RESEARCHERS at Virginia Polytechnic Institute & State University found new evidence that biofilms -- bacteria that adhere to surfaces and build protective coatings -- are at work when it comes to the survival of salmonella.
One out of every six Americans becomes ill from eating contaminated food each year, with more than 1 million illnesses caused by salmonella bacteria, according to the Centers for Disease Control & Prevention. Finding out what makes salmonella resistant to antibacterial measures could help curb outbreaks.
Researchers affiliated with the Fralin Life Science Institute at Virginia Tech 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 available online in the International Journal of Food Microbiology.
"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 Virginia Tech College of Agriculture & 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."
Outbreaks of salmonella linked to dried foods such as nuts, cereals, spices, powdered milk and pet foods have been associated with more than 900 illnesses in the last five years. These foods were previously thought to be safe because it was assumed that the dry nature of the product stopped 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," Ponder said.
In dry conditions, salmonella cease to reproduce but instead turn on genes that produce a biofilm, which protects 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 free-floating cells treated under 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 & Drug Administration regulations by highlighting the need for better sanitation and new strategies to reduce biofilm formation on equipment, which will hopefully decrease the likelihood of future outbreaks.
The Fralin Life Science Institute enables and enhances collaborative efforts in research, education and outreach within the Virginia Tech life science community through strategic investments.