A study led by Haley Oliver, assistant professor of food science, found that 6.8 percent of samples taken in 15 delis before daily operation had begun tested positive for L. monocytogenes (mah-noh-sy-TAH’-gin-eez).
In a second sampling phase, 9.5 percent of
samples taken in 30 delis during operation over six months tested
positive for the bacteria. In 12 delis, the same subtypes of the
bacteria cropped up in several of the monthly samplings, which could
mean that L. monocytogenes can persist in growth niches over time.
"This is a public health challenge," Oliver said.
"These data suggest that failure to thoroughly execute cleaning and
sanitation protocols is allowing L. monocytogenes to persist in
some stores. We can't in good conscience tell people with weak immune
systems that it is safe to eat at the deli."
In healthy individuals, eating food contaminated with L. monocytogenes
may lead to common food poisoning symptoms such as diarrhea or an upset
stomach. But the bacteria can cause listeriosis - a serious systemic
infection - in immunocompromised people such as the elderly, infants and
children, pregnant women and people with HIV. In severe cases, L. monocytogenes
can pass through the intestinal membrane and into the bloodstream or
cross the blood-brain barrier. The bacteria can also cross the placental
barrier in pregnant women, which can trigger abortion.
Ready-to-eat deli meats are the food most associated with L. monocytogenes, which can grow at refrigerator temperatures, unlike Salmonella and E. coli.
Stringent control measures and inspections have tamped down the presence of L. monocytogenes at meat processing plants, but there are no regulations specific to Listeria
for retail delis. Recent risk assessments suggest that up to 83 percent
of listeriosis cases linked to deli meats are attributable to products
contaminated at retail.
"It's kind of the Wild West," Oliver said. "Manufacturing has a zero-tolerance policy for Listeria,
but that dissipates at the retail level. The challenge of developing
systematic cleaning procedures for a wide variety of delis - which are
less restricted environments than processing plants - can make Listeria harder to control."
Oliver and her team tested for L. monocytogenes and other Listeria
species in 30 delis in national supermarket chains in three states. The
researchers swabbed surfaces that come into frequent contact with food,
such as meat slicers and counters, and surfaces that typically do not.
About 30 percent of the delis never tested
positive for the pathogen, while some delis tested positive in 35
percent of the samples collected over six months.
"The prevalence of L. monocytogenes is
higher than we expected in a significant percentage of delis, and the
bacteria is persisting in these delis over time," Oliver said.
Most of the positive samples were collected from
surfaces that usually do not come into contact with food, examples being
floors, drains and squeegees. But the bacteria can be transferred
unintentionally from these surfaces to food, Oliver said.
While the percentage of L. monocytogenes
found on food contact surfaces was low, "these numbers would never be
acceptable in manufacturing," she said. "The reason we haven't had a
listeriosis outbreak tied to a deli is because it's a disease with a
long incubation time, and it's difficult to track to a source. There are
only about 1,600 listeriosis cases a year. But the likelihood of death
is huge."
The research team tested 442 of the L. monocytogenes
isolates collected from delis to determine how virulent the isolates
were - that is, how great the likelihood was that they could cause
disease. They found that less than 3 percent of the isolates had a lower
potential for virulence.
"The vast majority of the isolates were 'hot' - comparable to wild-type L. monocytogenes," Oliver said. "These are particularly cause for concern."
She said that delis' standard sanitation
operating procedures can keep the bacteria at bay only if the delis are
in good condition, thoroughly cleaned and have sloped floors. But
cleaning and sanitation may not effectively manage Listeria in a deli with structural damage such as missing grout, loose wall coverings or a drain that is not working properly. L. monocytogenes can flourish when it finds a moist niche that is infrequently cleaned.
Delis with contamination problems should
"minimize the 'stuff' in the deli," Oliver said, to make it possible to
clean the area thoroughly and train employees on how to maintain a
sterile environment.
Consumers with vulnerable immune systems should
buy prepackaged deli meats or heat ready-to-eat meats to 165 degrees,
she said. Meat contaminated with L. monocytogenes will not show signs of spoilage, such as sliminess or odor.
"That's the challenge with pathogens such as Listeria, Salmonella, E. coli
and norovirus: They don't cause changes in the characteristics of the
products," she said. "Can you smell a food and tell if it's safe?
Absolutely not."
The study was a collaboration between researchers
at Purdue University, Cornell University, the U.S. Department of
Agriculture and North Carolina Agricultural and Technical State
University.
The paper was published in the Journal of Food Protection. The abstract is available at http://www.ingentaconnect.com/content/iafp/jfp/2014/00000077/00000011/art00012.
The USDA’s Food Safety and Inspection Service provided funding for the research.
A second study that tested the virulence potential of the strains of L. monocytogenes found in retail delis was published in Foodborne Pathogens and Disease. The abstract is available at http://www.ncbi.nlm.nih.gov/pubmed/25569840. That research was funded by the Purdue University Food Marketing Institute and the USDA-Agricultural Research Service.
Writer: Natalie van Hoose, 765-496-2050, nvanhoos@purdue.edu
Source: Haley Oliver, 765-496-3913, hfoliver@purdue.edu
Related website:
Purdue University Department of Food Science: https://ag.purdue.edu/foodsci/Pages/default.aspx