Escherichia or E. coli: Intestinal Flora and Bacterial Infection
A Surprising Bacterium
While you were in school you may have learned that Escherichia coli living in our large intestine is a helpful bacterium with many benefits. On the other hand, you've probably also heard reports describing dangerous cases of foodborne illness or food poisoning caused by the microbe, which may sometimes be fatal. How can the bacterium be both beneficial and harmful?
The reason for the conflicting reports is that there are many different types of E. coli, which are known as strains. While the strains are similar enough to be given the same species name, there are important differences between them. These differences enable some strains to co-exist with us inside our intestine and others to be dangerous if they enter the body.
Escherichia coli Cells
Esherichia coli was officially discovered in 1885 by Theodor Escherich, a German pediatrician. He discovered the microbe in the feces of healthy babies. He called the new organism Bacterium coli because it was a bacterium that lived in the colon (the longest part of the large intestine).
E. coli exists as a microscopic, rod-shaped cell. The cells of most strains of the bacterium are motile and have thin, whip-like extensions on their surface called flagella. The extensions create a beating motion that moves the microbes through their liquid environment. Flagella are most obvious under certain viewing conditions. Since they are attached at several places on the surface of a cell, the bacteria sometimes tumble as they swim.
The bacteria stop swimming when necessary and develop numerous fimbriae. Like flagella, fimbriae extend from the cell surface. They are shorter and thinner than flagella, however. Bacteria use their fimbriae to attach to other bacteria or to the surfaces of objects.
One site of E. coli attachment is the mucus lining the interior of our large intestine. Here the bacterium absorbs nutrients from its surroundings in order to survive. In laboratories it does well using glucose as a food source. The microbe is a facultative anaerobe—that is, it can survive with or without oxygen.
E. coli can sense many aspects of its environment. It can detect certain chemicals and move towards or away from them. It can also detect changing temperature and pH and respond to these stimuli.
Swimming and Tumbling
Researchers are discovering that despite their tiny size bacteria are more complex than was previously realized. For example, when bacteria are in a dense group a phenomenon called quorum sensing develops. During this process, the bacteria coordinate their behavior and respond in the same way and at the same time to a specific stimulus.
In the video below, a gene that turns a fluorescent protein on and off has been inserted into the E.coli cells. When only a few cells are present, the genes activate and deactivate the fluorescent protein at different times in each cell. When the bacteria are in a large enough group, the genes work in unison, producing the appearance of "blinking" bacteria.
Scientists find this an interesting behavior in its own right, but it may also be useful to humans. Bacteria might be able to function as environmental sensors, for example, flashing light at the same time to indicate a specific change or condition in the environment.
Quorum Sensing in a Group of Bacteria
How Do Bacteria Communicate During Quorum Sensing?
Quorum sensing can take place because bacteria communicate with each other via chemicals. Scientists have found that when the chemicals released during a behavior reach a specific concentration—which happens when the population is sufficiently dense—all the bacteria in the area perform the activity at the same time.
Quorum sensing may help bacteria to infect their host more efficiently. If researchers can learn more about the process, they may be able to interfere with it and reduce the severity of bacterial infections.
E.coli Swarming in a Group (One Cell Colored Red for Visibility)
Escherichia coli is a normal component of the intestinal flora, also known as the gut flora or the intestinal microbiome. The flora consists of the bacteria, yeasts, and protozoa that live in our large intestine. The intestine is thought to contain about ninety trillion individual microbe cells, most of them bacteria. We actually contain more bacterial cells than human cells. This is possible because bacterial cells are generally smaller than human ones and because the bacteria live in body cavities, such as the intestine.
E.coli isn't the most numerous bacterium in our large intestine, but it does seem to be consistently present. In fact, its presence in the environment is used as an indicator of fecal pollution. One specific benefit of the bacterium is that it makes vitamin K, which we absorb. We need this vitamin in order for our blood to clot when we're wounded. The organism also makes some B-complex vitamins.
Benefits of the intestinal Microbiome
Researchers are finding that many members of our intestinal flora are beneficial for our health. They make substances that our bodies absorb and use, help to digest food, and interfere with the growth of harmful bacteria. Some have additional benefits, such as helping our immune systems to work better. Investigating the ways in which gut bacteria affect the body is a very active area of scientific research.
Adding useful bacteria to the large intestine may be beneficial. This is the idea behind the use of probiotic supplements and the addition of probiotics to foods and drinks. "Probiotics" are microbes that are thought to support our lives.
Some exciting evidence shows the importance of a healthy microbiome in the large intestine. In some serious and debilitating cases of a Clostridium difficile gut infection, the medical administration of a fecal transplant has been very helpful. The feces is obtained from someone who is healthy and has a normal collection of intestinal flora. The normal bacteria multiply in the recipient's intestine and overcome the effects of the harmful one.
All fecal transplants must be performed by a doctor. It's essential that the donor's feces is checked for the presence of dangerous bacteria.
Genetic Engineering in E. coli
E. coli in Biotechnology
E. coli affects our lives in another way. It's a popular organism in biotechnology labs, where scientists use the bacterium to study the process of combining genes from different cells and organisms. This process is often known as genetic engineering. The goal of the scientists is to better understand how genes work and to develop techniques that will benefit humans in some way.
One example of genetic engineering involving E. coli cells is their use in insulin production. Scientists have incorporated the human gene that directs the manufacture of insulin into the DNA of E. coli. Insulin is an essential hormone in our bodies. The genetically altered bacteria make insulin, which is given to people with Type 1 diabetes. People with this condition are unable to make their own hormone.
Food Poisoning and Infection by E. coli
E. coli lives in the intestines of both humans and animals. A person can become infected by pathogenic strains of the bacterium by eating contaminated food or drinks or after touching feces— even a very small amount—that has come from an infected person or animal. Pathogenic bacteria can cause disease.
One type of food that may contain dangerous strains of the microbe is undercooked beef. Bacteria in the cow's intestine can be transferred to the meat during the butchering process. Raw vegetables, sprouts, and fruits can be contaminated if they come into contact with animal manure or contaminated water. Raw milk products, raw nuts, and unpasteurized fruit juices may also cause E. coli food poisoning.
E.coli may be transferred to people when they touch an object contaminated by human or animal feces and then touch their mouth or nose. Fecal contamination of items in our environment is more common than many people realize. Even a tiny amount of feces can contain many bacteria.
Possible Symptoms of an Infection
Possible symptoms of an E. coli infection include stomach cramps, nausea, vomiting, and diarrhea, which may be bloody. Some people get no symptoms from the infection. In other people the infection may give a few days of discomfort and then disappear. It may sometimes be serious, however, and even deadly. Symptoms generally appear after an incubation period of several days.
The people who are most likely to suffer severe effects from the bacterial infection are those that are very old or very young or people with weakened immune systems. Anyone with diarrhea that doesn't quickly disappear or is bloody should visit a doctor. This advice is also important for someone who has severe vomiting or a high fever.
E.coli and Disease
Pathogenic E. coli strains affect the intestine, the urinary tract, the respiratory tract or, in the case of newborn babies, the meninges (the three membranes that surround the brain). The most common site of infection is the large intestine, however.
The bacteria have features that help them to infect an area and stay there so that they can't be removed. For example, some strains make adhesins, structures on the cell membrane that help them stick to surfaces. Some produce invasins, proteins which enable the bacteria to enter human cells. Others produce toxins, chemicals that harm the host in some way.
Scientists are finding that although all E. coli are classified in the same species, there is actually a considerable amount of variability in their genes and in the molecules on their surfaces. A number of labeling systems have been developed to identify the different strains. For example, E. coli 0157:H7 is a strain that causes disease, E. coli K-12 is a strain that's used in laboratory experiments, and UPEC E. coli (Uropathogenc Escherichia coli) causes urinary tract infections. A special classification system is sometimes used for strains of E. coli that produce diarrhea, as shown in the table below.
Strains of Escherichia coli That Cause Diarrhea
Action in the Intestine
Enterotoxigenic E. coli
Attaches to the intestine by fimbriae and produces a toxin
Traveler's diarrhea (or Montezuma's revenge), with no fever
Enteroinvasive E. coli
Enters and destroys cells lining the colon
Watery diarrhea with fever
Enteropathogenic E. coli
Attaches to the intestinal lining via a protein called intimin
Watery diarrhea, which may be bloody
Enteroaggregative E. coli
Forms clumps on the lining of the intestine and produces a toxin
Diarrhea, which may be prolonged
Enterohemorrhagic E. coli (or Shiga Toxin-Producing E. coli)
EHEC (or STEC)
Attaches via intimin and produces Shiga toxin
Bloody diarrhea, possible kidney damage
Anyone who is experiencing prolonged or bloody diarrhea or diarrhea accompanied by other symptoms should visit a doctor for a diagnosis and treatment.
Preventing Bacterial Infections
Some simple safety procedures will greatly reduce the chance of an E. coli infection.
- Cook meat to a safe temperature, using a digital thermometer to monitor the temperature.
- Keep raw meat separate from other foods.
- Wash your hands before and after preparing food.
- Wash countertops and utensils thoroughly after food preparation.
- Wash and scrub raw vegetables and fruits before eating, paying particular attention to peels with crevices, such as the rind of cantaloupes. Bacteria can pass into the flesh of a cantaloupe as it's cut.
- Drink and eat only pasteurized fruit juices and dairy products
- Drink from a safe water supply
- Wash your hands after contact with animals.
- Follow good hygiene techniques in high-risk areas, such as public restrooms and bathrooms.
Although it's important to protect ourselves from pathogenic E. coli, it's good to know that some strains of the bacterium are beneficial. Bacteria can be helpful as well as harmful.
Rutherford, S. T., & Bassler, B. L. (2012). Bacterial Quorum Sensing: Its Role in Virulence and Possibilities for Its Control. Cold Spring Harbor Perspectives in Medicine, 2(11), a012427. http://doi.org/10.1101/cshperspect.a012427
E. coli facts and news from the CDC (Centers for Disease Control and Prevention)
E. coli infection information from the NIH (National Institutes of Health)
Questions & Answers
© 2012 Linda Crampton