Escherichia coli and genetics

ALSO KNOWN AS: E. coli

SIGNIFICANCE: In the late nineteenth century, so many babies were dying of diarrhea that German pediatrician Theodor Escherich suspected microorganisms to be the cause. Struggling to understand how the stools of healthy infants differed from those of sick babies with watery and bloody excretions, he cultured microorganisms from the diapers of each. In 1885, he cultured a rod-shaped, gram-negative facultative aerobic bacteria from a baby showing no signs of illness. The organism was named Escherichia coli in 1919, after Escherich's death. Today, more is documented and understood about E. coli than any other life form on Earth. A single E. coli and its progeny, isolated from a diphtheria patient in 1922, was introduced into research laboratories the world over. Scientists working with E. coli K-12 have demonstrated how genes work to direct the physiology and biochemistry of life, to evolve life through natural selection, and to engineer genetic modifications that impact the quality and the context of people’s daily lives.

Model Life-Form in Nature and the Research Laboratory

Newborns of all endotherms (warm-blooded animals including pigs, cows, chickens, elephants, and humans) enter the world nearly sterile. During their trip through the birth canal and within the first forty hours, the infant is seeded with E. coli and other beneficial bacteria required for establishing protective normal flora. These organisms occupy space and prevent pathogens from gaining a foothold. E. coli and other intestinal normal flora aid in digestion and produce nutrients and vitamins such as B12 and K essential for coagulation.

While most E. coli strains play an essential and helpful role in a healthy intestine, some of the more than three thousand strains of enteroinvasive E. coli bore into the intestinal wall, causing diarrhea. Enterotoxigenic E. coli produce toxins causing travelers’ diarrhea. Others mutate, like the enterohemorrhagic E. coli O157:H7, first described as the cause of a food contamination outbreak in 1982. It results in hemorrhagic colitis and bloody stools, sometimes leading to kidney failure and death.

If the organism escapes the digestive tract, where it shares a peaceful and productive coexistence with a human, it can alter and cause meningitis or endocarditis. E. coli causes nearly 90 percent of all urinary tract infections.

E. coli shares genes with all life forms. To understand the genetics and molecular biology of E. coli is to begin to understand all life. Before E. coli K-12’s genome was published in 1997 and its sequence of 4,377 genes and 4,639,221 base pairs were known, it emerged as the preferred model in biochemical genetics, molecular biology, and biotechnology research the world over. Its twenty-minute generation time, minimal food requirements, genetic variation, and expression of classic metabolic pathways with a single chromosome catapulted E. coli to the top of the experimental chart.

Genetics and Molecular Biology

Francis Crick, James D. Watson, and Maurice Wilkins, based in part on the research of Rosalind Franklin, proposed the groundbreaking three-dimensional molecular double helix structure of deoxyribonucleic acid (DNA), a form of complementary base sequences that suggested genetic and molecular function. DNA structure alone fails to explain how DNA works, however. E. coli has proven essential to unraveling many of these secrets, including replication and function.

Feeding E. coli a varied diet of different forms of nitrogen, Matthew Meselson and Franklin Stahl demonstrated that DNA replicates when the double helix is pulled apart. Each old strand of the base pairs—cytosine with guanine and thymine with adenine—serves as a for a new complementary strand.

François Jacob and Jacques Monod used E. coli to demonstrate gene expression, the mechanism by which genes are switched on and off through operons. E. coli prefers to metabolize glucose as its energy source. When the glucose supply is exhausted, protein production to break down glucose is switched off. If lactose is available, then the lac operon will direct specific genes to produce proteins required to metabolize the lactose.

Evolutionary Evidence for Natural Selection

Max Delbrück and Salvador Luria observed that some E. coli survived attacks from phages. Luria devised studies that supported the notion that some organisms randomly mutate to resist the phage attack through natural selection.

George Beedle and Edward Tatum demonstrated that genes control the synthesis in cells through chains of chemical reactions. Joshua Lederberg worked with Beedle and Tatum to show that bacteria can sexually exchange genetic material, resulting in genetic recombination. Lederberg further demonstrated that genetic material can be introduced into and change the bacteria, resulting in genetic transduction. The newly mutated and reengineered E. coli passed this genetic information on to their offspring.

Beginning with a single E. coli in 1988, Richard Linski’s laboratory has maintained continuous cultures for more than forty thousand generations. Species have evolved with regard to energy requirements, size, and rates of mutation.

Growing resistance to antibiotics by E. coli and other microorganisms, as well as to some therapeutic treatments for cancer, suggests that cells mutate, and those that resist annihilation reproduce and resurge in their competition for survival.

Genetic Engineering and Industrial Biotechnology

E. coli’s influence in modern life continues to expand. Examples range from use in monitoring contamination in water and food supplies to production of pharmaceutical products and research tools.

The insulin gene is recombined with a plasmid and introduced into E. coli to trick the organism into producing therapeutic insulin. The technique, recombinant DNA, is responsible for the production of growth hormone, somatostatin, and antibiotics such as erythromycin and vancomycin.

E. coli strains are specific to the animal the organism inhabits, a characteristic useful as a principal indicator of fecal pollution and sources of intestinal infections.

E. coli played a role in the development of green fluorescent protein (GFP) as a protein marker. In 1962, Osamu Shimomura isolated GFP from the jellyfish Acquorea victoria.Martin Chalfie used a GFP clone to demonstrate the expression of green fluorescence in E. coli and Caenorhabditis elegans. Roger Tsien coaxed the GFP into expressing a spectrum of fluorescence that is used to tag specific proteins. With this tool, researchers create specific genetic tags to identify protein location, movement, and interactions.

In a key development that provided further insight into the potential role of E. coli in genetic engineering and industrial biotechnology, researchers from the University of California (UC) Davis School of Veterinary in 2024 published the results of a study during which they examined the whole genome sequence of multidrug-resistant E. coli in pre-weaned dairy calves. The study revealed that the E. coli samples in question had a high level of genomic diversity and potential host-driven adaptations. This discovery set the stage for future work examining the relationship between genomic diversity and other factors, including disease and antimicrobial exposure

Impact

All life forms share basic genetic codes, so unraveling the mechanisms of E. coli applies often to the human genome as well. Researchers are beginning to appreciate that organisms such as E. coli work together within their own colonies and in competition with others. Colonies of organisms form biofilms that create a competitive advantage and protect them. These complex relationships are just beginning to emerge.

E. coli’s role in biotechnology continues to emerge and promises to reverse environmental pollution, degrade cellulose, provide food and energy sources, produce antibiotics and vaccines, and detect and treat cancer.

Key Terms

• gene cloningisolation and replication of individual DNA fragments
• model organisma life-form selected as a focus of study, results from which are applied to other processes; selected for its short generation time, relative structural simplicity, rich history, ease of manipulation, basic growth requirements, and small size
• normal florabacteria that colonize the body surfaces (skin, conjunctiva, nose, pharynx, mouth, intestines, anterior urethra, and vagina)

Bibliography

Blattner, F. R., et al. “The Complete Genome Sequence of Escherichia coli K-12.” Science 5 Sept. 1997: 1453–62. Print.

Donnenberg, Michael. Escherichia coli: Pathotypes and Principles of Pathogenesis. 2d ed. Amsterdam: Academic, 2013. Print.

“E. coli (Escherichia coli).” Centers for Disease Control and Prevention. CDC, 21 Dec. 2015. Web. 22 Jan. 2016.

“E. coli Infections.” MedlinePlus. Natl. Lib. of Medicine, 15 Jan. 2016. Web. 22 Jan. 2016.

Evans, Thomas C., and Ming-Qun Xu. Heterologous Gene Expression in E. coli: Methods and Protocols. New York: Humana, 2011. Print.

Lee, Katie Y., et al. "Whole Genome Sequence Analysis Reveals High Genomic Diversity and Potential Host-Driven Adaptations Among Multidrug-Resistant Escherichia coli from Pre-Weaned Dairy Calves." Frontiers Microbiology, vol. 15, 2 Sept. 2024, doi.org/10.3389/fmicb.2024.1420300. Accessed 5 Sept. 2024.

Perna, N. T., et al. “Genome Sequence of Enterohaemorrhagic Escherichia coli O157:H7.” Nature 409.6819 (2001): 529–33. Print.

Zimmer, Carl. Microcosm: E. coli and the New Science of Life. London: Heinemann, 2008. Print.