by Eric Lannak

Created on: January 30, 2011

MRSA (Methicillin-Resistant Staphylococcus Aureus) is a type of staph that’s become resistant to common antibiotics.  It’s often said MRSA developed from overuse of antibiotics which killed weaker strains and made it easier for stronger strains to prevail.

This "natural selection" process may play a small part in the process, but scientists now believe MRSA would have come about no matter what we did. This is because staph can develop drug resistances in two ways, known as “vertical gene transfer” and “horizontal gene transfer”.

BUILT TO MUTATE

Bacteria naturally evolve faster than higher life forms. Their life cycle is much shorter and they reproduce far more often, creating more chances for their DNA to change.  Over a few years and millions of infections, a huge number of mutations will occur. When bacteria multiply and pass mutations on to their offspring, it’s known as a “vertical gene transfer”. Natural selection theories suggest this is the main way drug resistance develops.

But many bacteria (S. Aureus included) have genes that ENCOURAGE mutation in certain portions of their DNA, mainly those addressing interaction with the environment. It’s believed this allows them to adapt quickly to new and challenging living conditions, an important ability in an organism that exists almost everywhere.

How fast can they evolve drug resistance? Amazingly, an experiment in the 1950’s showed S. Aureus can develop resistance even before a drug is encountered.

THE LEDERBERG EXPERIMENT

When penicillin was discovered in 1928 it killed S. Aureus, and by the 1940’s it was used widely to treat staph infections. But by World War II some staph weren’t responding to it anymore, and by the mid-1950’s most staph infections were penicillin-resistant.

Esther and Joshua Lederberg were researching mutations, and grew a strain of S. Aureus from a single microbe known to be susceptible to penicillin. They sprinkled some of the bacteria on a culture dish and let new S. Aureus colonies grow, and then washed the dish with penicillin.

Lo and behold, all the bacteria didn’t die. In just the space of a few days, a penicillin-resistant strain had been born, one that had never encountered the drug. This proved that mutations occur randomly, instead of occurring as a response to a need, and also that staph can develop drug resistances “in the wild”.

METHICILLIN

In 1959

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