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Powerpoint Presentation: Antibiotic Resistance

Evolution Index

The origins of life
Evolution and Fixity
Natural selection
Lamarkian Evolution
Industrial melanism
Palaeontology : The study of fossils
The C-14 Decay Curve
In Search of Deep Time
Evolution of the Horse
Punctuated Equilibrium
The classification of living organisms : Taxonomy
Kingdoms
Humans: Neotonous, bipedal African apes
The Hominids
The Changing Trees of Human Evolution
Genetic verses Cultural Evolution
Phenylketonuria (PKU) Fact Sheet
Cystic Fibrocis (CF) Fact Sheet

Topic Chapters Index

References

Robert Miller Bacterial gene swapping in nature Sci Am Jan 1998
Stuart Levy The challenge of antibiotic resistance Sci Am March 1998
K.C.Nicolaou & N.C. Boddy Behind enemy lines Sci Am May 2001

 

EVOLUTION

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Antibiotic Resistance

Fast breeders

Bacteria reproduce very quickly. Under ideal conditions, Eschericia coli can complete a life cycle in 30 minutes.

 

Sex in bacteria

Bacteria do not show sexual reproduction as such but they do exchange genes forming new combinations. The principle way in which the bacteria exchange genes is by conjugation. This involves the transfer of genetic material (either from the main DNA loop or plasmids) via a cytoplasmic bridge between the two organisms.

This can be done between unrelated species of bacteria. Recent studies on bacteria in the wild show that it definitely occurs there - in the soil, in freshwater and oceans and inside living organisms.

 

The magic bullet

Antibiotics revolutionised medicine by improving the survival rates of patients suffering from infections.

The first antibiotic, penicillin, was discovered by Alexander Fleming in 1929. It was later isolated by Florey and Chain. It was not extensively used until the 2nd World War when it was used to treat war wounds.

After World War II many more antibiotics were developed. Today about 150 types are used. Most are inhibitors of the protein synthesis, blocking the 70S ribosome, which is characteristic of prokaryotes.

 

Resistance

It took less than 20 years for bacteria to show signs of resistance. Staphylococcus aureus, which causes blood poisoning and pneumonia, started to show resistance in the 1950s.

Today there are different strains of S. aureus resistant to every form of antibiotic in use.

 

Multiple resistance

It seems that some resistance was already naturally present in bacterial populations but the presence of antibiotics in their environment in higher concentrations increased the pressure by natural selection.

Resistant bacteria that survived, rapidly multiplied. They passed their resistant genes on to other bacteria (both disease causing pathogens and non-pathogens)

Resistance genes are often associated with transposons, genes that easily move from one bacterium to another.

Many bacteria also possess integrons, pieces of DNA that accumulate new genes. Gradually a strain of a bacterium can build up a whole range of resistance genes. This is multiple resistance. These may then be passed on in a group to other strains or other species.

 

Antibiotics promote resistance

If a patient taking a course of antibiotic treatment does not complete it or forgets to take the doses regularly, resistant strains get a chance to build up.

The antibiotics also kill innocent bystanders bacteria which are non-pathogens. This creates more space for the resistant pathogens to expand.

The use of antibiotics also promotes antibiotic resistance in non-pathogens too. These non-pathogens may later pass their resistance genes on to pathogens.

 

Resistance gets around

When antibiotics are used on a person, the numbers of antibiotic resistant bacteria increase in other members of the family. In places where antibiotics are used extensively (e.g. hospitals and farms - 70% of antibiotic production is put in animal feed) antibiotic resistant strains increase in numbers.

 

Antibiotic use

Viral infections are not stopped by antibiotics. Yet doctors still prescribe (or are coerced into prescribing) antibiotics to treat them.

 

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