
• Bactericidal


  Antibiotics that kill bacterial cells
• Bacteriostatic


  Antibiotics that inhibit bacterial growth processes
• Broad-spectrum antibiotics


  Act on a wide range of bacteria
• Narrow-spectrum antibiotics


  Act on a very small number of bacteria
• Doctors often prescribe broad-spectrum antibiotics (e.g. Amoxicillin) unless a culture has been taken to prove the need for a narrow-spectrum antibiotic
• Bacteria have a single loop of DNA with only one copy of each gene so when a new allele arises it is immediately displayed in the phenotype
• When an antibiotic is present

  Individuals with the allele for antibiotic resistance have a massive selective advantage so they are more likely to survive, reproduce and pass genome (including resistance alleles)
• When an antibiotic is present

  Those without alleles are less likely to die and reproduce
• Over several generations, the entire population of bacteria may be antibiotic-resistant
• Antibiotic resistance is an example of natural selection
• 1. lactamase (also known as penicillinase)


  An enzyme produced by some pathogenic bacteria that breaks down penicillin
• Bacteria evolve rapidly as they reproduce quickly and acquire random mutations- some of which confer resistance
• Consequences of antibiotic resistance

  • Overuse of antibiotics and antibiotics being prescribed when not necessary
  • Large scale use of antibiotics in farming to prevent disease when livestock are kept in close quarters, even when animals are not sick
  • Patients failing to complete the full course of antibiotics prescribed by doctors
• Bacteria living where there is widespread use of many different antibiotics may have plasmids containing resistance genes for several different antibiotics, giving them multiple resistance and presenting a significant problem for doctors
• Resistance may first appear in a non-pathogenic bacterium, but then be passed on to a pathogenic species by horizontal transmission
• MRSA (Methicillin-resistant Staphylococcus aureus)


  A strain of Staphylococcus aureus that has developed resistance to the antibiotic methicillin
• Some MRSA strains have also become resistant to other antibiotics (e.g. penicillin)
• Clostridium difficile is a bacteria present in the human gut, and a course of antibiotics can kill the 'friendly' gut bacteria, allowing C.diff to increase in numbers and cause diarrhoea and fever
• Ways to prevent the incidence of antibiotic resistance increasing

  • Tighter controls on the sale of antibiotics without a doctor's prescription
  • Doctors avoiding the overuse of antibiotics, prescribing them only when needed
  • Antibiotics not being used in non-serious infections that the immune system will 'clear up'
  • Patients finishing the entire course of antibiotics prescribed
  • Antibiotics not being used for viral infections
  • Reducing the use of 'wide-spectrum' antibiotics and using 'narrow-spectrum' antibiotics instead
  • Changing the type of antibiotics prescribed to reduce the chance of a resistant strain developing
  • Reducing and more tightly controlling the use of antibiotics in industries such as agriculture
• Ensuring good hygiene practices such as handwashing and the use of hand sanitisers can reduce the spread of resistant strains of bacteria, such as MRSA, in hospitals
• Isolating infected patients can prevent the spread of resistant strains, in particular in surgical wards where MRSA can infect surgical wounds
• Horizontal gene transfer


  The transfer of alleles for antibiotic resistance between different species of bacteria via plasmids