Escherichia coli (or E coli for short) is no stranger to people these days. Every so often, E coli crops up on the news and more often than not the reports sombrely tell of the latest death toll attributed to its presence in poorly prepared food or as the lead player in the latest meningitis scare. Most people have first-hand experience of the role E coli plays in diarrhoea1.
E coli was first discovered in the human colon by a German scientist, named Theodor Escherich, in 1885. Escherich also showed that some strains of E coli are responsible for diarrhoea and gastroenteritis2. Although the bacterium was initially called Bacterium coli, the name was later changed to Escherichia coli, in honour of its discoverer. Technically speaking, E coli is a rod-shaped, gram-negative, facultatively anaerobic bacterium in the family Enterobacteriaceae.
As microbiology expanded as a scientific field, people learned of the roles that micro-organisms played in life - more often than not, as the cause of disease. However, E coli isn't all monster. What most people don't know is that E coli is not only a pathogen; it is an indispensable part of the global ecosystem, vital to the maintenance of human health and a crucial tool in the advancement of science and understanding.
E coli - Foe
Even the most innocuous micro-organism around is capable of causing different types of disease when circumstances permit. Staphylococcus aureus, a normal resident of the skin, can cause toxic shock syndrome and skin infections, among other things. Streptococcus pyogenes, which you will find in anybody's throat, is the cause of sore throats, scarlet fever and, in its most virulent form, the horrifying flesh-eating disease known as necrotizing fasciitis. Disturbingly, these bacteria play second fiddle to E coli as far as infections are concerned.
One would be amazed, or even horrified, to learn of the multitude of diseases that E coli is capable of causing. There is probably no part of the human body that it is not capable of infecting; it causes many illnesses, ranging from gastrointestinal tract-related complications such as diarrhoea, dysentery and Haemolytic Uremic Syndrome (HUS) to urinary tract infections, pneumonia and even meningitis.
Haemolytic Uremic Syndrome
Haemolytic Uremic Syndrome (HUS) is E coli's worst possible manifestation. Caused by the infamous strain E coli O157:H7, the bacterium is transmitted to humans by way of undercooked beef, raw milk and apple cider, among other things. Once inside the human body, it colonises and multiplies inside the gastrointestinal tract, producing a deadly toxin that kills the cells lining the tract. If the bacterium is not stopped, it continues to spread and wreaks havoc, perforating the colon and causing it to haemorrhage, spreading infection throughout the abdominal cavity. Eventually, blood transfusion and haemodialysis are required to save the person's life.
Also, this bacterium readily infects the intestinal tracts of farm animals, causing their intestine linings to inflame. When farmers started giving their livestock antibiotics to ward off infection, this unknowingly favoured a group of E coli that had acquired a drug-resistance gene from the bacterium that causes dysentery, Shigella dysenteriae. This drug-resistance caused antibiotic therapy to fail. Even worse, the bacterium harbours a bacteria-infecting virus (known as 'bacteriophage') that carries the gene for a toxin also found in Shigella. Thus, when antibiotics therapy is carried out, the phage is induced to go into the cell-breaking (lytic) cycle, which causes multiple copies of this toxin gene to be produced, and subsequently the amount of toxin produced escalates. This, if you have been reading carefully, is exactly the opposite of what drug therapy is supposed to do.
Nosocomial infections are infections acquired in the hospital. The hospital is meant to be a place of rest for a patient, a sanctuary where they can seek comfort and healing from those who know best what to do with their conditions. Unfortunately, because the hospital is also a place where sick people with infectious diseases go to, it is inevitably a reservoir for some of the nastiest bugs around.
According to hospital statistics, roughly five per cent of hospitalised patients contract infections during their stay; 10% of this occurs in the intensive care unit. Approximately 50% of all patients who require catheterisation for more than five days develop bladder infections. Abnormalities or obstruction of the urinary tract or faecal incontinence are also optimal conditions for infection. More often than not, E coli is to blame.
Urinary tract infections are very, very painful things. The number one symptom of a urinary tract infection is an abominably intense burning sensation while urinating. Other symptoms include the need to urinate frequently and a feeling of fullness over the bladder, no matter how many times one has visited the lavatory. Happily, in many early cases of infection, removal of the catheter solves the problem as the bacteria is then forced to multiply in areas where stagnant urine collects, and is subsequently flushed out by repeated calls of nature. However, if they are not removed they may cause a lot of trouble as many hospital-acquired strains of E coli are drug-resistant.
Besides urinary tract infections, E coli is also reportedly responsible for 10% of surgical wound infections and 6 per cent each of respiratory tract infections and septicaemia3 in the hospital environment.
There is a reason why doctors warn you not to drink unboiled water in developing tropical countries - travellers' diarrhoea can be a nasty nuisance if you're going to be tromping around half the world.
Travellers' diarrhoea is caused by enterotoxigenic E coli, which is so named because it produces toxins once it colonises the epithelial cells of the small intestine. These toxins are probably 'borrowed' from another nasty diarrhoea-causing bacterium, Vibrio cholerae (which causes cholera and can be found in clams living in contaminated waters). These toxins cause massive quantities of water to be lost from the body through the anus, and in severe cases can cause dehydration and electrolyte imbalance, both of which can lead to death if the disease is not stopped with antibiotics in time.
Interestingly, during the 1991 Gulf War, E coli was the leading cause of diarrhoea among US troops, more than half of whom suffered two or more episodes of the disease.
One of the diseases that rogue strains of E coli are capable of causing is meningitis.
Meningitis a very serious matter, especially if it strikes the very young. The mortality rate for E coli meningitis in infants is pretty dire: 40 - 80%. Many survivors are left with neurological or developmental abnormalities. Studies have indicated that pregnant mothers are more susceptible to colonisation by a strain of E coli with a particular antigen4 known as K1; the infection is subsequently spread from mother to child. It is still unknown how K1 predisposes infants to developing meningitis. Interestingly, a large portion of this antigen seems to be very closely related to that of another meningitis-causing bacterium, Neisseria meningitides.
It can only be called fortunate that the pregnant mothers themselves are not predisposed to catching meningitis from E coli; this bacterium has rarely been identified as the cause for adult meningitis.
E coli - Friend
The humble E coli has always been the microbe of choice in research. This goes back to the 1950s, when a group of scientists who were interested in studying basic biological processes decided that the best way to go about their research was to use a common, simple, free-living micro-organism, and base all their studies on that. Needless to say, they chose E coli. From that day on, this rapidly-reproducing bacterium has been the model organism of studies on everything from metabolic pathways to genetic regulation and DNA replication.
E coli Are a Part of the Natural Gut Flora
There are far more bacterial cells inside a human than human cells5. A small number of these bactera are friendly E coli and they're there to help. One of the main functions of gut flora is protection. Without the help of E coli our gut would be overrun by harmful bacteria and fungi. The friendly and rapidly-replicating E coli out-compete pathogens; they even inhibit the growth of pathogenic bacteria by secreting substances that are toxic to non-indigenous bacteria. An interesting side-note: E coli is responsible for the delightful aroma of faeces6!
The natural gut flora also secrete vitamins B12 and K as metabolic by-products. These vitamins are essential for our nutrition. Lactose-fermenting bacteria such as E coli produce lactase, the enzyme that breaks down lactose, a sugar found in milk and other dairy products. These microbes help to confer lactose-tolerance7.
E coli Are an Integral Part of the Ecosystem
Nitrogen is the most abundant gas in the atmosphere, constituting about 70% of the air that we breathe. In biological processes, nitrogen is an important element needed in the synthesis of proteins. Unfortunately, as Coleridge once lamented that water was abundant and yet undrinkable8, so this store of nitrogen is inaccessible to most, being in gaseous form. Indeed, the organisms on this Earth would probably get little, if any, access to nitrogen if not for the microbes in the soil that convert nitrogen gas into a form that is readily usable - ammonia.
Just as micro-organisms are needed to produce usable forms of nitrogen, they are also important in returning nitrogen to the environment. E coli is one of the many hard-working microbes that recycle nitrogen and return it to nature. Even though it can only carry out the first step of the process (dissimilative nitrate reduction), it plays an important role in a world where one can afford to waste nothing. You may not think much of this role, but this process takes place largely in waterlogged areas where oxygen supply is minimal - a condition that the oxygen-loving E coli clearly does not favour.
Sulphur is another element important for life and growth. Plants that are deprived of sulphur grow wilted and yellow. Many types of micro-organisms depend on sulphur for growth, deriving energy from chemical reactions. Sulphur is also found in a number of amino acids, which are the building blocks of protein. There is great need for micro-organisms to do the job of recycling sulphur in the environment. E coli is deeply involved in this process. However, the importance of E coli in the environment is not limited to its ability to recycle elements and compounds. Researchers are using isotope studies of various elements, including sulphur, to assess the extent of microbial activity in the environment, monitor important links in the sulphur cycle and track the flow of these elements in the food webs of ecosystems.
Genetic engineering is the most recent tool of the ancient practice of biotechnology. It came about in the early 1970s, due to the simultaneous development of E coli transformation and the discovery of enzymes in E coli that are able to cut and rejoin DNA at will.
When a researcher 'transforms' a bacterium such as E coli, they permanently alter its genetic make-up. In genetic manipulation, transformation refers specifically to the successful insertion of 'foreign' DNA (or more accurately 'exogenous' DNA) into the bacterium. In the early 1970s, E coli was the best characterised bacterium, and the only bacterium known to possess plasmids. Plasmids are little circular rings of DNA, independent of the bacterium's single chromosome. They can even replicate themselves on their own, which, for biologists, is a very useful thing indeed. The plasmid of E coli can be easily purified with a very simple technique.
A class of enzymes called 'restriction endonucleases' was discovered in E coli. Scientists found that these enzymes can cut DNA at specific base-pairs. When a specific restriction enzyme recognises a specific DNA base-pair sequence, it cleaves it in a specific and predictable way. For example; EcoR1 recognises 'GTTAAC' (and its complementary strand) and cuts the DNA apart between the 'G' and the first 'A'. Sma1 recognises 'CCCGGG' and cleaves in between the third 'C' and the first 'G'. There are literally thousands of different enzymes one can choose from, and all cut in their own particular way.
Once the purified plasmid DNA is cut with your endonuclease of choice (physically done by incubating the enzyme and DNA in a buffer at 37°C for a while) it is possible, with the help of another enzyme called 'T4 DNA Ligase' to join (or 'ligate') them back together again. More usefully, foreign DNA can be added to the cut plasmid DNA and DNA ligase to create a chimaeric construct9 that can be used to transform other E coli bacteria in culture. This is the heart of genetic engineering: cutting plasmid DNA, inserting exogenous DNA, ligating, and transforming. From this point, E coli will replicate at an exponential rate, copying, alongside its own DNA, your chimaeric plasmid in bulk. This is appropriately known as 'gene cloning'.
In 1978, a company named Genentech was the first to exploit genetic engineering technology to make human insulin. Before Genentech, medical insulin had to be extracted from pig pancreas, a costly and undesirable procedure. Genentech took the human gene responsible for insulin, inserted it into a plasmid and transformed our friend E coli. The bacteria happily synthesised huge amounts of pure human insulin, making this life-saving compound affordable and accessible. Genentech went on to clone and synthesis the human growth hormone, Activase (a product for dissolving blood clots in heart attack victims), Factor VIII (for haemophilia therapy) and other products, with the same technology.
Today, genetic engineering is routine worldwide, in even the most basic laboratories. It is an extremely powerful tool that has revolutionised the pharmaceutical industry, enabling the development of numerous invaluable drugs.
Further Biological Studies Based Upon E coli
E coli also has a little bit of a celebrity status as a model for researchers. Here are some examples of studies that scientists have used E coli as a model for:
There are four ways by which chemicals can enter a cell. One of them is called group translocation, which is a process in which a given substance is chemically altered in the course of passage across the membrane. E coli was the key to understanding the phosphotransferase system, the best-studied case of group translocation, involving transport of various sugars to which are added phosphate molecules while being moved.
E coli was an important model in the study of electron transport chains, which is a process by which energy is produced in a cell.
All of the glamorous genome sequencing projects such as the Human Genome Project were done with E coli as the primary workhorse. Huge plasmids called bacterial artificial chromosomes (BACs) containing the DNA you want to sequence are made, cloned and purified with the help of E coli. Entire libraries of E coli, which consist of hundreds of thousands of different colonies of E coli clones, are made with enough variation in their BACs to cover the entire genomes of whatever organism you want. The BACs are purified, chopped up and sequenced automatically. The sequences fragments are pasted back together in a computer to assemble the genome.
In pathogenecity studies, much attention has been devoted to a macromolecule on the surface of E coli called the fimbrae, which plays a key role in the attachment of the organism to the host cell. By studying cases of diarrhoea caused by E coli, researchers were able to uncover evidence of the specific interaction between the gut layer and the pathogen.
E coli has also provided researchers with a working model of a long whip-like propeller called 'flagella'. By rotating on its axis, this structure gives a bacterium its motility, and thus its ability to reach different regions of its microenvironment, which is crucial in the microbe's life-or-death struggle for survival.
Escherichia coli is a nasty, brain-infecting, bladder-colonising, intestine-attacking, pneumonia-causing, horrible little bacterial pest. It is the root cause of a lot of human illness and suffering, and because of this has a rotten reputation amongst the general public.
However, E coli isn't a completely malevolent creature. There is a flipside to the death and misery it wreaks. E coli is the best understood bacterium, a scientific model for all life, and as such has had a leading role in vastly increasing our understanding of the living world. E coli is a vital inhabitant of our intestines, both aiding digestion and protecting us from other infections. E coli is a keystone of the world's ecosystems. E coli is a very useful tool for the biological sciences, most notably the remarkable technique of genetic engineering, which ushered in a new dawn in pharmaceutical research and development.
Is the bacterium E coli a friend, or is it a foe? As this entry demonstrates, it's most certainly both.
Water, water everywhere and all the boards did shrink; water, water everywhere, nor any drop to drink.9Named after the chimera; a mythological beast with the head of a lion, the body of a goat, and the tail of a serpent.
- 'Rime of the Ancient Mariner' by Samuel Taylor Coleridge