Bacterial Mutations and Selection

Mutations are heritable changes in the DNA sequence, and in bacteria they arise spontaneously during growth, independently of whether the change happens to be useful. When the environment then favours cells carrying a particular variant, selection increases its frequency in the population, which is how bacteria adapt, including by acquiring resistance to antibiotics.

Definition

A bacterial mutation is a heritable change in the genome sequence of a bacterium; selection is the differential survival and reproduction of cells according to the traits their genotypes confer in a given environment.

Scope

This topic covers the origin and types of bacterial mutation, the classic evidence that mutations are pre-existing rather than directed, how selection acts on bacterial populations, and mechanisms that modulate mutation rate. It is a reference overview of population-genetic mechanisms and does not provide clinical instructions.

Core questions

• Do bacterial mutations arise before, or in response to, a selecting agent?
• What types of mutation occur, and what mechanisms generate them?
• How does selection change the frequency of a variant in a bacterial population?
• How is mutation rate regulated, and can stress increase it?

Key concepts

• Spontaneous mutation
• Point mutations, insertions, and deletions
• Fluctuation test
• Pre-existing versus directed mutation
• Positive and negative selection
• Mutation rate and mutator strains
• Stress-induced and adaptive mutation
• Contingency loci and phase variation

Key theories

Random pre-existing mutation (Luria-Delbruck)

Fluctuation analysis showed that the number of resistant cells varies widely between parallel cultures, which is expected only if resistance mutations occur randomly during growth before exposure, not as a directed response to the selecting agent.

Mechanisms

Mutations arise from replication errors, spontaneous chemical changes to DNA, and the action of mobile elements, producing point substitutions, insertions, deletions, and rearrangements. The Luria-Delbruck fluctuation test established that such mutations occur randomly during growth, independent of the selecting agent, so that selection acts on variation that already exists in the population. Selection then enriches advantageous variants and removes deleterious ones. Mutation supply is not uniform: mutator strains with defective repair raise the overall rate, certain contingency loci with simple sequence repeats mutate at high frequency to allow reversible phase variation as Moxon and colleagues describe, and stress responses can transiently elevate mutation, a phenomenon Rosenberg reviews under adaptive mutation. Together these processes let bacterial populations generate and fix heritable change.

Clinical relevance

Spontaneous mutation followed by selection is a core route to antimicrobial resistance, and mutation-rate variation and phase variation contribute to persistence and immune evasion. This entry explains how variation is generated and selected and is not a guide to therapy.

History

The Luria-Delbruck experiment of 1943 resolved a long-standing question by showing that bacterial mutations are spontaneous and pre-existing rather than induced by selection, a result honoured with a share of the Nobel Prize and foundational to bacterial population genetics. Later work on mutator phenotypes, contingency loci, and stress-induced mutation, reviewed by Moxon and by Rosenberg, refined the understanding of how mutation supply itself is shaped.

Debates

Is there a distinct, regulated 'adaptive' or stress-induced mutation mechanism?

Some experiments suggest that stress can transiently elevate mutation in ways that increase the chance of useful change, but how directed or general this is, and how it fits with the random-mutation framework, remains debated.

Key figures

• Salvador Luria
• Max Delbruck
• Susan Rosenberg
• Richard Moxon

Seminal works

• luria-delbruck-1943
• rosenberg-2001
• moxon-2006

Frequently asked questions

Do bacteria mutate in response to antibiotics?

The classic evidence shows resistance mutations arise spontaneously during growth before the antibiotic is encountered; the antibiotic then selects the already-resistant cells rather than causing the specific mutation.

What is a mutator strain?

A mutator strain has a higher-than-normal mutation rate, usually because a DNA-repair or proofreading function is defective, which can speed adaptation but also accumulate harmful mutations.