The U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) has told producers of ready-to-eat foods not to veer from their regular manufacturing, sanitation and food-safety procedures when federal inspectors test for Listeria monocytogenes in their processing plants.

According to the notice published Wednesday, routine sampling to test for Listeria is conducted about once every four years at most facilities that make ready-to-eat foods, such as hot dogs and luncheon meats, or after a problem is suspected.

But too many establishments are making temporary changes before inspectors in the monitoring program arrive, or during the sampling, presumably to reduce the chances of positive test results, the FSIS notice implies.

“By altering routine practices, establishments may make changes that are not consistent with their documented food-safety system and that impede FSIS’s ability to assess the safety of the product,” said the notice, which was signed by Daniel Engeljohn, assistant administrator for the Office of Policy and Program Development.   

Some of the examples of altered practices:

– Increasing the use of sanitizer only during the testing

– “Drastically” reducing the typical production time (i.e. by more than two hours in a typical eight-hour shift), the lot size or the number of workers handling the product

– Selectively not processing higher-risk products, such as sliced meats

– Not using particular equipment previously associated with a positive test for Listeria

“Such practices can interfere with FSIS’s assessment of routine conditions or corrective actions at the establishment and may limit FSIS’s ability to determine whether post-lethality exposed RTE meat and poultry products are not adulterated,” Engeljohn wrote.

If a company changes practices without a “supportable rationale,” FSIS personnel should notify their district offices and reschedule the sampling, the notice said.

Food makers who violate the notice may face suspension of inspection, which would halt production. FSIS will review reports from plants where testing has been conducted to see if further action is needed, Engeljohn wrote.

Listeria monocytogenes is one of the most dangerous foodborne pathogens in the U.S. food supply. Although listeriosis is infrequent, relative to other foodborne infections such as those caused by Salmonella, it has a higher rate of hospitalizations and fatalities.

Listeria is killed by pasteurization and cooking; however, in some ready-to-eat foods, such as deli meats, contamination can occur after factory cooking but before packaging. Unlike most bacteria, Listeria can survive and multiply under refrigeration.

When Listeria bacteria get into a food-processing factory or on food-processing equipment, they can live there for years, and create the persistent potential for contamination.