WASHINGTONNew research published in the journal Applied and Environmental Microbiology suggests Salmonella bacteria can form biofilms on the surfaces of a food processing facilities, making it extremely difficult to kill.

According to the Centers for Disease Control and Prevention (CDC), more than 1 million cases of Salmonella occur annually in the United States, resulting in 23,000 hospitalizations and 450 fatalities. Researchers from National University of Ireland, Galway conducted a study to investigate whether certain disinfectants can kill Salmonella biofilms on a variety of hard surfaces in a food processing setting. The impetus for the study was the 2008 outbreak of Salmonella Agona in Europe that sickened 160 people in 10 countries. The outbreak eventually was traced back to meat from a major food-processing facility.

It seems that Salmonella Agona entered into the environment in the part of the facility where meat that was already cooked was being handled, and it had survived and contaminated the cooked meat," said researcher Mary Corcoran. We were interested in determining if this particular Salmonella, that caused the outbreak, might have something special about it that makes it better at surviving in the environment of a food processing facility. Was it better at forming a dense biofilm or was it more resistant to disinfectants than other Salmonella?"

In this study, the researchers found it was not possible to kill the Salmonella cells using any of the three disinfectants if the biofilm was allowed to grow for seven days before the disinfectant was applied. They also noted that even soaking the biofilms in disinfectant for 90 minutes failed to kill them. They also found all of the types of Salmonella they looked at were able to adopt the specialized biofilm lifestyle on all of the test surfaces, including glass, stainless steel, glazed tile, and plastic. They also noted the biofilm of Salmonella gets more dense over time, and becomes more firmly attached to the surface.

Corcoran warns that food processing facilities must take strict care to keep Salmonella out of the clean areas where cooked foods get further processing and packaged.

People need to question whether disinfectants that are promoted as killing various types of bacteria are really as effective in real life situations where biofilms can form as they are claimed to be based on experiments that do not use biofilms. A lot of the time, the disinfectant may add very little, if anything, to good cleaning and appropriate food handling practices," she said. There is a need for more research to define better methods for killing Salmonella biofilms."