Salmonella is an extremely common type of bacteria. These rod-shaped organisms can be found in both cold-blooded and warm-blooded animals across the world. They are also one of the most common causes of sickness in human beings.
Salmonella poisoning can infect people in one of two ways. It is most often spread from animals to people through the food supply. This is how the bacteria can cause the nauseating disease of gastroenteritis. Typhoid-causing Salmonella, however, is usually spread from person to person.
In this article, you will learn all about Salmonella. You'll find out how it can make you sick, how the bacteria are fighting back against antibiotic medicine and what we can do about it, how to keep your grub safe from these illness-causing organisms, and even what happens when Salmonella grows in space.
Salmonella symptoms vary depending on the type of Salmonella that has caused the infection. Most Salmonella infections lead to problems with digestion known as gastroenteritis, though some strains of the bacteria can cause typhoid fever.
For most infected people, symptoms develop from 12 hours to three days after eating food containing the bacteria. Symptoms often include abdominal cramps, diarrhea, and fever. These symptoms usually disappear after two to seven days, though returning to normal bathroom habits can take several months. However, in a few rare cases, Salmonella infections can lead to long-term joint pain known as reactive arthritis, which can over time develop into chronic arthritis.
Not all Salmonella bacteria are the same. In fact there are more than 2,500 types of Salmonella. Each type is identified and labeled as a different serotype. Some of these serotypes will only infect one particular animal, or only exist in one specific place. Of these, less than 100 are responsible for the majority of human infections.
Knowing the serotype of a given organism is important for scientists who want to observe and control the spread of outbreaks. Since the 1960s, the US government has paid researchers to discover outbreaks of Salmonella with the hope of tracking the source of the infection. Depending on the serotype, a Salmonella outbreak could be mild or extremely severe.
The scientists who watch for Salmonella outbreaks are concerned about three specific groups of serotypes. These three groups are responsible for sickening millions of people across the world each year, and some of those illnesses result in death.
Non-Typhoidal (S. typhimurium and S. enteritidis)
Non-typhoidal Salmonella are the serotypes that sicken the most people. As the name suggests, these bacteria do not cause typhoid fever. Instead, they lead to diarrhea and related symptoms. Of all the varieties of non-typhoidal Salmonella, there are two that are responsible for half of all human infections in the United States: S. typhimurium and S. enteritidis. S. typhimurium is usually caused by food from animals. S. enteritidis is usually caused by infected eggs and poultry.
Every year in the United States, non-typhoidal Salmonella sickens an estimated 1.2 million people. Of these, 23,000 are admitted to the hospital and 450 die.
Typhoidal Salmonella typhi infections cause typhoid fever. Someone with typhoid fever will need to take antibiotics. It is important to take the full round of antibiotics and not stop just because you begin to feel better—this disease is notorious for being passed to others after a person's symptoms decline. We will describe the symptoms and treatment for typhoid more completely later in this article.
Foodborne diseases as a whole infect 550 million people each year. According to the German government, Salmonella infections account for about 90 percent of foodborne infections in recent years. The CDC estimates that approximately 1.2 million illnesses occur due to non—typhoidal Salmonella per year in the US.
According to the World Health Organization, most cases of Salmonella infection are mild. However there are three groups of people who need to be particularly careful. These are the elderly, young children, and anyone whose immune system is weakened, either due to HIV/AIDS, cancer treatment, or some other cause.
Salmonellosis symptoms range from mild to severe, and treatments should be dictated accordingly. For those with mild symptoms, the WHO recommends no antibiotic treatment, as these bacteria are becoming increasingly resistant to these drugs, and administering them for every patient is likely to lead to further resistance. However, anyone in a particularly vulnerable group, including infants and those older than 65 years old, should receive antibacterial medications. That's also true for anyone whose infection has spread beyond the digestive system.
Antibacterial resistance is a real and growing problem when it comes to this infectious bacterium. For non-typhoidal serotypes, about 5% are resistant to five or more drugs as of 2011. About two out of every three typhoidal serotypes show some level of resistance to ciprofloxacin in the United States. Typhoidal serotypes show resistance to other common antimicrobial drugs in other parts of the world, according to the CDC.
Antibacterial Soap: What’s the Harm?
You may reach for antibacterial soap to keep your family safe from harm. Killing more bacteria has got to keep them safer, right? Not necessarily, according to the Food and Drug Administration. The FDA says there is no evidence washing up with plain soap and water is any less effective at preventing illness.
What's more, studies have shown that using these antiseptic soaps can kill off certain bacteria, but they leave other bacteria alone. By doing so, they may be contributing to the rise of so-called “super bugs,” which are more resistant to antibiotics and more potentially damaging to human health. So while our first instinct may be to use antimicrobial soaps for better health, in the long run these soaps may be causing more harm than good.
Compared to gastroenteritis from Salmonella poisoning, typhoid fever is very rare. About 5,700 cases occur in the United States every year, and about 75% of those are contracted overseas. Compare that with more than 1 million sickened in the U.S. by nontyphoidal salmonella bacteria.
In developing regions, typhoid fever remains a serious and relatively common disease. Worldwide, about 21.5 million people are afflicted with it each year, resulting in 200,000 deaths. Each year another 2 million people worldwide are estimated to contract the related disease paratyphoid fever.
Typhoid Fever Symptoms
Typhoid comes on gradually. Symptoms don't develop for about a week after infection. Then it takes about three to four days for a low-grade fever to rise, sometimes as high as 104 degrees. These fevers tend to be worse in the afternoon and evening.
Along with fever, almost all people with typhoid experience headaches, loss of appetite, and the soreness and uneasiness associated with an approaching sickness known as malaise. Many people experience diarrhea, constipation, and pain in their abdomens, and their spleen and liver may swell. Sometimes people with this condition develop rosy spots that move from place to place along their trunk.
Treatment for typhoid is crucial. Without treatment, the fever lasts for about a month and as many as 30% of those infected will die from the disease. Antibiotics are used to eradicate the disease, though this takes about three to five days, during which time a patient's symptoms may actually get worse. This effort is hampered by the increasing resistance Salmonella bacteria have shown to antimicrobial medicine.
There are two vaccines that help lower your chances of contracting typhoid fever. One is a shot of the inactivated (killed) bacteria, while the other is an oral medicine of a weakened form of the microbe. The shot can be administered to people older than 2, but needs a booster after two years. The oral medicine is for ages 6 and up, and needs a booster every five years.
The vaccines are not 100% effective, though. If you are traveling to an area where typhoid is a known problem, you will want to take additional precautions. Be careful about where you find your food and water—these are common sources of infection. Also, do your best to avoid anyone sickened by the disease. This isn't perfectly effective either, though, because someone who is contagious may not have symptoms.
Salmonella infections are usually caused by food poisoning. In fact, about 95% of infections come from the foods we eat. Animal products such as beef, pork, chicken, milk, and eggs are particularly susceptible, but vegetables can also harbor the disease.
To preventing food poisoning, every point in the food chain needs to be carefully controlled. Harvesting, butchering, processing, and preparation should all involve proper hygiene to ensure safety. The WHO provides the following five points for food handlers both at home and in restaurants to prevent infection:
- Keep food clean
- Cook meats and vegetables completely and thoroughly
- Maintain ingredients at safe temperatures
- Keep raw and cooked items separate in the refrigerator and elsewhere
- Make sure your water and materials are clean and safe
Salmonella infections from raw eggs is a major cause of salmonellosis food poisoning. These infections are usually caused by Salmonella enteritidis (SE). SE accounts for as much as 80% of egg contamination. This is because SE can infect the ovaries of an egg-laying hen without the hen showing any outward signs of infection.
In a major assessment of US egg production and SE, the USDA concluded in 2005 that 1 in every 3,600 shelled eggs has some amount of the bacteria. Of these contaminated eggs, 85% pose no risk to humans if they are properly stored and cooked. That frequency of infection is probably lower today. In 2010 the Food and Drug Administration set higher standards for egg producers. These standards are expected to reduce SE food poisoning from eggs by 60%.
Pasteurization and Storing Temperature
As of 2005, the USDA estimated that less than one half of 1% of all eggs produced in America are pasteurized. In order to be pasteurized, an egg must be covered in hot water long enough to destroy SE bacteria, but not long enough to cook the liquid inside the egg.
If US egg producers began pasteurizing their eggs, illnesses would drop from about 130,000 per year down to between 41,000 and 19,000, depending on the method of pasteurization, according to USDA estimates.
Proper storage makes a big difference in protecting consumers from food poisoning as well. If every US egg were stored below 45 degrees Fahrenheit within 12 hours of being laid, yearly illnesses would drop from 130,000 to an estimated 28,000.
If you're cooking meat, how can you be reliably safe from food poisoning? One of the most important steps is to bring your food to the proper temperature. A good internal thermometer is crucial to get this right. The proper temperature depends on what you're cooking:
- For lamb, veal, pork, and beef, bring the internal temp to at least 145 degrees Fahrenheit.
- Ground meats, including beef and pork, should be heated to 160 F.
- Poultry (including chicken and turkey) should be heated to 165 F.
- Fish should be heated to 145 F.
- Any sauces, gravies, or soups should be brought to a boil.
- Leftovers should be heated to at least 165 F.
Even when you aren't eating eggs or animal products, you may still be at risk of Salmonella food poisoning. This bacterium can be found in fruits, vegetables, peanut butter, and some processed foods as well.
Be especially careful during the summer. As temperatures rise, they create ideal circumstances for the bacteria to thrive. Picnics and backyard barbecues are especially at risk of an unwanted visit from this sneaky pathogen. Make sure you keep your hot foods hot and your cold foods cold. Refrigerate all leftovers right away. Never let your food sit for more than two hours—or more than one hour if it's hotter than 90 degrees Fahrenheit outside.
You should also follow basic safety precautions year round. That means keeping your hands, cutting boards, counters and utensils clean, keeping animal products separate from other foods both inside the fridge and out, and never preparing food if you have diarrhea or if you have been vomiting.
Just about any animal can be a carrier for these bacteria. That includes cats and dogs. However some are at a higher risk of carrying this disease than others. Some of these are water frogs, turtles, and baby chickens and ducks (chicks and ducklings).
Reptiles and amphibians have been singled out by the CDC for the Salmonella they carry, which poses a particular risk to children under 5. Small turtles were responsible for a 2015 outbreak that sickened more than 200 people. Some of the sick people hadn't even touched the turtles.
Because pets can become carriers, it is crucial that you wash your hands carefully and completely with soap and water after petting or feeding an animal. Animal areas should be cleaned and disinfected regularly by adults or children ages 6 and over. Avoid eating or drinking around animals that are at a high risk of carrying the disease.
When researchers studied the impact of space on microorganisms by growing Salmonella on a space shuttle, they discovered something startling. Compared to samples of the bacterium grown on Earth, the space Salmonella was deadlier.
This was proven when some of the space sample was injected into hundreds of lab mice. The mice died two days earlier and with lower doses than others who received terrestrially grown bacteria.
Discovering this strange fact has changed the way scientists think about long-term space travel, such as a manned trip to Mars. It's impossible to completely disinfect a spacecraft because so many bacteria are found on humans already. That means we will need to come up with a way to fight off these especially dangerous organisms if we want to travel beyond Earth's orbit.
For all the terrible, harmful things this bacterium causes, it's worth considering one way they may actually be used for our benefit. A scientist in California is studying whether he can turn these nasty bugs into cancer fighters. Professor Jeff Hasty of UC San Diego has engineered the bacteria to remove their disease-causing tendencies and replace them with the power to attack cancer tumors.
If they are as successful in humans as they have been in mice, these engineered cells could have further medical uses beyond cancer treatment. They could be programmed to release drugs at predictable times, which could help people with common ailments like diabetes and high blood pressure.
IMAGES PROVIDED BY:
- Tom Le Goff / Digital Vision / Getty Images
- Science Source / Photo Researchers, Inc
- Veronique Beranger / Riser Collection / Getty Images
- The Atlantic: "How Salmonella could be used to kill cancer."
- Bundesinstitut für Risikobewertung: "Salmonella and their importance as pathogens."
- CDC: "Infectious Diseases Related to Travel," "NARMS 2012 Human Isolates Final Report—Tables," "Salmonella Atlas," "Salmonella Infection," "Salmonella is a Sneaky Germ: Seven Tips for Safer Eating," "Salmonella Questions and Answers," "Take Care with Pet Reptiles and Amphibians," "Typhoid Fever," "Typhoid Fever Vaccine."
- FDA: "Antibacterial Soap? You Can Skip It—Use Plain Soap and Water."
- MIT Technology Review: "Deadly Bacteria from Outer Space."
- Nature: "Synchronized cycles of bacterial lysis for in vivo delivery."
- USDA: "Risk Assessment of Salmonella Enteritidis in Shell Eggs and Salmonella spp. In Egg Products."
- WHO: “Salmonella (non-typhoidal).”