Escherichia Coli and the Development of Bacterial Genetics

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Escherichia Coli and the Development of Bacterial Genetics

Claire G Cupples, Simon Fraser University, Burnaby, British Columbia, Canada

Published online: June 2020

DOI: 10.1002/9780470015902.a0000820.pub2

Full Article on Wiley Online Library

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Abstract

Genetic analysis of bacteriophage and bacteria, particularly Escherichia coli, was a key to identifying DNA as the genetic material in the middle of the twentieth century, laying the foundations for the modern science of molecular biology in the process. Key experiments have become textbook classics, and most of those who performed them won Nobel Prizes for their work. The techniques that they developed, particularly conjugation and transduction, are still used in bacterial strain construction and for mapping of mutant genes. The advantages of large population size and rapid growth, combined with modern techniques of genome sequencing, the use of green fluorescent protein to measure gene expression and protein localisation, and one‐step strain construction will ensure that Escherichia coli remains central to answering the big biological questions of the twenty‐first century.

Key Concepts

Escherichia coli and bacteriophage genetics enabled the identification of DNA as the genetic material in the mid‐twentieth century.
Key experiments conducted by Luria and Delbrück; Hershey and Chase; Avery, MacLeod and McCarty are textbook classics.
The first step towards understanding how gene expression is controlled came from Jacob and Monod's genetic studies of the Escherichia coli lac operon.
The genetic techniques of sexual conjugation and bacteriophage‐mediated transduction, pioneered by Joshua and Esther Lederberg, were the primary tools for chromosomal gene mapping and strain construction in Escherichia coli until recently.
Well‐characterised genetics combined with large population numbers and rapid growth ensure that Escherichia coli will remain a favourite model organism for scientists tackling the big biological questions of the twenty‐first century.

Keywords: strain construction; conjugation; transduction; transformation; transposition; gene mapping; gene regulation; bacteriophage

	Figure 1. (a) The fluctuation test. Small numbers of phage‐sensitive Escherichia coli are taken from one large culture and used to set up seven smaller cultures. Cells from six of these subcultures are placed on agar plates in the presence of an excess number of phage. Meanwhile, the seventh subculture is divided into six fractions each of which are also placed on a phage‐containing plate. The numbers of phage‐resistant bacterial colonies from the six independent subcultures are quite variable. The numbers for the six samples which all came from the same subculture are much more consistent. (b) The origin of ‘jackpots’. Bacteria reproduce by dividing in two. Any mutation (grey) which occurs in one of the first few cells to appear in the culture will be present in all of its many descendants (left panel). A cell ‘born’ late in the culture period will leave few descendants carrying the mutation (right panel).
	Figure 2. A bacterial conjugation experiment. The strain in tube A requires proline for growth (P⁻), while that in tube B requires tryptophan (W⁻). Therefore, neither will grow on selective medium lacking both amino acids. However, the W⁺ strain is an Hfr strain, so it transfers part of its chromosome to the W⁻ strain when the two are mixed together. Some of the progeny are now both W⁺ and P⁺, so they are able to grow on the selective medium.
	Figure 3. A bacterial transduction experiment. The strain in tube A is a proline prototroph (P⁺), shown by growth on a medium lacking proline. That in tube E is a proline auxotroph (P⁻), which shows no growth on proline‐deficient medium. Bacteria from A are mixed with a transducing phage (from tube B). Following infection and lysis in tube C, the new phage particles (without bacteria) are mixed in tube D with the auxotrophic strain from tube E. When the surviving bacteria are plated on proline‐deficient media, colonies appear. The phage has transferred the genetic information for proline synthesis from the prototroph to the auxotroph.
	Figure 4. (a) The PaJaMo experiment. Cross between an I⁺Z⁺ Hfr strain containing functional genes for the Lac repressor and β‐galactosidase and an F⁻ strain with non‐functional lac genes (I⁻Z⁻). A copy of the functional lac genes transfers from the Hfr to the F⁻, with the lacZ gene entering before the lacI gene. Streptomycin was added to prevent gene expression in the donor strain; the recipient strain is streptomycin resistant. (b) Entry of the lacZ gene from the donor strain results in immediate production of β‐galactosidase in the recipient strain. However, subsequent entry of the lacI gene represses further β‐galactosidase production unless inducer is added also. b: Data from Pardee AB et al. () The genetic control and cytoplasmic expression of “inducibility” in the synthesis of β‐galactosidase in Escherichia coli. Journal of Molecular Biology 1: 165–178.

Article Title:	Escherichia Coli and the Development of Bacterial Genetics
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