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.”

It’s That Time of Year Again

It’s a few days after a major holiday (Memorial Day in the United States), and clinical microbiologists knows what that means. It’s foodborne illness season! According to the CDC, Norovirus and Salmonella are the biggest culprits, but several organisms can be implicated.

If your lab doesn’t recover Salmonella, Campylobacter, or E. coli O157:H7 often, consider brushing up on the identifying characteristics of these organisms. (Do you know which one doesn’t ferment sorbitol?) It’s also helpful to keep the patient history (in particular, their travel history) in mind when reading enteric cultures or performing a microscopic ova and parasite examination. Also, now is a good time to be sure your reporting procedures (including local public health contact information) are up to date.

Check out the CDC’s website for more information on foodborne outbreaks, including how many people are affected.



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


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.





‘Tis the Season

The holiday season is rife with celebrations. Tree Trimmings! Presents! Gatherings! And let’s not forget the food. Turkey! Dressing (with or without oysters)! Cookies and its glorious dough! An unfortunate side effect of holiday celebrations is food poisoning, specifically those caused by, Salmonella, Campylobacter, and Vibrio.

According to the CDC, Salmonella and Campylobacter are in the top five pathogens with Salmonella being the biggest culprit in hospitalization. If you want to prevent Salmonella poisoning—or think you might already have it—here’s a handy guide to causes, symptoms, and treatment.

Raw eggs and mishandled poultry aren’t the only causes of food poisoning, though. Shellfish can be a concern, as is undercooked beef and unpasteurized dairy products. The Mayo Clinic has a wonderful chart describing the major food poisoning pathogens. A noticeable omission is Bacillus cereus, which breeds quite nicely in leftover rice.

For laboratory professionals, foodborne illnesses are a common cause of laboratory-acquired hospital infections. Be vigilant when handling enteric specimens and enteric cultures. Observe basic lab safety—use personal protective equipment, don’t use personal electronics in the lab, and be obsessive about washing your hands. Don’t let Salmonella or one of his buddies ruin your holiday season.



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