Antibiotics vs. Bacteria: “Evolutionary Arms Race”

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“Since the mass production of penicillin began 60 years ago, humans have waged an escalating chemical war against bacteria.”  Many strains of bacteria carry circular bits of DNA called plasmids, which are composed of drug resistant genes that fight off antibiotics. Plasmids are inter-exchangeable among bacteria, therefore allowing for the allocation of these defense mechanisms. Prior to the antibiotic era, there were no traces of these resistance-bearing plasmids in the bacteria, which suggest that gene resistance has evolved recently. 

The idea that bacteria has evolved drug resistance in response to modern antibiotics has been a long-standing notion among many evolutionary biologists. However, recent studies conducted by molecular evolutionist Barry Hall of the University of Rochester in New York and colleagues have challenged this notion. The team reconstructed the family history of the OXA ß-lactamase genes, a penicillin-fighting group common to nasty Salmonella and Staphylococcus bacteria, to determine the number of genetic changes accrued over time. They discovered that these genes hopped onto plasmids millions of years ago, way before humans ever discovered antibiotics.  

"First we thought we were responsible for the existence of resistance genes. Then we realized that some lived a plasmid-borne lifestyle for millions of years." Whether genes hopped onto plasmids millions of years ago or recently, antibiotics have accelerated an evolutionary arms race. The more drugs we throw at them, the more the bacteria evolves. 

Posted by Saad Choudhry (4)

Update (March 6, 2009):

One example of an evolutionary “arms race” would be the symbiosis between leaf cutter ants and fungus. Leaf cutter ants make their nests in underground chambers and come out to forage. They take toxic leafs and cultivate them for the fungus, which provides the protein necessary for the ant population survival.The fungus needs the leafs for survival. Mold can destroy the fungus, but the ants keep it under control - white cover of ants is tangled bacteria that produces half of antibiotics used by humans today. Ants use antibiotics to control pests in gardens - the mold has not evolved a counter because the bacteria itself keeps evolving (arms race between mold and bacteria on ants). Many researchers believe that medicine and bacteria are in an evolutionary arms race. This long-standing notion has been challenged by the idea that some resistant genes lived a plasmid-borne lifestyle for millions of years. However, researchers are still trying to determine why these resistant gene were present millions of years prior to the antibiotic area.