How to avoid food poisoning from E. coli and salmonella

Fresh off the E. coli and salmonella outbreaks this spring, Shelley Rankin, professor of microbiology at the School of Veterinary Medicine, explains key differences between the bacteria—and what to do to avert them.

iStock Lettuce Vegetable

Customers shopping at the supermarket lately may have felt a swell of anxiety while passing by heads of romaine lettuce or the egg case—the former, recalled due to an E. coli O157:H7 outbreak, and the latter linked to an outbreak of salmonella brandaerup infections. 

Surrounding all that concern, though, is a more fundamental question: What is the difference between salmonella and E. coli? 

Shelley Rankin, professor of microbiology at the School of Veterinary Medicine, details the differences, as well as teases out some general food safety concerns as the summer season of fresh foods and barbecues approaches.

What are the fundamental differences are between salmonella and E. coli as bacteria?

Salmonella and E. coli are both bacteria and they are fundamentally very similar. Salmonella actually evolved from E. coli, about 100 million years ago. E coli is much more heterogeneous; they are regular commensal gut organisms, which means they’re part of the normal healthy gut flora of pretty much every mammalian species. There are some very well-known pathogenic types of E. coli, though, and some of these can cause diarrhea. 

So, not all E. coli are the same? 

They’re not all the same. In terms of the E. coli strains that cause food poisoning, the big one we all know about is E. coli O157:H7. This organism is known to have a reservoir in cattle, predominantly, and most of the cows that carry it are unaffected by it. It’s not a pathogen for the animal, but it is in their GI tract. Food such as ground beef can become contaminated during food processing. If lots of people get sick from eating food contaminated with E. coli O157:H7, then before you know it, there are companies recalling millions of pounds of ground beef—or millions of pounds of spinach or lettuce. 

E. coli is a fecal pathogen and that’s how produce like lettuce becomes contaminated. So, think of the large romaine lettuce outbreak recently. That’s a secondary contamination from the environment that obviously comes from the animal to the environment, and then the produce becomes affected either because it’s been sprayed with contaminated water or because the soil is contaminated. It’s a very complicated animal-environmental-human interface, especially when it comes to fresh produce.

Do the bacteria live in the lettuce or on it? How does that work?

It’s usually on the lettuce. Surface contamination. It’s complicated; it depends where the lettuce has been grown and what the production system is. Some farms grow millions of heads of lettuce and they spray the plants continually, and sometimes that water can get contaminated. 

In other situations where produce is grown in open fields, the ground water can become contaminated with animal or bird feces, and produce can be contaminated that way. 

I’m curious why there’s such a long gap between ingesting something that has E. coli or salmonella, and when you get sick. They say it can take up to a week.

For both salmonella and E. coli, there’s a particular number of organisms that need to be ingested for infection to be initiated, and we call that the infective dose. Following ingestion, there will be diarrhea for a short period of time—just a day or two, usually, and then the diarrhea is gone. The time between ingestion and getting sick is a function of the infective dose. In humans, if you ingest a large number of salmonella bacteria, it takes about 12 to 24 hours for the symptoms of food poisoning to start. There’s obviously some die-off of bacteria in the stomach because of the acid, then the salmonella makes its way to the small intestine and then the large intestine, where it infects intestinal epithelial cells and continues to grow. That’s where the effects come in because the immune system recognizes there’s a foreign organism infecting the intestinal cells and the body will actively try to flush it out. 

And so what happens is there’s a large influx of water into the colon, which has to come out. And that comes out in the form of a liquid stool rather than a solid stool. If the number of organisms ingested is big enough, infection can be initiated in about 12 hours at a minimum, in adults. Many foodborne outbreaks occur in the summer when people go to parties or barbecues in the daytime and then get sick in the middle of the night. Or, early the next morning. It’s kind of a 24-hour window with salmonella, but it can take a little longer in some people. 

With E. coli O157:H7, the infective dose is documented to be a little lower. Again, though it does takes some time for infection to be initiated. Typically, it takes three to four days for diarrhea to start after ingestion. The organism has to infect the intestine and the immune system has to recognize it’s there, and the effect, diarrhea, is the body’s way of ejecting the organism from the system. Flushing it out.

Maybe it’s a silly question, but if eggs have a shell, how do they get contaminated with salmonella?

It’s actually very interesting. The University of Pennsylvania School of Veterinary Medicine has, since around 1998, been involved with the Pennsylvania Egg Quality Assurance Program. Over the years, the program evolved, and in 2010 the FDA launched the federal egg safety program. That means that all the states that have egg production are enrolled into this food safety program. The organism we’re looking for in shell eggs, generally speaking, is called salmonella enteritidis. It has been known for many years that a lot of salmonella enteritidis infections come from eggs. 

Research has shown that there is transovarian transmission of salmonella enteritidis from the chicken to the egg. The birds get infected from the environment they live in, usually from rodent feces, and then the organism migrates to the ovaries of the hen. We know that for salmonella enteritidis infections in humans, the organism was inside of the egg. The current egg-associated outbreak of salmonella braenderup came as surprise to me because I don’t believe that transovarian transmission has been documented in the scientific literature for that organism, which may mean that these eggs were contaminated on the outside of the shell. 

The outbreak is still under investigation, but salmonella braenderup is an interesting organism for a salmonella outbreak associated with eggs. 

But generally speaking, we don’t all need to be washing the shell of our eggs, right?

No, we don’t. The producers should have taken care of that for you.

Any food safety tips as we approach farmers market season?

The United States food production system is unquestionably the safest in the world. And conventional production is very tightly regulated and controlled. When you buy something at the supermarket, there’s an assumption that all the government food safety regulation ‘boxes’ have been checked during the production process. When you have smallholders, if you will, people who have a few—or many—backyard birds laying eggs, or that are raising birds that are going to be sold at live bird markets, then their overall contribution to agriculture is very small. However, that does not mean these local products are safer. Because there are no controls in place. 

It’s a very complicated, very interesting, and very exciting discussion for those of us in the food safety field. In the minds of many consumers, food should be produced by small family farms, and should be sourced and distributed locally. By implication, that suggests that agricultural products produced in conventional large agricultural facilities, and distributed nationwide, may be less safe, but that’s not the case. There are many more constraints and regulations in conventional food production systems, and they are there simply to ensure safe products for consumers. The consumer’s role in food safety is critical. How we store and handle our food, regardless of source, is a risk factor associated with getting sick.  

What can people watch for while barbecuing outdoors? 

Every year, in the summertime, people get sick from chicken, and burgers, and hot dogs that were sitting in the sun, or on the countertop, an hour or two before they were put on the grill. People don’t always appreciate that there’s always a low level of contamination on many raw food products, just by nature of their production. Organisms like salmonella and E. coli double in number every 30 minutes when food is not refrigerated. If there were 200 to begin with, then after 30 minutes you have 400, and so on. If those products are not cooked right, then yeah, there’s a chance you’re going to get sick.


Consuming raw, or undercooked food is a known food safety hazard and there are many tips to prevent food poisoning. The following  are recommendations from the CDC.

Clean: Wash your hands and work surfaces often. Germs can survive in many places around your kitchen, including your hands, utensils, cutting boards, and countertops.

Separate: Separate raw meat, poultry, seafood, and eggs from ready-to-eat foods and produce in your shopping cart, refrigerator, and meal preparation area.

Cook: Cook food to the right internal temperature to kill harmful bacteria. Use a food thermometer when you barbecue.

Chill: Keep your refrigerator below 40 degrees Fahrenheit. Refrigerate leftovers within 2 hours of cooking (or within one hour if above 90 degrees Fahrenheit outside).