The study, led by Assistant Professor of food science Haley Oliver, took swabs samples from food contact and non-food contact surfaces in 30 delis located in supermarkets in three states. The swabs were then tested for the presence of L. monocytogenes.
The results revealed that 6.8% of samples taken in 15 delis before daily operation began tested positive, while a second sampling phase recorded 9.5% of positive samples in 30 delis during operation over a six month period. In 12 delis, the same L. monocytogenes subtypes were found in several monthly samples, suggesting that they could persist in the environment for some time.
Although most of the positive samples came from non-food contact surfaces, the researchers point out that these could act as reservoirs for recontamination of food contact surfaces and equipment. Furthermore, virulence testing found that the majority of the 442 isolates recovered were capable of causing human infection.
Commenting on the results, Oliver said, “This is a public health challenge. These data suggest that failure to thoroughly execute cleaning and sanitation protocols is allowing L. monocytogenes to persist in some stores. We cannot in good conscience tell people with weak immune systems that it is safe to eat at the deli.”
A report of the study is published in the Journal of Food Protection and an abstract can be found here.