There was a time not long ago when experts relied heavily on patient interviews to identify any food or foods contaminated with Listeria monocytogenes, which is the species of listeria that causes illness in people. Health officials would ask patients with listeria infections what they’d eaten, and these food logs would be compared to identify potential culprits. (1) RELATED: The Signs and Symptoms That May Mean You Have a Listeria Infection Health officials also use a form of analysis called pulsed-field gel electrophoresis (PFGE) to identify related strains of listeria. They look at DNA fragments of suspected contaminated foods or molecules after sending an electrical current through it, and then compare it with molecules known to be Listeria monocytogenes. (2) But this form of analysis isn’t always accurate. (3) According to the Centers for Disease Control and Prevention (CDC), with PFGE, unrelated listeria germs can sometimes look similar, and closely related ones can look different. (3) RELATED: Everything You Need to Know About Listeria Infection While patient interviews and PFGE are still an important part of outbreak management, whole genome sequencing has become the most powerful tool. (4) Each of these strains has a subtly different genetic blueprint. Since 2013, public health officials have been able to use whole genome sequencing to examine that blueprint in order to precisely identify the exact strain of Listeria monocytogenes that is making a person sick. (3) Why is this helpful? Not all listeria-related outbreaks are linked to a specific food product. Some may start on a piece of food-processing equipment that handles multiple foods. If those contaminated foods are then included in different food products and sent all over the country, it can be very difficult for experts to track those seemingly unconnected listeria contaminations back to their shared source. Whole genome sequencing allows officials to connect dots in ways they couldn’t before. If the same exact strain of listeria turns up in people with listeria illnesses in Ohio and also in Nevada — people who haven’t eaten the same foods — officials know to dig deeper. Thanks to the sequencing technique, “more outbreaks have been identified with fewer cases and in foods not previously a focus of investigation,” according to a study in Food Control. (5) The CDC’s whole genome sequencing program has been so successful that the agency has expanded its program to include other illness-causing bacteria — including Campylobacter and E. coli. (3) Most bacteria cannot withstand the acidic environment of the stomach and upper intestine. But Listeria monocytogenes is different. Not only can it survive the perils of the upper GI tract, but it has the ability to invade its host’s cells and multiply, despite the absence of nutrients found inside those cells. From there, listeria can work its way into a patient’s bloodstream, where it causes invasive and sometimes deadly infections. (6) Donaldson says her research in part investigates how illness-causing listeria bacteria “sense” oxygen variations in the GI tract, and how this may help them survive (and cause problems for its hosts). It’s difficult to defeat a foe if you don’t understand how it operates. So understanding all these processes behind listeria bacteria’s success is important. RELATED: How Doctors Currently Treat Listeria Infection Donaldson’s work and the research of others using whole genome sequencing should help inform the development of new and better treatments. “If we can figure out how listeria respond to the GI tract, we can design therapeutics targeting these mechanisms to block the ability of listeria to get into the blood,” she says. Even when a food is contaminated with listeria bacteria, its levels of harmful bacteria are often not high enough to cause sickness in humans — at least not at first. But Listeria monocytogenes has the ability to stay alive and spread even at refrigeration temperatures. So as a contaminated food sits around in a refrigerator or freezer, its listeria bacteria continue to reproduce until their levels are great enough to cause infection. (7) Using whole genome sequencing, experts can better understand how listeria is able to survive and spread in refrigerated conditions. They can also gain insights into the ways listeria contaminates food-processing equipment — even when that equipment is thoroughly cleaned and treated with disinfectant. With these insights, they may be able to prevent food contamination, and so prevent people from ever being exposed to harmful levels of the bacteria.
