5.12 - Antibiotic Resistance

How antibiotic resistance arises

• Bacteria reproduce very quickly, so they can evolve in a short time.
• Like all organisms, a bacterium can develop a random mutation in its DNA. Very occasionally this makes the bacterium less affected by a particular antibiotic - a drug that kills bacteria or stops them reproducing.
• The mutation produces a new allele (sometimes called a new variant) for antibiotic resistance.
• The mutation happens by chance - the antibiotic does not cause it. So some bacteria in a population are already resistant before any antibiotic is used.

Natural selection builds up a resistant strain

This is natural selection acting on bacteria, with the antibiotic as the selection pressure. Because bacteria reproduce so fast, it can be watched happening within a few generations:

1. The antibiotic kills the bacteria that are not resistant, known as susceptible bacteria, but a resistant bacterium survives.
2. Resistance is a large survival advantage - the resistant bacterium lives longer and reproduces many more times, even inside a host being treated for the infection.
3. It passes the resistance allele on to all of its offspring.
4. Because bacteria reproduce so rapidly, the proportion of resistant bacteria rises quickly until a whole new antibiotic-resistant strain has built up.

Why a resistant strain is a serious problem

• People have no immunity to it and the usual antibiotic no longer works, so the infection is hard to treat and spreads easily from person to person.
• Bacteria that have become resistant to many different antibiotics are called superbugs.
• An example is MRSA, and such superbugs are becoming more common.