New research by Paul Fildes and K. Whitaker challenges the theory that strains of bacteria can be “trained” to mutate by withholding a metabolite necessary for their regular function. In particular, they consider the case of bacteria typhosum, which needs tryptophan in order to reproduce. Earlier researchers had grown the bacteria in a medium somewhat deficient in tryptophan and observed the growth of mutant strains of the bacteria which did not need tryptophan in order to reproduce.
Fildes and Whitaker argue that the withholding of tryptophan did not induce these mutant strains of bacteria. Rather, these mutants were already present in the original sample of bacteria typhosum, albeit in a concentration too small to detect. In experimenting with the bacteria grown in agar cultures, they found that by plating out huge quantities of the bacteria, one could locate mutant strains. Because of the possibility that the lack of uniformity of the agar cultures had in fact trained mutant strains, they conducted similar experiments with liquid cultures and again found that mutant strains of the bacteria were present in the original sampling. From these experiments, Fildes and Whitaker conclude that the mutants are of genetic origin and are not induced by environmental training. They asserted that the concentration of tryptophan is unrelated to the appearance of these mutants in the bacteria.

To confirm these results, Fildes and Whitaker used an innovative plating technique using pile fabrics, such as velvet or velveteen, to accurately imprint the growth found on an original agar plate to a series of replica agar plates. The process entails taking the original agar plate, inverting it onto the velvet while using light finger pressure to transfer growth, and then imprinting the fabric, with its pattern of growth, on the new agar plates. By using this replica plating method, Fildes and Whitaker demonstrated that the mutants were in fact genetically present or preadapted, as the locations of the mutant strains of bacteria on the replica plates were identical to the locations of the mutant strains on the original agar plate.

Which of the following most accurately states the purpose of the passage?

A) To defend a scientific hypothesis from attack by an innovative technique

B) To describe a process by which bacteria can be trained to mutate

C) To present the results of an experiment designed to test an established theory

D) To argue against an established protocol on the grounds that it is outdated

E) To challenge a scientific technique used to prove a questionable theory Which of the following most accurately states the purpose of the passage?