Describe the development of strains of antibiotic resistant bacteria as an example of natural selection

The emergence of antibiotic-resistant bacteria is a prime example of natural selection in action. This process illustrates how populations of bacteria can evolve over time in response to selective pressures imposed by the use of antibiotics. Here’s a detailed description of how this phenomenon occurs:

1. Genetic Variation Within Bacterial Populations

Bacterial populations exhibit genetic variation due to mutations and horizontal gene transfer. Mutations can occur during DNA replication, leading to random changes in the genetic material. Additionally, bacteria can acquire genes from other bacteria through mechanisms such as transformation, transduction, and conjugation, which allows them to gain new traits, including antibiotic resistance.

2. Use of Antibiotics Creates Selective Pressure

When antibiotics are administered to treat infections, they create a selective pressure on bacterial populations. Most bacteria are susceptible to the antibiotic and will be killed off; however, some individuals may possess mutations or acquired genes that confer resistance to the drug. For example, a population of Escherichia coli might contain a small number of cells that have developed resistance through a mutation that alters the target site of the antibiotic or enhances the efflux pump mechanisms that expel the drug .

3. Survival and Reproduction of Resistant Bacteria

The resistant bacteria survive the antibiotic treatment while their susceptible counterparts die off. These survivors reproduce, leading to an increase in the proportion of resistant individuals in the population. This process is a clear demonstration of natural selection: the antibiotic acts as a selective agent that favors those bacteria with advantageous traits (resistance) .

4. Rapid Increase in Resistant Strains

As resistant bacteria continue to reproduce, they can quickly dominate the population. Over time, this results in a strain of bacteria that is predominantly resistant to the antibiotic used. The rapid reproduction rate of bacteria allows for swift changes in population dynamics; within just a few generations, a previously susceptible strain can evolve into one that is resistant.

5. Horizontal Gene Transfer and Further Resistance Development

In addition to mutations, horizontal gene transfer plays a significant role in spreading antibiotic resistance among bacterial populations. Resistant strains can transfer their resistance genes to other bacteria, even those that are not closely related. This gene sharing can occur through plasmids—small DNA molecules that replicate independently within bacterial cells—or through other mobile genetic elements . As a result, multiple strains can acquire resistance traits from one another, leading to the emergence of multi-drug resistant bacteria.