Natural Selection & Evolution

Antibiotic resistance is one of the most important real-world examples of natural selection, and arguably the most serious medical problem facing modern healthcare.

What antibiotics are

Antibiotics are drugs that kill bacteria (or stop them growing). They work by targeting features of bacterial cells that human cells do not have:

• The cell wall (penicillin and related drugs prevent bacteria building their wall properly)
• The bacterial ribosome (different from the human ribosome; some antibiotics target it specifically)
• Enzymes only bacteria have

Antibiotics have saved millions of lives since the discovery of penicillin in 1928. Before antibiotics, even a minor infection could be fatal.

How resistance develops

In any population of bacteria, there is genetic variation from random mutations. Occasionally a mutation produces a protein that makes the bacterium resistant to a particular antibiotic (for example, by changing the shape of the antibiotic's target so the antibiotic can no longer bind).

When the bacterial population is exposed to that antibiotic:

1. Variation: a few bacteria in the population have the resistance allele from random mutation.
2. Differential survival: bacteria without the resistance allele are killed. Bacteria with the resistance allele survive.
3. Differential reproduction: the surviving bacteria multiply rapidly (a single bacterium can become millions in a day), all carrying the resistance allele.
4. Allele frequency change: within just a few days, almost the whole population has resistance to the antibiotic. The antibiotic no longer works.

Because bacteria reproduce so quickly, this whole process can happen in days or weeks, not the centuries it might take in a slower-reproducing species.

Resistance is amplified by misuse of antibiotics

Several human behaviours make resistance worse:

• Overprescribing: prescribing antibiotics for viral infections (which they don't treat), or "just in case", exposes huge numbers of bacteria to the drugs.
• Patients not finishing their course: stopping antibiotics as soon as you feel better leaves the slightly less susceptible bacteria alive, ready to multiply and resist next time.
• Use of antibiotics in farming: many farm animals are routinely given low-dose antibiotics to promote growth. This creates a constant low-level selection pressure for resistant bacteria in the animal population, which can then transfer to humans.

Famous resistant bacteria

• MRSA (Methicillin-Resistant Staphylococcus aureus): resistant to many common antibiotics; a major cause of hospital infections.
• Multidrug-Resistant Tuberculosis (MDR-TB): resistant to the standard first-line TB drugs.
• Carbapenem-resistant Enterobacteriaceae (CRE): resistant to one of the strongest classes of antibiotics; very difficult to treat.

What can be done

• Use antibiotics only when needed and finish the prescribed course
• Develop new antibiotics, although this has slowed because it is not very profitable for drug companies
• Reduce antibiotic use in farming
• Improve hygiene in hospitals to prevent the spread of resistant strains
• Vaccination to prevent bacterial infections in the first place