For each bacterial species, like E. coli, there can be many different strains. In this lesson we will discuss the common pathogenic strains of E. coli.
Bacteria: Friend or Foe?
I think we can all agree that each person is entitled to the right of being innocent until proven guilty, but what about bacteria? Should these microscopic creatures enjoy the same right? So many people, when they hear the word bacteria, whip out their favorite antibacterial disinfectant and embark on an eradication mission.
In this lesson, I am going to argue that yes, in fact, we should allow bacteria to be innocent until proven guilty. In the microbial world, there is no clear-cut boundary between innocent, friendly microbes and guilty, disease-causing microbes. We will discuss everyone's favorite microbe, E. coli, and how it can be both a friend and a foe.
The first thing we have to understand is what is meant by a bacterial strain. Let's start by defining, or at least attempting to define, a bacterial species. There is some controversy, but basically, two bacterial cells are considered a member of the same species if their genomes are highly similar - so something like 95% identical. But this leaves 5% of the genome that can be different. This might not seem like a lot, but nature can accomplish a lot with that 5%.
The variability in 5% of the genome within a single species can give rise to many different strains of that species. Different strains of the same species can have noticeably different attributes - for example, a difference in metabolic ability, like the ability to break down a specific sugar. These differences between bacterial strains are due to each strain having different genes that code for different attributes.
Let's apply what we have learned about bacterial species and strains to better understand the bacterial species Escherichia coli. E. coli is a single species, but within the species, there are greater than 20 recognized strains.
Some of these strains are definitely friends of humans. In fact, within hours of a human birth, E. coli will colonize the gastrointestinal tract. It is highly successful in this environment, cozying up in the mucus that lines the intestinal walls. Here, it can live for decades with mutual benefit, a symbiosis between itself and the human host. The only time these friendly E. coli cause a problem is when the host becomes immunocompromised or the intestines are damaged in some way.
Now, on the other hand, there are also strains of E. coli that are definitely not friends of humans. These strains carry genes that give them the ability to not only colonize the human intestines like the friendlier strains but also to cause disease in the host. These strains are defined by different virulence genes they carry and are referred to as pathotypes. A pathotype is a group of strains that cause the same disease because they share a set of virulence genes. Let's discuss the most common E. coli pathotypes.
E. coli Pathotypes
EPEC, or Enteropathogenic E. coli, was the first pathotype described. It is primarily associated with causing diarrhea in children in developing countries. The EPEC pathotype is able to bind tightly to the intestinal cell wall and causes extensive damage to the microvilli, sloughing them off and exposing the intestinal cells beneath. EPEC pathotypes release an enterotoxin, a toxin that is active in the gastrointestinal tract of the host. Diarrhea is caused by a combination of enterotoxin release, intestinal cell damage and inflammation and loss of microvilli.
ETEC, or Enterotoxigenic E. coli, is the disease commonly referred to as traveler's diarrhea. ETEC causes diarrhea in children in developing countries and is the main cause of diarrhea in travelers to developing countries. Infection results in watery diarrhea that can vary in severity. ETEC cells are able to bind to the intestinal walls, where they release toxins that cause a massive influx of water into the intestine, resulting in diarrhea.
EHEC, or Enterohemorraghic E. coli, was first recognized in 1982. It is able to cause bloody diarrhea. The primary reservoir for EHEC is the cow intestine, giving rise to fears of eating undercooked hamburgers that could potentially be contaminated with EHEC. These are the E. coli outbreaks you hear about most often in the news and are a subject of their own lesson.
There are a few additional pathotypes, nicknamed enteroaggregative, enteroinvasive and diffusely adherent, that affect the intestines, as well as uropathogenic and meningitis-associated E. coli that affect the urinary tract and the meninges.
Diagnosis, Treatment and Prevention
You can imagine that since there are so many pathotypes, it is a multi-step process to diagnose a specific pathotype. In general, when diagnosis is required, E. coli are cultured from the feces and then analyzed using serological and molecular methods to determine the specific pathotype present. Once the pathotype is determined, then doctors have a better sense of the potential severity of the illness.
Treatment of E. coli infections typically involves supportive therapy - so, for example, replacing fluids lost due to diarrhea and dehydration and resting to allow recovery. In severe cases, antibiotics can be used to shorten the severity and duration of infection.
Since all E. coli pathotypes use the intestine as a route of entry into the host, anything that is consumed can potentially be contaminated. To prevent infection, food should be cooked thoroughly, raw food should be washed with clean water, and basic hygiene measures should be taken to break the fecal-oral cycle of infection. For travelers to areas where water sanitation standards are non-existent or unknown, it is best to avoid local water sources and only drink bottled beverages.
E. coli is a single species, but within the species, there are greater than 20 recognized strains. Some of these strains are perfectly friendly and are important symbionts in the human gut. Other strains carry weapons in the form of genes that cause disease.
Disease-causing strains are defined by the difference in virulence genes they carry and are referred to as pathotypes. A pathotype is a group of strains that cause the same disease because they share a set of virulence genes. Three of the major pathotypes include EPEC (Enteropathogenic E. coli), ETEC (Enterotoxigenic E. coli) and EHEC (Enterohemorraghic E. coli). These pathotypes have different genes that result in different forms of host invasion and different disease symptoms and severity.
E. coli pathotypes invade through the intestine, so exposure is primarily through contaminated food and water sources. Basic sanitation and hygiene measures and thoroughly washing and cooking food is the best way to prevent exposure. For the unlucky people that ingest one of the E. coli pathotypes, supportive therapy is usually sufficient to allow recovery. In more severe cases, antibiotics can be used to limit the duration and severity of the disease.
After seeing this video, students should be able to identify which E. coli is good and which is problematic. They should also understand how E. coli gets inside us and does its damage.