Caramel Apple Microenvironments and Listeria monocytogenes

Just in time for Halloween: a recently-published study in mBio discusses the 2014 outbreak of Listeria monocytogenes tied to caramel apples. There’s not enough water in caramel and an apple is too acidic to support this bacterium, so how did this outbreak occur? From the study: “Our findings suggest that the 2014 listeriosis outbreak associated with caramel-coated apples can be explained by growth of L.¬†monocytogenes occurring at the interface between two foods which, by themselves, are inhibitory to pathogen growth.”

Listeria monocytogenes

The FDA is currently reporting an outbreak of Listeria monocytogenes in some Hispanic-style cheeses. While Listeria isn’t listed in the top five pathogens that cause food poisoning, it’s number four on the list of foodborne pathogens that cause death. It also causes meningitis, encephalitis, and septicemia; in pregnant women, it can cross the placenta and cause abortion, stillbirth, or premature birth. Perhaps now would be a good time for a refresher course in this bacterium.

Listeria grows on blood agar; this growth can be enhanced by cold enrichment. Selective enrichment–inhibiting other organisms while bolstering the growth of Listeria–is recommended if the specimen is food or environmental in nature. Other characteristics include:

  • Short gram-positive rods
  • motile
  • Beta-hemolytic
  • Smooth, light gray, 1-2 mm colonies after 24-48 hours of incubation at 37 degrees C
  • Will grow at 4 degrees C
  • facilitative anaerobe
  • catalase positive
  • oxidase negative

Listeria can be hard to identify, not because it’s fastidious, but because it can be confused with organisms such as Group B Streptococcus, Erysipelothrix, and corynebacterium. Don’t let that happen to you!



Kelly Swails, MT(ASCP), is a laboratory professional, recovering microbiologist, and web editor for Lab Medicine.