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“PhD, eh? Cool! What do you work on?"

“Escherichia coli" rings a vague bell in many heads. And if I say the famous abbreviation “E. coli" to break the silence that follows, many of these vague faces light up in recognition. A brave soul then utters, what most are wondering, “Isn’t it like a virus or something which causes a fever of some sort?" My insides squirm with a strong desire to blurt “No, it ain’t no virus and it does a lot more than causing a fever". Earth is home to billions of different kinds of bacteria and yet only a few, one of them being E. coli, have attained some fame. Why? What makes E. coli special? Not so fast! Let us begin from the beginning.


German-Austrian paediatrician, Theodor Escherich, is credited with the discovery of this bacterium. He isolated a new bacterium from the faeces of his patients suffering from diarrhoea, then studied it carefully under the microscope, noted its properties and selflessly named it Bacterium coli commune. Escherich noticed one very important property: its immense speed to grow in laboratory conditions. By the time of Escherich’s death in 1911, his discovery was already being used in several laboratories and in 1919 the bacterium was re-christened as Escherichia coli in his honour.

Size and shape

We, microbiologists, are not unlike fashion pundits when it comes to our subject of interest. Size, shape and colour are the first things we like to notice. The protagonist of our story, the bacterium Escherichia coli, can be usually found shaped like a rod approximately 1-3 µm long and 0.5 µm wide; a single grain of salt can comfortably harbour hundreds of E. coli bacteria. The E. coli, however, can also change its shape. When the environmental conditions are unfavourable —less food, high temperatures, etc.—the bacterium can increase its length many times or form long chains or even adopt an attractive L shape too. This skill—of changing its shape and size—is not a rare talent in the bacterial world, and is known as “pleomorphism". The colour identification in case of microbes is much simpler though, unlike the daunting array of begonia, beige and bisque that a fashion critic usually frets over. The very small size of bacteria demands staining procedures before they are put under the microscope.

Danish bacteriologist Hans Christian Gram developed a simple technique to stain bacteria, which at the same time classifies them into two kinds: Gram positive (violet in colour) and Gram negative (pink in colour).

E. coli appears vivid pink after Gram staining and hence it is a Gram negative rod.


If you want to hunt for an E. coli, you need to look within. This is factual advice and not a philosophical one, as E. coli resides in the gut of all mammals, including humans. You might assure me that you haven’t got any diarrhoea currently. But E. coli not only lives in the gut of a healthy human being, but also contributes to keeping its host healthy.

Within a year after birth, it occupies part of the mucous-rich lining (yum! if you were a bacterium) of the large intestine, along with other neighbours. If not for these early settlers, our intestine would be a holiday resort for disease-causing bacteria. But this cooperative relation between host and bacteria can be formed with only some kind of E. coli and not with others.

What we know as “E. coli" actually contains a diverse group which shares the properties of being “E. coli"—for instance, the shape of a rod or a certain gene. On the other hand, these kinds differ in other aspects which will decide whether a particular E. coli can cause an infection or not, and more importantly where will that infection occur. Urinary tract? Digestive tract? Skin? Gastrointestinal infections are the most common kind, and food or water sources contaminated with animal waste matter are the likely culprits. In fact, absence of disease-causing E. coli is used as one of the indicators of clean water. In spite of its versatile infective abilities, most groups of E. coli are rarely life-threatening and the associated mortalities remain low.

Why so famous?

But then, if it isn’t a deadly pathogen, why so famous? The part of the answer lies in the question. The fact that most varieties of E. coli are benign make the bacterium an excellent lab pet. You can perform experiments without a hell lot of sophistication.

Additionally, it grows very fast. A single bacterium placed on a nutritious jelly can form more than millions of its own kind within 24 hours. This makes it further amenable for experimentation.

Unravelling the questions of biology demands that you look inside the living being. And when it comes to the building blocks of life, humans, mice, plants, E. coli, and pretty much everything that lives, have vast similarities. Now, imagine a scientist in the beginning of 20th century. He wants to study why kids are similar to parents. But it is impossible to study it in humans or cows or even mice for that matter.

Then he sees the same is true for E. coli. The daughter cell has properties similar to the mother cell. It is easier to look at them under the microscope, follow multiple generations, even break the envelope of the cells and take out what is inside for further investigation. And while he works with one of the million things inside that envelope, someone else can investigate some other thing inside the E. coli, using the same lab protocols. Putting their findings together will be much less hassle due to the use of same kind of bacteria. In fact, this is exactly what happened. As more and more people started using the same “model system", more and more was known about it and more people wanted to use it further.

The number of biological insights assisted by use of E. coli is thus humungous—a complex, long chain-like molecule called DNA is the basis of heredity which can harbour errors; the position of the errors is necessarily random; enzymes are machines that help draw energy from food; viruses can infect bacteria (yes, there exist such a thing); bacteria have sex (again, yes)—the list is never-ending. These insights have further allowed us to utilize bacteria for our benefit. We effectively control the cellular “factories" of bacteria that can ferment sugars to form yogurt or alcohol or idli. We can utilize the same machinery to make antibiotics and insulin. E. coli is undoubtedly the most studied bacterium on our planet today and our planet is a better place for humans, as a result!

“Anything found to be true of E. coli must also be true of elephants," Jacques Monad, a French biochemist and a Nobel prize winner, had said. Nineteenth century research primarily exploited these similarities to understand how life works and how we can mould these systems for our own use.

Recently, which means 20-odd years in the slow world of science, we have also started asking “what if" questions. Answers to these questions can allow us to predict the possible outcomes of certain conditions. For instance, large changes in climatic conditions are now evident, over short as well as long time periods. Such fluctuations can affect all living beings, albeit to different extents.

What if the environment changes every day? What are the ways in which living beings can respond to such changes? How are these responses relevant to us human beings? These and many such questions can be answered by conducting a ‘laboratory evolution experiment’. Imagine Age of Empires with real populations of E. coli and real environments created by the experimenter (sounds like fun, doesn’t it). These evolution experiments, when repeated enough number of times, can provide very important insights. For example, if E. coli is exposed to ever-changing doses of salts and acids, as it might in sewage water, it can evolve to make cellular machines which will protect the cell from such insults. These evolved machines are nothing but pumps which can remove the unwanted. But then, the same machines can also protect the E. coli against the antibiotics that we use for treating the infections. The changes in the environment can surprisingly result in serious ramifications for us.

What next?

Of course many properties of life cannot be studied using E. coli. How does an entire animal or plant develop from an embryo? Why do there exist only two sexes in sexually reproducing animals? How did photosynthesis evolve? And so many more. In fact, with golden age of biology in effect, scientists want to know more about the “non E. coli" species and use them for research or efficient production of fermented foods. Some might argue that E. coli is at the end of its days of glory. But many grad students are still found gawking at the E.coliwiki for hours and coli-poems are still being written with deep passion. Younger model systems (with better growth rates) still have a long way to go before they replace E. coli. And whatever the future may be, E. coli has made a permanent mark on 20th century biology.

This is the first in series of stories by scientists on their field of work.

Shraddha Karve is postdoctoral scientist at University of Zurich.

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