by Matt Shipman
Salmonella. You don’t want it, but odds are good that you have had it – or, actually, them.
Even people who aren’t scientists have usually heard of Salmonella. Most people associate it with food poisoning – and spending a lot of time in the bathroom.
That’s because Salmonella bacteria cause an infection called “salmonellosis,” which is one of the most common causes of foodborne illness in the United States. Symptoms of salmonellosis include fever, abdominal cramps and diarrhea and can take weeks to get over. And there are more than 40,000 reported cases of salmonellosis in the U.S. every year. However, the U.S. Centers for Disease Control & Prevention (CDC) thinks that only 1 in 40 people will go to the doctor when they get salmonellosis, and estimates that more than one million people are affected by salmonellosis every year (including an average of 400 deaths).
But even though people talk about Salmonella as if it’s just one thing, it’s not. Salmonella actually refers to a collection of related bacteria. Scientists have identified more than 2,500 different strains of Salmonella; more than 1,400 are known to cause illness.
One of those strains, called Salmonella typhi, actually causes typhoid fever – which is fairly rare in the modern United States, but still affects millions of people around the world every year. Some researchers think typhoid fever is what killed Alexander the Great – so clearly humans have been living with Salmonella for thousands of years.
The good news is that you are very unlikely to find S. Typhi in your house – especially if you live in the United States – and S. Typhi doesn’t cause salmonellosis anyway.
The bad news is that there are many other types of Salmonella that DO cause salmonellosis – and they can be found on all kinds of different things. In 2012 alone, salmonellosis outbreaks investigated by the CDC included multistate outbreaks caused by: a strain called S. Bredeney in peanut butter; S. Typhimurium in hedgehogs(!); S. Braenderup in mangoes; and both S. Newport and S. Typhimurium in cantaloupes.
So where do Salmonella come from?
Salmonella bacteria have evolved over thousands of years, changing to give themselves a better chance of surviving in a wide range of environments. For example, at some point in the very distant past Salmonella first evolved the ability to infect the intestinal tract of other organisms – where they grow and reproduce. Later, some Salmonella organisms developed the ability to infect warm-blooded vertebrates, like humans (or hedgehogs).
Because there are so many different strains of Salmonella, there are types that can infect an amazing array of different creatures – including mammals, birds, reptiles and even insects. Some have even specialized in infecting certain animals – S. Typhi, for example, has no natural host other than humans.
Salmonella spread through the feces of infected animals, including people. If an infected bird poops in a pond that is used to irrigate crops, some of the Salmonella in the poop could end up on those crops. When another animal, such as a human, eats the food, they’re also eating the Salmonella bacteria. Those bacteria then invade the walls of that person’s intestines, which triggers inflammation and diarrhea. The bacteria reproduce, and can travel through the blood to other parts of the body. The bacteria also release toxic substances, which travel through the blood and make you sick.
In the case of the hedgehog-linked outbreak in 2012, people were infected by Salmonella after touching hedgehogs (who were infected and had walked through – or rolled in – their own poop). The bacteria got on their hands and then, ultimately, into their mouths. (Washing your hands is important!)
This, of course, is really gross. But it’s also pretty amazing. One of the reasons that salmonellosis is relatively common is that Salmonella bacteria are really tough. They can survive in soil or any dry environment for a long time, and they can also survive in a wide range of temperatures. Putting them in a refrigerator won’t kill them – it will just stop them from growing. And they can withstand temperatures up to 133 degrees Fahrenheit. In fact, their ideal temperature is right around 98.6 degrees Fahrenheit – our body temperature which, like the last porridge Goldilocks tried, appears to be just right.
Andreas J. Bäumler, Renée M. Tsolis, Thomas A. Ficht, and L. Garry Adams. Evolution of Host Adaptation in Salmonella enterica. Infect Immun. 1998 October; 66(10): 4579–4587.
About the Author
Matt Shipman is a science writer and public information officer at North Carolina State University. Before coming to NC State he covered the nexus of science, politics and policy as a reporter for Inside EPA, Water Policy Report and Risk Policy Report. His science communication blog, Communication Breakdown, is part of the SciLogs network presented by Spektrum der Wissenschaft in association with Nature.com.