Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
I was doing some background research for this lesson and happened on a website article discussing foodborne illness caused by the bacteria Salmonella. Lots of readers were making comments on the article, and as you can imagine, the comments got pretty interesting. There were people cracking jokes, some of which were pretty funny, but not appropriate here. Other posts were busy refuting the accuracy of the information presented. And, of course, just to prove I am giving a good representation of the comments, there was one post trying to sell me generic pharmaceuticals at rock bottom prices.
But, one post in particular caught my eye. The reader claimed that if you want to avoid foodborne illness, you must become a vegetarian. At first, this might sound like a reasonable idea. Most of the bacteria that cause foodborne illness originate from the animals you are eating. We are constantly reminded not to eat raw or undercooked meats, raw eggs and unpasteurized milk. Switching to a vegetarian diet would seem to eliminate this threat. So, is this anonymous poster onto something? Let's take a quick look at the data.
On the Centers for Disease Control and Prevention website, they have a list of 49 selected Salmonella outbreak investigations since 2006. I took a minute and crunched the numbers. There were 12 investigations into contaminated meats, like chicken, eggs and ground beef. This represents about a quarter of the total. About another quarter were investigations into live animal sources, like pet shop turtles, frogs and live chickens. But nearly half, 21 cases, identified a non-animal based product as the source of the outbreak. This includes alfalfa sprouts, peanut butter, pine nuts and cantaloupe.
The take-home message here is that simply avoiding animal products won't keep you safe. The vegetables on your table may be more dangerous, simply because people are less careful with them than they are with their meats. So keep this in mind as we take a closer look at the bacteria Salmonella.
Salmonella is a genus of gram-negative, facultative aerobic, rod-shaped bacteria normally found in the gut of animals. Gram-negative refers to Salmonella having a cell wall with a thin peptidoglycan layer and an outer membrane, making these cells appear red in the classic Gram stain. Being a facultative aerobe, Salmonella can grow with or without oxygen. In fact, Salmonella normally lives in the anaerobic small and large intestines of animals, including mammals, birds and reptiles. It is then passed out of the intestines and can be found in feces, and is normally present in sewage.
Food products can become contaminated by fecal material, spreading the bacteria to consumers. This contamination can be from the food animal itself, from fertilizers sprayed on food crops or from food handlers. Raw poultry, raw eggs, unpasteurized dairy products and contaminated raw vegetables are frequent sources of exposure.
The bacteria can also be found on the surfaces of live animals, especially reptiles and birds. Who hasn't heard the horror stories about little kids getting Salmonella from putting turtles in their mouths? This might sound like a joke, but it has happened. Simply handling these reptiles, then eating without washing your hands can also lead to infection.
The species of Salmonella most often responsible for foodborne illness is S. enterica. Within this species, there are around 1,400 subspecies, also called serotypes. Within these serotypes, S. enterica serovar Typhimurium is the most common cause of illness, even though almost all are pathogenic to humans.
Salmonellosis is the term used to describe any symptoms caused by a foodborne infection by the bacteria Salmonella. In a food infection, the symptoms are caused by actively growing bacteria in the body. A person must consume a very large dose of Salmonella for symptoms to occur, often millions of cells. The bacteria pass through the stomach and begin to colonize the walls of the small and large intestine. The Salmonella bacteria inject a cocktail of proteins and toxins into the intestinal cells, which forces the cell to engulf the bacteria.
Once inside, the Salmonella cells release more toxins that cause a rush of fluid into the intestines and stimulates inflammation and a host immune response. One of the toxins released by growing and dying Salmonella is endotoxin, which consists of lipopolysaccharides, a major component of the outer membrane of gram-negative bacteria. The release of endotoxin during cell growth and death contributes to the fever and inflammation associated with Salmonellosis.
Consistent with the features of a foodborne infection, the symptoms of Salmonellosis don't start right away. It usually takes the bacteria between 8 and 48 hours to invade enough intestinal cells and cause enough inflammation for symptoms to develop. Very suddenly, you will get a headache and chills from your immune response. This is followed pretty closely by abdominal cramps, vomiting and diarrhea from the increased intestinal fluids. Many people will develop a fever that lasts for several days.
The symptoms of Salmonellosis are pretty typical for a foodborne illness. So, in order to positively diagnose Salmonella infection, doctors usually start with a complete history, to try and pin down what you might have eaten that made you sick. The next step is to try and culture the bacteria from a sample of your fecal material. There are several special types of growth media that can be used to differentiate Salmonella from other bacteria, based on Salmonella's ability to use the element sulfur during metabolism. The sulfur makes the Salmonella colonies appear a characteristic dark, blackish color.
Treatment is often unnecessary. The symptoms usually resolve on their own in two to seven days. But, the patient can continue to shed Salmonella in the feces for weeks. Some patients can even shed the bacteria for years, becoming chronic carriers, potentially infecting many new people. Contrary to what you might think, antibiotics are actually considered a bad idea. The use of antibiotics has not been proven to shorten the illness, and has even been linked to causing a chronic carrier state. In the worst cases, the diarrhea can become severe and profuse. These patients often require intravenous fluids to prevent dehydration, but usually recover without any problems.
In the United States, there are around 42,000 reported cases of Salmonellosis every year, with potentially many more unreported cases. Approximately 400 of these people die from the disease. These deaths are usually a result of complications from dehydration, and impact the very old, the very young and the immunocompromised, predominantly. As with many other foodborne illnesses, prevention involves thoroughly cooking your food, avoiding desserts with raw eggs, washing your fruits and vegetables and practicing proper personal hygiene.
Let's take a minute and review some of the key points of Salmonella. Salmonella is a genus of gram-negative, facultative aerobic, rod-shaped bacteria normally found in the gut of animals. People get Salmonellosis by eating foods contaminated by fecal matter containing the bacteria. Major sources include raw poultry, raw eggs, unpasteurized dairy products and contaminated vegetables.
Once eaten, the bacteria invade the cells that line the intestinal walls, causing inflammation. This leads to the classic symptoms of headache, abdominal cramping, vomiting, diarrhea and fever. This is an example of a food infection, since the symptoms are a result of the actively growing Salmonella in the gut. The symptoms usually clear up on their own, so treatment is often unnecessary. For some patients with severe diarrhea, intravenous fluids can be given to avoid dehydration. To prevent getting Salmonellosis, you should always thoroughly cook your food, avoid eating raw eggs, and wash your hands after handling live animals, raw meat or using the bathroom.
After viewing this lesson, you should be able to explain Salmonella, including what it does to the human body and how to prevent getting it.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons