Natural selection explains more than just how giraffes evolved long necks. It has very practical importance. In the last century of widespread use of antibiotics, bacterial populations evolved resistance to these life-saving drugs.

Why this is important…If you have heard of MRSA, it is probably something that scares you. Many microorganisms have become resistant to antibiotics, and MRSA is just one. It is important that we keep antibiotics as a tool for fighting infections. The best way to do that is to understand how microbes become resistant to antibiotics.

Optional: The Centers for Disease Control and Prevention (CDC) have created a short video explaining the urgency of the The Threat of Antibiotic Resistance.

Foundations

Antibiotics are important drugs that allow us to kill bacteria when they become a problem. Most of these drugs come from fungi, which make them for the same purpose – to fight off the bacteria around them. Each antibiotic (penicillins, cephalosporins, tetracyclines, etc.) works a little differently, but many work by attacking the cell wall of the bacterium, leaving it vulnerable.

Since bacteria have a cell wall that is chemically different from other organisms, antibiotics can attack bacteria with few side-effects to our own cells. Bacteria do not have complex immune systems like we do, but they do have molecular “machinery” that can disable antibiotics. Not all bacteria have these abilities, but the more we use antibiotics, the more bacteria acquire these resistant properties. This makes our antibiotics ineffective and bacterial infections harder to treat.

Key terms

Antibiotic – in this context, it refers to drugs that specifically target bacteria; in other words, an anti-bacterial agent. In a broader sense, it can also be used to refer to drugs that harm any living thing.

Details

Antibiotics only treat bacterial infections! Since antibiotics (at least the types we are discussing here) work by destroying the cell wall unique to bacteria (peptidoglycan) they don’t treat other infections. Different drugs must be used to treat infections resulting from fungi, viruses or other microorganisms.

Over the last century, antibiotic use has led to resistant varieties (or strains) of bacteria. Many doctors prescribed antibiotics “just in case” an illness was bacterial. This meant that all the bacteria in the body, helpful and harmful types, were being killed off by antibiotics. For many years, antibiotics were also routinely put into hand soaps. This had the same effect – it killed off bacteria that were susceptible to that antibiotic, leaving only the more resistant varieties to survive and reproduce.

A common misconception is that antibiotics cause bacteria to mutate or fight back in order to survive. This is wrong. The real explanation is actually much simpler – it’s natural selection. Bacteria are not clever enough to know how to mutate, they simply survive if they can.

How does antibiotic resistance develop through natural selection? A population of bacteria has genetic variation – some individuals are naturally resistant to antibiotics, while others are not. When the bacteria are exposed to an antibiotic, many of them will die – mostly the non-resistant varieties. The survivors keep reproducing. Since bacteria reproduce by cloning, their offspring will be resistant to the antibiotic. The more times the bacteria are exposed to the antibiotic, the more this cycle repeats. Eventually, the bacteria remaining are mostly the ones that are unaffected by the antibiotic.

Optional: For additional information about how antibiotic resistance developed, watch this animated TED-Ed talk. What causes antibiotic resistance? - Kevin Wu

2-15 Antibiotic resistance.png

FIGURE 2-15. Development of antibiotic resistance. Timeline depicting how a population of bacteria might become more resistant to an antibiotic drug over time. Non-resistant bacteria cells are plain, blue cells. Resistant strains are red with an R. (“Antibiotic resistance” by J. Buegge is licensed under CC BY-SA 4.0)

MRSA is just one type of antibiotic-resistant bacteria, but a powerful one. The acronym describes Methicillin-Resistant Staphylococcus aureus – a common species of bacterium (“staph”) that is resistant to a group of antibiotics after years of exposure to methicillin drugs.

Learning objectives

Apply the theory of natural selection to specific examples (such as developing resistance)