Once Salmonella bacteria get into a food processing facility and have an opportunity to form a biofilm on surfaces, it is likely to be extraordinarily difficult, if not impossible, to kill it, according to research published ahead of print in the journal, Applied and Environmental Microbiology.

Researchers from National University of Ireland, Galway conducted a study in which they attempted to kill Salmonella biofilms on a variety of hard surfaces, using three types of disinfectant.

"We found that 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," said Mary Corcoran, a researcher on the study. Even soaking the biofilms in disinfectant for an hour and a half failed to kill them

The impetus for the study was a European outbreak in which 160 people in 10 countries became ill with gastroenteritis (vomiting and diarrhoea) from the Agona serotype of Salmonella, said Dr Corcoran. That outbreak was traced to meat from a major food-processing facility.

She continued: "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."

"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?"

This image shows photos of biofilms on surfaces at low magnification (300) and high magnification (3000) using electron microscope.
Photo courtesy of National University of Ireland, Galway

The research uncovered nothing special about that specific strain.

Dr Corcoran said: "We found that all of the types of Salmonella we looked at were able to adopt the specialized biofilm lifestyle on all of the surfaces we looked at, including glass, stainless steel, glazed tile, and plastic, and that the biofilm of Salmonella gets more dense over time, and becomes more firmly attached to the surface."

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

She added: "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. There is a need for more research to define better methods for killing Salmonella biofilms."

In the US, an estimated million-plus cases of Salmonella occur annually, with 23,000 admitted to hospital and 450 fatalities reported each year, according to the Centers for Disease Control and Prevention.