Antibiotic Resistance (Cambridge (CIE) AS Biology): Revision Note

Resistance to antibiotics

• Genetic variation exists between individuals in a bacterial population; this is the result of random mutation

• A chance mutation might give rise to a new allele that provides resistance to an antibiotic

  • E.g. some pathogenic bacteria have become resistant to penicillin as they have acquired an allele that codes for the production of an enzyme which breaks down penicillin

• When the bacterial population is treated with this antibiotic any bacteria with the resistance allele do not die

  • The antibiotic acts as a selection pressure

• The resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead

• The allele for antibiotic resistance is passed on with much greater frequency to the next generation

  • This can occur especially quickly in bacterial populations, due to:

    • short generation times

    • all offspring being clones of their parents

    • horizontal gene transfer

• The antibiotic resistance allele increases in frequency, and a higher proportion of the bacterial population is resistant to the antibiotic

• This is an example of evolution by natural selection

• Humans have caused an increase in the occurrence of antibiotic resistance; this is because treating infections with antibiotics provides a selection pressure which drives natural selection

• This problem is made worse when antibiotics are used improperly, e.g.

  • For treatment of non-serious infections

  • For treatment of viral infections

  • Routine treatment to animals in agriculture