Food Microbiology- Bacteria Causing Intestinal Infections

Reading Guide Ch 24. Read sections 24.1, and 24.4. Also look at figure 24.6, Table 24.2(about H. pylori) Food Microbiology- bacteria causing intestinal infections

The second part of this material focuses on the organisms that can cause food poisoning. In this section we will focus on the specifics about each organism, what type of symptoms they cause, the food that they are most likely associated with, and specific mechanisms of pathogenesis. Before talking about the organisms let’s look at the features present in the digestive tract to prevent infection. Once food is eaten, the stomach coats the bolus in gastric juice as it is mechanically mixed and digested by the layers of the stomach contracting and almost “wringing” the food. The pH of the gastric juice is around 1.0 so any organisms present in food are easily destroyed by this very low pH. Individuals who don’t make enough gastric juice or are taking acid reducing drugs (as a treatment for ulcers) will not have a good layer of protection from food poisoning. So if the gastric juice can protect from food poisoning, why does it happen at all? While there is no easy answer to this question there are some factors that play a role in increasing the chance of food poisoning. For instance, we know that in order to cause food poisoning (food borne infection) you need to eat bacteria, lots of bacteria. When so many cells enter the stomach there is a greater chance that not all of them will be destroyed. In order for bacteria to cause disease (foodborne infection) they need to gain entry into the body and then find a location (small or large intestine) to bind and grow. Pay attention to how the organism attaches, does it use pili, adhesions, flagella, or even a capsule? Also when organisms grow in the intestines they may produce toxins that cause changes to the epithelial tissue or even destroy cells. Notice if there are special toxins made by an organism and how the toxin works.

Let’s begin with a quick look at the bacteria. The first category to cover are the organisms that cause food borne intoxication. The organisms S. aureus and C. botulinum are examples of food borne intoxication. Remember that these organisms cause food poisoning by creating toxins in food, not by ingestion of the organisms directly. The toxins that are made and secreted by the organisms are called exotoxins since they are secreted out of the cell. There are different actions for these toxins, for example, some are enterotoxins that target the GI tract and others are neurotoxins and have their effect on the nervous system. I have covered a discussion of the two examples of food borne intoxication in the reading guide Part 1 of food microbiology. The rest of this reading guide is covering discussion of the organisms that cause food borne infection.

Organisms that cause foodborne infection are bugs that you ingest in food and then the organisms grow inside and may produce more toxins and lead to the common symptoms of food poisoning. Campylobacter jejuni is an organism found naturally occurring on poultry and in raw (unpasteurized) milk. It is a gram negative curved rod, microaerophile, with one or more polar flagella. The flagella are used for attachment to the surface of epithelial cells. This organism produces a toxin known as a cytotoxin which leads to epithelial cell death. Symptoms caused by this organism are bloody diarrhea with mucus which can last for a week. Salmonella is a genus of organisms that consists of over 2400 serovars. All of these are gram negative rods, facultative anaerobes, with peritrichous flagella. The nomenclature for this group has identified two species of Salmonella, Salmonella enterica and Salmonella bongori. Of these two species they are further divided into serotypes, S. enterica is divided into 6 groups. Salmonella are found to be part of the normal flora of poultry and are present on reptiles (turtles, lizards, and snakes). Infection of this organism is due to contamination of fecal material from these animals and often requires ingestion of a large number of organisms. Salmonella can bind to epithelial cells using an adhesion and activate a Type III secretion system which facilitates endocytosis of the bacterial cell into the epithelial cell. The organism (Salmonella enterica Typhi) can also exhibit phase variation of flagella genes and survive inside the macrophage if ingested! Wow!

Salmonella Typhi is an organism transmitted from human to human and the cause of Typhoid Fever. It is very rare in the US, most cases are the result of people traveling to countries where sanitation practices are not good. This organism has a much longer incubation of 2 weeks and the symptoms are different-note that you have a high fever, chills, and a headache for a week, then diarrhea.

Shigella is an organism that is a gram negative non-motile rod and a facultative anaerobe. It is also an organism that is transmitted from other humans with transmission occurring via the five F’s (see lecture notes). It is found worldwide and in the US there are about 21,500 cases each year. The symptoms of shigellosis are dysentery….what does this mean??(see p595) You will see the passage of small volume bloody stools about 3-4 days after a person ingests this organism. It is not destroyed by stomach acid and only takes 10 cells to cause disease….YIKES! The organism is taken up by M cells (see pg. 358 in textbook) in the digestive tract and then are ingested by macrophages. They multiply in the macrophage, lyse the macrophage, and enter the epithelial cells by inducing endocytosis. Once inside the cells they begin to build “actin” tails and become motile! The organism uses the Type III secretion system (pg 386-387 in textbook) to modify the arrangement of the actin fibers in the cell and generates this “actin” tail. Shigella dysenteriae is a species that is more virulent because it produces the Shiga toxin. This toxin is a cytotoxin since it inhibits protein synthesis. There are also strains of E. coli which produce the Shiga toxin.

Vibrio cholerae is a curved gram negative rod able to tolerate strong alkaline and high salt conditions. It is killed by stomach acid so you must eats LOTS of them to get sick, mostly through ingestion of contaminated shellfish or undercooked fish. They attach to the small intestine using pili and other adhesions. They produce the cholera toxin, an A-B toxin which causes the levels of cAMP in the cell to increase. This increase in cAMP levels promotes an active pumping of other ions and electrolytes out of the cell and into the intestinal lumen. Where ions (especially sodium go) water follows…so it is easy to see how this leads to a huge loss in fluids. Patients can lose up to 3-5 gallons of fluids per day. The symptoms of this type of food poisoning are rice-watery stools with often a sudden onset of explosive watery diarrhea. Symptoms will resolve without antibiotic treatment but most patients do need fluid supplements.

The non-cholera Vibrios are two species of Vibrio that are also associated with food poisoning. Vibrio parahemolyticus is a halophile, motile by single polar flagella. Ingestion of this bug will lead to vomiting and diarrhea about 12 hours after ingestion and last about 2-5 days. Food most often associated are fish and undercooked or raw shellfish. Vibrio vulnificus is the most virulent of the non-cholera vibrios, found to cause food poisoning with ingestion of fish and undercooked shellfish. It has also been found to cause tissue wounds (cellulitis) and can cause septicemia in patients who are immunosuppressed or have liver disease.

The textbook highlights the general characteristics of the different strains of E. coli on pages 590-591. The strain ETEC is responsible for Traveler’s diarrhea and produces toxins that are similar in function to the cholera toxin. The STEC is the E. coli that was famous here in Seattle…the Jack-in-the-Box E. coli. This organism produces two toxins that are similar to the Shiga toxin. The STEC release the toxin in the large intestine and do not invade the epithelial cells (like Shigella).

Listeria monocytogenes is a motile, faculatative anerobe, gram postive rod that can grow at 4oC. It is found in natural waters and vegetation as well as intestines of humans and animals. It can be found to contaminated unpasteurized cheeses, lunchmeat, hot dogs, or coleslaw. When ingested symptoms appear like the flu with a headache and stiff neck as signs of meningitis. GI symptoms are not always present. The organism enters the body through the intestinal epithelium, macrophages uptake it and it can continue to grow and migrate through the body in these cells. If the organism penetrates the meninges, meningitis can occur which is fatal in 70% of cases.

Clostridium perfringens is the last bug on my list, and another obligate anaerobe, endospore former, and gram positive rod. It is most often the cause of food poisoning from foods that are rich in proteins and made in large pots/volumes, like stews and soups. The endospore germinates after cooking and the cells grow as the food is left at cooler temperature. Once ingested (the bugs) it can produce an enterotoxin that leads to watery diarrhea. Symptoms appear 7-15 hours after ingestion and resolves in about 24 hours. Short lived at least! =)