Pathogen & Host 'Evolutionary Arms Race'

• Increased number of antiobiotic resistant bacteria
• Multiple antibiotics may be needed to destroy
• More an antibiotic is used the more it acts as a selection pressure
• Greater the selection pressure the greater the likelihood of resistant strain of bacteria developing

Host evolves to combat pathogen --> pathogen evolves new method to overcome immune system --> repeat

Bacterial populations evolve very quickly as:

• Reproduce very quickly
• Large population size (so lots of cells have mutations)
• Large quantity random mutations occur that:
  • Produce enzymes that breaks down the antibiotic
  • Enables them to use different metabolic pathway for reactions inhibited by the antibiotic

Driven by selection pressure --> the immune system & antibiotics

Bacteria with mutation may be at disadvantage when antibiotic not present - e.g. reproduce at a slower rate as they are using resources to produce enzymes that are no longer needed

Vertical Evolution: Advantageous gene passed down from one generation to next by binary fission

Horizontal Evolution: Advantageous gene carried in plasmid passed onto another bacterium by conjugation

1. One strand of the plasmid carrying the gene is transferred between conjugating bacteria

2. Each bacteria replicates the strand to make a complete plasmid

3. Both bacterium now resistant

1. Random mutation results in an advantageous allele

2. Advantageous allele may: produce enzyme that breaks down the antibiotic OR make the bacteria naturally resistant to the antibiotic

3. Antibiotic acts as a selection pressure

4. Resistant bacteria survive & reproduce by binary fission, passing the resistance allele on to their offspring

5. Resistant allele also passed on by horizontal evolution to conjugating bacteria

6. Increased allele frequency for resistant allele in population

7. Antiobiotic no longer effective