Sanitation methods brought into question with Listeria study

Haley Oliver, assistant professor of food science who led the study, said the prevalence of L. monocytogenes was higher than expected in a significant percentage of delis and the bacteria is persisting over time.

“There is a disconnect between improving management of Listeria in manufacturing compared to retail delis. Everyone has the goal of high standards of sanitation but when you have 2,500 stores, it is harder than a few manufacturing plants,” ​she told FoodQualityNews.

“Customers want to see an open environment and that means we don’t control the air flow and there is also the complexities of foods that you have in delis. ​

“For the large stores sanitation standard operating procedures (SSOPs) are harder because of the time, the FDA Food Code says food contact surfaces should be cleaned and sanitized every four hours.​

“For non- food contact surfaces such as floors, walls and drains it is dictated by the company for the most part.”​

Non food contact surfaces of equipment shall be cleaned at a frequency necessary to preclude accumulation of soil residues, according to the code.

Most of the positive samples were from surfaces that usually do not come into contact with food, such as floors, drains and squeegees but bacteria can be transferred unintentionally from these to food, said Oliver.

Methodology and findings​

Researchers swabbed surfaces that come into frequent contact with food, such as meat slicers and counters, and surfaces that typically do not such as floors, walls and drains.

To understand ecology and transmission of Listeria in retail delicatessens, food and nonfood contact surfaces were tested in 30 sites in three US states (New York, North Carolina and Indiana).

In phase I, seven sponge samples were collected monthly for three months in 15 delis (five delis per state) prior to start of daily operation; in phase II, 28 food contact and nonfood contact sites were sampled in each of the 30 delis during daily operation for six months.

Among the 314 samples collected during phase I, 6.8% were positive for L. monocytogenes. Among 4,503 samples collected during phase II, 9.5% were positive for L. monocytogenes.

Nine of 30 delis showed low L. monocytogenes prevalence (<1%) for all surfaces.

A total of 245 Listeria isolates, including 184 Listeria innocua, 48 Listeria seeligeri, and 13 Listeria welshimeri were characterized.

Researchers tested 442 of the isolates to determine how virulent the isolates were - how great the likelihood was they could cause disease and found less than 3% had a lower potential for virulence.

About 30% of delis never tested positive while some tested positive in 35% of samples collected over six months.

Pulsed-field gel electrophoresis (PFGE) was used to characterize 446 L. monocytogenes isolates from the data collected in 2010.

It showed for 12 of 30 delis, one or more PFGE types were isolated on at least three separate occasions, providing evidence for persistence of a given L. monocytogenes subtype in the delis.

Reaction to data​

Finding Listeria in the environment of delis was not a surprise, said Oliver.

“It is common to find multiple species of Listeria. We can suspect it is likely a biofilm if it has the same footprint from month to month, if it is difficult to clean it facilities growth and in a high moisture environment it will grow unlike other pathogens.​ 

“Some stores have more challenges than others, we need to identify the risk factors, is it the floor, equipment or that the surface can’t be cleaned because of the equipment design.​

 ​“It takes resources and labour hours to clean and it needs to be part of a culture to get the return on investment.​

“There are recommendations which says it takes 20 minutes to clean a slicer, when we took one apart it took eight hours, there is certainly room for improvement in equipment design.”​

Ready-to-eat deli meats are most associated with L. monocytogenes, which can grow at refrigerator temperatures, unlike Salmonella and E. coli.

Stringent control measures and inspections have lowered its presence at meat processing plants, but there are no regulations specific to retail delis.

Since collecting the data for the studies referenced here, researchers have expanded work to include 100 delis and expect the next paper to be published around May.

Oliver said findings did only present a snapshot in time in a few locations but with the expansion to 100 delis the same trend of food contact and non-food contact surfaces was emerging so was representative of what is seen in a wider context. 

The study was between Purdue University, Cornell University, the US Department of Agriculture and North Carolina Agricultural and Technical State University.

It was published in the Journal of Food Protection and funded by the USDA’s Food Safety and Inspection Service. 

A second study that tested virulence potential of the strains of L. monocytogenes was published in Foodborne Pathogens and Disease. 

This was funded by Purdue University Food Marketing Institute and the USDA-Agricultural Research Service.