Hello Everyone! I hope you're all well and ready to explore the intriguing world of foodborne pathogens. Over the course of the next 10 lectures, we’ll be delving into individual, representative foodborne bacteria to understand them better. Kicking off our series, we will begin with the Enterohaemorrhagic E. coli, a type of bacteria that can cause serious illness. We'll be discussing its symptoms, incubation period, causes, and routes of transmission, aiming to present the information in a manner that’s easy to comprehend without the need to memorise every detail individually.
Understanding each bacterium's habitat is essential, and we'll focus on it primarily. By grasping where these bacteria reside, you’ll find that other characteristics will follow like a row of falling dominos, making them easy to understand in a sequential manner.
Now, the domino effect approach we are going to use requires a basic understanding of Gram staining and the properties of microorganisms, which have been explained in the foundational series on our blog. If you read the below article beforehand, the domino effect understanding will become much more straightforward to you.
So, I’d recommend that you first familiarise yourself with the foundational topics before we dive deeper. This will help in achieving a clearer, holistic understanding of the topics that we are going to discuss, in a way that’s both engaging and informative.
Understanding Various Characteristics through a Domino Effect from Habitats
Firstly, let’s talk about Enterohaemorrhagic E. coli. It's a serotype of E. coli. While it is different from regular E. coli in terms of its pathogenicity, its other physiological characteristics are quite similar. So, we will first discuss its characteristics without distinguishing it from other E. coli.
１．Habitat: Enterohaemorrhagic E. coli primarily resides in the intestines of cows, a watery environment. Thus, we can understand that it is a Gram-negative bacterium, suited to such environments.
２．Infection Type: Being Gram-negative means it causes infectious food poisoning. If you want to understand the why behind these two points, I recommend reading the above basic article.
３．Temperature Preference: Since it resides in the warm intestines of cows, it can’t multiply in environments like refrigerators, which are below 8°C.
４．Resistance to Dryness and Heat: Gram-negative bacteria aren’t generally strong against such stresses, and Enterohaemorrhagic E. coli is no exception. If you want to understand the why behind these points, please refer to our foundational article discussing how Gram-positive and Gram-negative bacteria differ in surviving in dry and wet conditions.
５． Oxygen and Growth: Enterohaemorrhagic E. coli lives in cow’s intestines, where oxygen is scarce. Therefore, it’s a facultative anaerobic bacterium that can grow even without oxygen.
６．Acid resistance: Generally, facultative anaerobes are resilient against acids, as they produce a lot of organic acids around them due to fermentation metabolism in anoxic conditions. E. coli, including Enterohaemorrhagic E. coli, are especially acid-resistant. EMB agar is used to detect E. coli, and only the colonies that shine golden are E. coli. This color difference is also related to E. coli being particularly acid-resistant.
We use something called EMB agar plates for this purpose. Now, here’s the cool part! On these EMB agar plates, only the colonies of bacteria that shine in a golden colour are E. coli! If it doesn’t shine golden, then it’s a different type within the E. coli group.
Now, you might be wondering, why this golden shine? Well, this is because E. coli is especially strong against acids. The surrounding pH of E. coli colonies that form on EMB agar plates is lower compared to other colonies from the E. coli group. As a result, the dyes added to the medium, Eosin Y and Methylene Blue, cluster around the E. coli colonies. This makes the E. coli colonies shine golden around the edges.
Remember, these golden shining colonies signify the presence of E. coli due to their unique ability to withstand acidic environments. Isn’t science fascinating?
7．Drug Resistance: As mentioned, Gram-negative bacteria, with their outer membranes, demonstrate strong resistance against antimicrobial agents. Hydrophilic polysaccharide portions of the outer membrane repel antimicrobial agents with hydrophobic functional groups.
So, as we have discussed, Gram-negative bacteria show generally strong resistance to antibacterial agents. It’s because they have this outer layer, and the hydrophilic part of this layer, made of polysaccharides, can repel antibacterial agents that have hydrophobic functional groups. It’s like having a shield that keeps unwanted things away!
8. Selective Growth
Given this, we can add compounds like bile salts and Brilliant Green, which have hydrophobic functional groups, to the selective medium. This way, we can exclude Gram-positive bacteria and selectively allow only Gram-negative bacteria to grow. It’s like creating an environment where only the ones we want can thrive!
I hope you can see how these pieces of information connect like dominoes falling one after the other. Understanding each step can help you see the whole picture clearer and make learning about bacteria a bit more exciting!
What is Serotypes?
Enterohemorrhagic E. coli is one of the serotypes found in E. coli. Among E. coli, only limited serotypes, like O157 and O26, have the ability to cause diseases in humans. Let me briefly explain what serotypes are. Since E. coli is a Gram-negative bacterium, it has an outer membrane on the cell surface, as mentioned before. This outer membrane is basically similar to the cell membrane, having a structure like a lipid bilayer. The difference is that it has chains of polysaccharides on the outside.
The structure of these polysaccharide chains varies, defining the serotype of each E. coli. This is the outer membrane antigen, or O antigen.
Apart from the O antigen, the H antigen is also a factor in determining E. coli serotypes; the H antigen is due to differences in the structure of the flagellar surface layer proteins.
To determine the serotype, we recognize the differences in the structure of these polysaccharide chains through serum reactions.
By leveraging the highly specific nature of antigen-antibody reactions, we can identify specific serotypes within E. coli.
Initially, most cases of enterohemorrhagic E. coli were mainly serotype O157, so food testing around the world has been centered around serotype O157. However, recent studies have revealed that not just O157, but diverse serotypes of enterohemorrhagic E. coli are causing food poisoning around the world. Therefore, it's being suggested globally that we need to reconsider our testing systems which are primarily focused on E. coli serotype O157.
So, when we understand serotypes, we understand how diverse E. coli can be and why it’s crucial to identify and study them. It’s like having different models of cars; they might look different and have different features, but they are all cars. And, in this case, understanding those ‘models’ can help us in fighting against diseases more effectively!
The Two Patterns of Infection by Infectious Foodborne Bacteria
Before we delve into the pathogenic mechanisms of E. coli, let’s briefly touch upon the general mechanisms of pathogenicity of infectious foodborne bacteria on intestinal epithelial cells. Infectious foodborne bacteria can be broadly categorized into two types.
The first type is where the microbes do not invade the intestinal epithelial cells. Instead, they produce toxins on the surface of these cells. This type can be called “infectious toxigenic.” Examples of this type include E. coli O157 and Vibrio cholerae. However, these bacteria are not classified as toxigenic foodborne bacteria. As mentioned before, the definition of toxigenic foodborne bacteria is that microbes produce toxins in the food. Infectious toxigenic bacteria start producing toxins only when they are on the surface of intestinal epithelial cells. These toxins are not secreted in the food. The expression of toxin genes and thus toxin production only occur within the host. It’s in this sense that these microbes are defined as infectious foodborne bacteria.
The other type is where the microbes do invade the intestinal epithelial cells. This type is represented by bacteria like Salmonella and Listeria. We can refer to this type of bacteria as “infectious invasive.” I will elaborate on the mechanisms of invasion for each of these foodborne bacteria when discussing them individually.
So, in summary, we have bacteria that make toxins on our cells but don't go inside them, like E. coli O157, and then we have those sneaky ones that actually invade our cells, like Salmonella. It’s pretty fascinating, and scary, how these tiny creatures have developed such different strategies to make us sick!
Infection Mechanism of Enterohemorrhagic E. coli
Now, let’s delve into the mechanism of infection of enterohemorrhagic E. coli. Compared to other strains, this one tends to cause more severe symptoms and has a higher fatality rate. Enterohemorrhagic E. coli destroys intestinal epithelial cells by firing a kind of projectile called Shiga toxin, leading to large amounts of blood flowing into the intestines. As a result, a typical symptom of E. coli O157 infection is bloody diarrhea.
Moreover, another significant characteristic of enterohemorrhagic E. coli is that when children, who have weaker immune systems compared to adults, get infected, the Shiga toxin doesn’t just destroy the intestinal epithelial cells. It travels through the blood, reaching all the way to the kidneys. Here, it employs the same mechanism to destroy the cells in the kidneys.
Now, the kidneys have a critical role in cleaning up impurities in our blood, so when the cells in the kidneys that have this cleaning function are destroyed, it can lead to a condition called hemolytic uremic syndrome (HUS). This condition is life-threatening and can lead to death in severe cases.
In a nutshell, this is how the bacteria work: they invade our bodies, damage our intestinal cells causing bloody diarrhea, and in severe cases, the toxin can even travel to the kidneys, causing even more severe problems, especially in children. It’s crucial that we understand these mechanisms so we can find ways to fight back and protect ourselves and our loved ones from such harmful bacteria.
Which Foods are Risky?
Let's talk about the foods that are most likely to pose a risk of enterohemorrhagic E. coli infection. The most common sources are beef and beef-related products. Additionally, vegetables contaminated with enterohemorrhagic E. coli, often found near cattle farms, are also frequent causes of food poisoning.
In simple terms, when we are thinking about risky foods, we need to be careful with beef and related products and also watch out for vegetables, especially those from areas near cattle farms.
Lasting Damage such as Kidney Dysfunction
Even after recovery, many people experience lingering effects, some requiring kidney dialysis for life. When young children get infected and fall into severe conditions due to hemolytic uremic syndrome (HUS), even if they survive, they may have to live with serious impairments such as high-level brain dysfunction and kidney dysfunction for the rest of their lives.
Beyond the rights and functions of children, in adults, there are cases where individuals suffer from conditions like hypertension as a consequence of being infected with enterohemorrhagic E. coli.
So, the consequences of these bacteria are not just immediate illness. They can have long-lasting impacts, affecting the quality of life for those who get infected, especially children. Understanding the sources of risk and the potential long-term consequences is vital for preventing infection and protecting our health and the health of our community.
