outline how bacteria become resistant to antibiotics as an example of natural selection

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Natural and Artificial Selection

Natural and Artificial Selection

Learning Objective

Outline how bacteria become resistant to antibiotics as an example of natural selection.

Key Concepts

  • Variation: Individuals within a population differ in traits.

  • Differential Survival and Reproduction: Some variants survive better in a given environment.

  • Inheritance: Beneficial traits are passed to offspring.

  • Population Change Over Time: The frequency of advantageous traits increases.

Artificial Selection – A Brief Contrast

In artificial selection, humans deliberately choose which individuals reproduce, accelerating the change in trait frequencies (e.g., breeding dogs for specific coat colours).

Natural Selection in Bacteria: Antibiotic Resistance

Antibiotic resistance provides a clear, real‑time illustration of natural selection. The process can be described in four stages:

  1. Mutation or Gene Acquisition: Random genetic changes (point mutations, plasmid uptake, transduction) create variants with reduced susceptibility to an antibiotic.

  2. Exposure to Antibiotic: The environment (e.g., a patient receiving treatment) imposes a selective pressure.

  3. Survival of Resistant Cells: Sensitive bacteria die, while resistant cells continue to grow and divide.

  4. Propagation of Resistance Genes: Resistant cells reproduce, and the resistance genes become more common in the bacterial population.

The overall change can be expressed mathematically using the Hardy–Weinberg principle modified for selection:

\$\Delta p = \frac{spq}{1 - sq}\$

where \$p\$ is the frequency of the resistant allele, \$q = 1-p\$, and \$s\$ is the selection coefficient representing the survival advantage conferred by resistance.

Mechanisms of Antibiotic Resistance

MechanismTypical Genetic BasisEffect on Antibiotic Action
Enzymatic degradationPlasmid‑encoded β‑lactamase genesAntibiotic is chemically destroyed before reaching its target.
Altered target sitePoint mutations in ribosomal RNA or DNA gyrase genesAntibiotic can no longer bind effectively.
Efflux pumpsChromosomal or plasmid genes encoding membrane transport proteinsAntibiotic is expelled from the cell, reducing intracellular concentration.
Reduced permeabilityMutations in porin proteinsAntibiotic entry into the cell is limited.

Suggested diagram: Flowchart showing the steps from random mutation to the spread of resistant bacteria in a treated patient.

Implications for Human Health

Understanding antibiotic resistance as natural selection highlights why misuse of antibiotics (e.g., over‑prescribing, incomplete courses) accelerates the evolution of resistant strains. Strategies to mitigate resistance include:

  • Prudent prescription practices.

  • Combination therapy to reduce the probability of simultaneous resistance.

  • Development of drugs targeting resistance mechanisms (e.g., β‑lactamase inhibitors).

Summary

Antibiotic resistance in bacteria exemplifies natural selection: random genetic variation provides some individuals with a survival advantage under the selective pressure of an antibiotic, and those individuals pass the advantageous traits to subsequent generations, leading to a population shift toward resistance.