Back To CourseMicrobiology 101: Intro to Microbiology
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
Have you ever heard of a staph infection? This bacterial infection, caused by Staphylococcus aureus, is usually a serious complication associated with hospital stays and surgeries. A person goes to the hospital for a routine procedure, like an appendectomy, but ends up dying from a staph infection.
Then there are the stories of young athletes getting a minor skin scrape, which gets infected, and suddenly they are in intensive care for a MRSA infection. MRSA stands for methicillin-resistant Staphylococcus aureus. This bacteria must be a killer, considering everything you hear about it is fraught with gloom and doom.
But have you ever heard of Staphylococcus aureus food poisoning? I bet not. This same species of bacteria associated with life-threatening infections causes nearly 200,000 confirmed cases of food poisoning in the United States every year. Because of the reputation of S. aureus, you might expect it to cause a devastating foodborne illness requiring hospitalization. You might also expect a pretty high body count. Well, you would be wrong. In fact, if you absolutely had to choose a foodborne illness to acquire in your lifetime, you might want to root for S. aureus.
Staphylococcus aureus is a Gram-positive, facultative anaerobic, coccus-shaped bacteria that usually grows in clusters. 'Gram-positive' refers to the thick layer of peptidoglycan present in the S. aureus cell wall, making these cells appear purple in the classic Gram stain. As a facultative anaerobe, S. aureus is not only able to survive in anaerobic conditions, but it also thrives in the presence of oxygen. The species name, 'aureus,' which means 'golden,' refers to a pigment produced by many S. aureus strains. This golden pigment plays a role in infection.
S. aureus is commonly found on the skin and in the respiratory tract of healthy humans and animals. Some estimates suggest that 25% of the population already has S. aureus somewhere on their body. Other estimates put that number closer to 100%.
Either way, S. aureus is an opportunistic pathogen, an organism that normally exists harmlessly on a host but can cause disease if conditions within the host change. This change can be a reduction in immune function, the opening of a wound or a disruption of the normal host microflora. This last one can occur when a person is put on antibiotics, and the normal bacterial populations are reduced or eliminated, allowing S. aureus to overgrow.
Since many people have S. aureus in their nose or on their skin already, it's probably not a surprise to learn that most people catch the bacteria from eating food contaminated by infected preparers and handlers. A poorly aimed sneeze by a deli worker can spread S. aureus all over your once-appetizing turkey sandwich. More frequently, though, small, seemingly insignificant skin sores and pimples caused by S. aureus shed the bacteria onto surfaces and foods.
The bacteria grows best on protein-rich foods like meat and fish. Ham is a frequent source of S. aureus, because this bacteria can survive and grow in the high salt content usually used as a preservative. Other bacteria can't tolerate the high salt, leaving S. aureus free to grow without a lot of competition by other bacterial species. In addition, creamy foods, like egg salad and cream-filled baked goods, are also common sources of S. aureus.
Once a food is contaminated with Staphylococcus aureus, the bacteria begins growing, dividing and producing exotoxins, which are toxins produced by bacteria and released into the environment. S. aureus is able to produce seven different exotoxins. Usually, with a foodborne illness, thoroughly cooking the food is enough to make it safe. This is not the case with S. aureus. Boiling and cooking will kill the bacteria, but the S. aureus exotoxins are very heat-stable. The exotoxins can even remain viable after being boiled for up to 30 minutes.
It is these toxins that cause the symptoms of Staphylococcus food poisoning. Remember, food poisoning, also called food intoxication, is a foodborne illness that results from consuming preformed microbial toxins. Consuming the bacterial cells will not contribute to the illness, and the food does not have to contain viable cells to cause disease.
Once consumed, the toxins pass through the stomach to the intestine. Symptoms usually start within 30 minutes to six hours of consumption. This quick turnaround usually means that patients can pinpoint the exact food that caused the illness. When the exotoxins come into contact with the cells that line the intestine, the host's body starts a massive immune response, causing inflammation. The inflammation and the exotoxins lead to a release of water into the intestine. What results is abdominal cramping, nausea, vomiting and diarrhea.
These symptoms last for several hours and most cases resolve on their own in a day or two. If the patient is very young or very old, the diarrhea can be severe enough to cause dehydration, but this doesn't happen very often. Dying from Staphylococcus food poisoning is also very rare, and the majority of deaths occur in the very old and very young as a result of dehydration.
The rapid onset of diarrhea is a hallmark of S. aureus infection. In fact, the symptoms, in conjunction with a probable source of the bacteria, are usually all a doctor needs to positively diagnose the illness as Staphylococcus. More in-depth testing is not done for isolated cases. If there is a suspected outbreak that requires a more concrete identification, officials will attempt to grow the bacteria from a fecal or vomit sample on a high-salt medium. Most common food contaminants won't be able to grow, but Staphylococcus aureus has no problem with the salt. Tests are also available to identify the exotoxins on foods to confirm the outbreak source.
Treatment is rarely needed. Most patients will fully recover by getting plenty of rest and drinking fluids. If an old or young patient develops a severe case of diarrhea leading to dehydration, they might require intravenous fluids. In either case, antibiotics are not needed. Remember, the symptoms are a result of consuming toxins, not actively growing bacteria, so there is no need for an antibiotic.
So, how can you prevent getting infected by bacteria that are probably already on your body somewhere? The best thing you can do to prevent Staphylococcus aureus gastrointestinal illness is to maintain good personal hygiene and always wash your hands before handling or eating food. Always handle food in a sanitary manner and be sure to wash all utensils and surfaces thoroughly. Refrigeration of food will slow the toxin production and growth of S. aureus but not eliminate the bacteria. As a result, any susceptible foods, like meat and egg salad, should be discarded, not put back in the fridge, if left unrefrigerated for more than a couple of hours.
Let's take a minute and review what we have just learned about Staphylococcus aureus food poisoning. Staphylococcus aureus is a Gram-positive, facultative anaerobic, coccus-shaped bacteria that usually grows in clusters. The bacteria are commonly found on the skin and in the respiratory passages of humans.
Respiratory droplets and infected skin lesions of food handlers can spread the bacteria to food. Meats, like ham, and creamy foods, like egg salad and cream-filled pastries, are great places for S. aureus to grow and produce exotoxins. Once consumed, the exotoxins cause intestinal inflammation, leading to abdominal cramping, nausea, vomiting and diarrhea. Since the symptoms are a result of the preformed toxins and not the actively growing bacteria, this is an example of food poisoning.
The illness usually runs its course in a day or two and goes away without treatment. In the very young and very old, the diarrhea can be severe enough to cause dehydration, requiring intravenous fluids. Severe cases like this are rare, and fatalities are rarer still. With good personal hygiene and sanitation, it is possible to prevent most cases of S. aureus food poisoning.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons