WEDNESDAY, Feb. 11, 2015 (HealthDay News) -- The next time you order that pastrami-on-rye at your local deli, you may get an unwanted ingredient -- the illness-inducing listeria bacterium.
That's the finding from a Purdue University study of dozens of delicatessens. Researchers say that on any given day, up to one in 10 deli swab samples tested positive for the Listeria monocytogenes germ.
"This is a public health challenge," study leader Haley Oliver, assistant professor of food science, said in a university news release.
"These data suggest that failure to thoroughly execute cleaning and sanitation protocols is allowing L. monocytogenes to persist in some stores," she added.
While listeria infection can cause serious but transient gastrointestinal illness in most people, the Purdue team noted that foodborne illness is potentially deadly in people with weakened immune systems. Those people include the elderly, infants and small children, and people living with HIV.
"We can't in good conscience tell people with weak immune systems that it is safe to eat at the deli," Oliver said.
In the study, Oliver's team first collected samples from 15 delis before they opened for the day. They examined swab samples from deli surfaces that came into contact with meat (such as slicers or countertops), as well as surfaces that did not, and found that nearly 7 percent of the samples tested positive for listeria bacteria.
A second round of testing at 30 delis over six months found that 9.5 percent of the samples tested positive for the bacteria. In 12 of the delis, the same subtypes of the bacteria were found in several of the monthly samplings. This suggests that the bacteria can persist in certain areas over time, the researchers said.
Only about 30 percent of delis never tested positive for listeria over the course of the study. But in some of the delis, samples came back positive for listeria about 35 percent of the time.
In most cases, positive samples came from surfaces that aren't usually in contact with food -- for example, floors, drains or squeegees. But the researchers noted that it's still easy to transmit the bacteria from these surfaces to a surface that's likely to touch food.
Ready-to-eat deli meats are most often associated with listeria, the study authors noted, since the germ can grow even when foods are refrigerated, unlike other bugs such as E. coli or salmonella.
The team also found that most of the listeria picked up on the samples was highly virulent, meaning it was likely to cause serious illness.
"These are particularly cause for concern," Oliver said.
While tight standards have reduced the presence of listeria in meat processing plants, there are no regulations specifically meant to control the bacteria in delis, the study authors said. They added that research suggests that up to 83 percent of listeria cases linked to deli meats are due to contamination at retail outlets.
"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."
So what's a wary consumer -- especially one with a compromised immune system -- to do? Oliver's team said buying your own pre-packaged cold cuts at a supermarket should cut the risk, or you can heat any ready-to-eat meat first to 165 degrees to cut the risk. Listeria-laden meats don't always look suspicious, so checking for odor or sliminess may not help.
As for delis, Oliver said that delis that are in good condition, have thorough cleaning policies and sloped floors are more likely to be listeria-free. Clogged drains and crumbling grout are warning signs, because listeria seeks out moist niches in which to grow, she noted.
The study was published recently in the Journal of Food Protection.
The U.S. National Library of Medicine has more about listeria infections.