The possibility that E. coli O157:H7 was a contaminant in cookie dough surprised even the most experienced microbiologists here in CDC’s Enteric Diseases Laboratory Branch. E. coli O157 is a common culprit of a severe diarrheal illness, usually caused by eating contaminated and undercooked ground beef or drinking unpasteurized apple juice. It shouldn’t have even been on the “Who’s Who” list of the top bacterial contaminants.

We are Gerry Gómez and Mike Humphrys, two microbiologists in the Enteric Diseases Laboratory Branch. We identify disease-causing bacteria from foods and from specimens taken from sick people.

As microbiologists we have to be flexible and creative in our approaches to isolate bacteria from all different kinds of foods. Our usual method is to mix the food with enrichment broth and see what grows after 18 hours of incubation. For this investigation, we made cookie dough slurry. Then we added tiny magnetic beads treated so they’ll stick to the antigen on the cell wall of E. coli bacteria. If present, E. coli cells would stick to the beads, and then we used a magnet to pull the beads (and any bacteria) out of the slurry. Now we had a better chance of isolating E. coli from the cookie dough.

An important part of our investigational process is collecting and sharing data from laboratories across the U.S. For the cookie dough outbreak, 13 laboratories tested 164 various cookie dough products.

E. coli growing on blood agar.

We found that the chocolate chip cookie dough that sick people had eaten didn’t come from only one batch.

We in EDLB never know what food products will come our way, but we’re willing to test just about anything to confirm a hypothesis. Even experienced microbiologists who have “seen it all” can be surprised and challenged by an old bacteria turning up in a new place.