Bacterial Mutation and Genetic Variation: Mechanisms and Experimental Evidence

Section 6.1 – Bacteria Mutate Spontaneously and Grow at Exponential Rate

Bacteriophages and Bacterial Resistance

Bacteriophages are viruses that infect bacteria. Early experiments in the 1940s, such as spraying Bacteriophage T1 onto E. coli cultures, showed that while most bacterial cells died, a few survived. This observation led to questions about the origin of bacterial resistance to phages.

• Bacteriophage: A virus that specifically infects bacteria.

• Immunity Hypothesis: Initial idea that bacteria acquire immunity after exposure to phage.

• Adaptation Hypothesis: Proposed that interaction with the phage induces resistance in bacteria.

• Spontaneous Mutation: The concept that mutations occur naturally and randomly, independent of exposure to phage.

Luria-Delbrück Fluctuation Test

Salvador Luria and Max Delbrück conducted experiments to determine whether bacterial resistance to phages was due to adaptive mutation or spontaneous mutation. Their fluctuation test provided evidence for the spontaneous mutation hypothesis and was foundational in modern microbiology.

• Spontaneous Mutation: Mutations that occur in the presence or absence of selective agents, serving as the primary source of genetic variation in bacteria.

• Selection: Growth of organisms under conditions where only mutants of interest can grow.

• Mutation: An alteration in DNA sequence.

• Induced Mutations: Result from exposure to external factors (mutagens), either natural or artificial.

Adaptive vs. Spontaneous Mutation Hypotheses

• Adaptive Mutation Hypothesis: Bacteria mutate in response to exposure to bacteriophage. Every cell has a constant chance of becoming resistant upon exposure, so the number of resistant cells should show little fluctuation among plates if conditions are constant.

• Spontaneous Mutation Hypothesis: Mutations occur at a low rate before exposure to phage. If mutation occurs early, many resistant cells will be present; if late, fewer will be present. This leads to high fluctuation in the number of resistant cells among plates.

• Key Finding: The Luria-Delbrück experiment showed high fluctuation, supporting the spontaneous mutation hypothesis.

Prototrophs vs. Auxotrophs

Bacteria can be classified based on their nutritional requirements:

• Prototroph (wild type): Can synthesize all essential organic compounds and grow on minimal medium (contains only a carbon source and inorganic ions).

• Auxotroph: Has lost the ability to synthesize one or more essential compounds due to mutation and must be provided with these compounds in the medium (requires a complete medium).

Bacterial Growth Phases

Bacterial populations grow in three distinct phases:

• Lag Phase: Slow growth as bacteria adapt to new conditions.

• Log (Exponential) Phase: Rapid, exponential increase in cell number.

• Stationary Phase: Growth slows and plateaus due to nutrient limitation.

Example: The classic bacterial growth curve shows these three phases, with cell number (log scale) plotted against time (hours).

Growth Phase	Description
Lag Phase	Slow growth; adaptation to environment
Log Phase	Exponential growth; rapid cell division
Stationary Phase	Growth rate slows; nutrients become limited

Key Terms and Concepts

• Mutation Rate: The frequency at which mutations occur in a given gene or organism.

• Genetic Variation: Differences in DNA sequences among individuals, providing the raw material for evolution.

• Selection Medium: A growth medium that allows only organisms with specific traits (e.g., mutants) to grow.

Summary Table: Adaptive vs. Spontaneous Mutation

Hypothesis	When Mutation Occurs	Expected Fluctuation	Experimental Evidence
Adaptive	In response to exposure	Low	Not supported
Spontaneous	Before exposure	High	Supported by Luria-Delbrück

Additional info: The Luria-Delbrück fluctuation test is a foundational experiment in genetics, demonstrating that mutations arise randomly and not as a direct response to environmental challenges. This principle underlies much of modern evolutionary theory and microbial genetics.