Correct Answer: (A) fluctuation test

The fluctuation test distinguishes spontaneous mutations (random, pre-existing) from induced mutations (environmentally triggered) by analyzing variance in resistant bacterial colonies across parallel cultures.​

Option Analysis

Fluctuation Test
Luria and Delbrück’s 1943 experiment grew multiple small bacterial cultures separately, then plated them on selective media (like phage or antibiotic). High variance (fluctuation) in resistant colonies proved mutations occurred randomly before selection (spontaneous), unlike uniform low variance expected from induced mutations post-exposure.​

Replica Plating
Lederbergs’ 1952 technique transfers colonies from a master plate to selective replicas using velvet. Resistant colonies appear at identical positions, confirming pre-existing spontaneous mutants, but it screens rather than statistically distinguishes mutation types.​

Disc Diffusion Test
Kirby-Bauer method assesses antibiotic susceptibility by measuring inhibition zones around disks on inoculated agar. It evaluates bacterial resistance phenotypes, not mutation origins (spontaneous vs. induced).​

Use-Dilution Test
AOAC procedure tests disinfectants on carrier surfaces with dried microbes, checking growth post-exposure. It measures antimicrobial efficacy, unrelated to distinguishing mutation mechanisms.​

Introduction to Mutations in Bacteria

Spontaneous and induced mutations in bacteria drive evolution and resistance, key for CSIR NET Life Sciences. Spontaneous mutations arise randomly during replication; induced ones result from mutagens like UV or chemicals.​

Fluctuation Test: Gold Standard Method

The fluctuation test by Luria-Delbrück (1943) resolves if mutations are spontaneous or induced. Parallel cultures show jackpot variance for spontaneous events vs. Poisson distribution for induced. This statistical proof earned a Nobel Prize.​

• Grow 50+ small E. coli cultures from single cells.

• Plate equal aliquots on T1 phage agar.

• High culture-to-culture variance confirms pre-selection spontaneous mutations.​

Replica Plating Technique Explained

Replica plating isolates pre-adapted mutants but complements, not replaces, fluctuation analysis. Lederbergs demonstrated resistance exists before antibiotics via positional colony matching.​

Why Other Tests Fail

Disc diffusion measures susceptibility zones, ignoring mutation timing. Use-dilution evaluates disinfectants on surfaces, not genetic events.​

CSIR NET Exam Relevance

This question tests molecular genetics foundations. Fluctuation test directly distinguishes via variance, essential for competitive exams like CSIR NET.​