Salmonella can be very difficult to remove from an establishment once it has become established. One study showed that a Salmonella strain survived for 10 years in a facility despite intensive cleaning and decommissioning of contaminated equipment. Two recent outbreaks in the US ultimately resulted in the shuttering of operations after those facilities were not able to eliminate Salmonella from the environment.
Biofilms may only be part of the answer. Aged bacterial cells may have increased resistance. We know that Salmonella has survived in products like peanut butter for months. We know that in dry products, Salmonella has increased heat resistance. For example, almonds roasted in oil require a process of 1.6 minutes at a temperature of 260ºF to achieve a 4 log kill.
Food processors beware: Salmonella biofilms incredibly resistant to powerful disinfectants
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," says 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 diarrhea) from the Agona serotype of Salmonella, says Corcoran. That outbreak was traced 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," says 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?"
The research uncovered nothing special about that specific strain.
"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 says.
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," says Corcoran. "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 hospitalizations and 450 fatalities reported each year, according to the Centers for Disease Control and Prevention.
Biofilm Helps Salmonella Survive Hostile Conditions
Science Daily Apr. 10 2013
Apr. 10, 2013 — Virginia Tech scientists have provided new evidence that biofilms -- bacteria that adhere to surfaces and build protective coatings -- are at work in the survival of the human pathogen Salmonella.
One out of every six Americans becomes ill from eating contaminated food each year, with over a million illnesses caused by Salmonella bacteria, according to the Centers for Disease Control and Prevention. Finding out what makes Salmonella resistant to antibacterial measures could help curb outbreaks.
Researchers affiliated with the Fralin Life Science Institute discovered that in addition to protecting Salmonella from heat-processing and sanitizers such as bleach, biofilms preserve the bacteria in extremely dry conditions, and again when the bacteria are subjected to normal digestive processes. The study is now online in the International Journal of Food Microbiology and will appear in the April issue.
"Biofilms are an increasing problem in food processing plants serving as a potential source of contamination," said Monica Ponder, an assistant professor of Food Science and Technology in the College of Agriculture and Life Sciences. "We have discovered that Salmonella in biofilms survive on dried foods much better than previously thought, and because of this are more likely to cause disease," said Ponder.
Outbreaks of Salmonella associated with dried foods such as nuts, cereals, spices, powdered milk and pet foods have been associated with over 900 illnesses in the last five years. These foods were previously thought to be safe because the dry nature of the product stops microbial growth.
"Most people expect to find Salmonella on raw meats but don't consider that it can survive on fruits, vegetables or dry products, which are not always cooked," said Ponder.
In moist conditions, Salmonella thrive and reproduce abundantly. If thrust into a dry environment, they cease to reproduce, but turn on genes which produce a biofilm, protecting them from the detrimental environment.
Researchers tested the resilience of the Salmonella biofilm by drying it and storing it in dry milk powder for up to 30 days. At various points it was tested in a simulated gastrointestinal system. Salmonella survived this long- term storage in large numbers but the biofilm Salmonella were more resilient than the free-floating cells treated to the same conditions.
The bacteria's stress response to the dry conditions also made it more likely to cause disease. Biofilms allowed the Salmonella to survive the harsh, acidic environment of the stomach, increasing its chances of reaching the intestines, where infection results in the symptoms associated with food poisoning.
This research may help shape Food and Drug Administration's regulations by highlighting the need for better sanitation and new strategies to reduce biofilm formation on equipment, thus hopefully decreasing the likelihood of another outbreak.