Scientists find ways to crack bacteria barrier
formations of bacteria on surfaces say their discoveries could lead
to new cleaning methods for fruit, vegetables and other foods
Fresh produce is catching up with chicken as a major culprit of Salmonella infections in the US, according to an analysis of food-poisoning outbreaks by a consumer group. Several outbreaks of food poisoning in recent years have been linked to cantaloupes, sprouts, lettuce, tomatoes, and other fresh produce.
Scientists from the US Agricultural Research Service (ARS) and the University of Navarra are studying how to improve the washing techniques used in the produce packing and processing industries. Food companies using fruit and vegetables for their products would also benefit from the research, which is available for companies to commercialise or use.
Conventional commercial washing and sanitizing methods to remove microbial contaminants from produce surfaces have been found to be marginally effective, the scientists stated. They are are conducting experiments to understand how these microorganisms survive and grow on produce surfaces even after exposure to sanitizing solutions like chlorine.
The human pathogen Salmonella is often the culprit behind produce-related outbreaks of foodborne illness. It's an important concern in ensuring the microbiological safety of fresh-cut cantaloupe, for example.
The bacteria quickly cling to the rind and form a "biofilm" - a hard to remove mass of microbes attached to a surface and to each other by complex sugars known as bacterial polymers. Salmonella cells present on the rind can be transferred to the melon's internal tissues during cutting.
"Gaining knowledge of factors that permit survival of microbial contaminants during washing and sanitizing will help us develop new technologies for removing or inactivating microbes on both fresh and minimally processed produce," stated Bassam Annous, a microbiologist with ARS.
Annous and others on the project found Salmonella cells produce fimbriae (hairlike structures) and cellulose that help them attach, colonise, and survive on a melon's surface. Once attached to the outside of the rind, the cells grow and excrete polymers, forming the biofilm, which shields the bacteria from harsh sanitizing solutions.
The US scientists will work with those in Spain to determine how Salmonella develop fimbriae as they grow and how they use them to cling to surfaces. They will also determine the role of specific bacterial polymers involved in surface attachment and biofilm formation by using Salmonella cells that are defective in producing fimbriae or cellulose.
The scientists are already finding ways to outsmart the bacterium. For example, Anous and Joseph Sites, a mechanical engineer, recently developed a commercial-scale surface-pasteurisation treatment that resulted in a 99.999 per cent reduction in a population of Salmonella on the surface of artificially contaminated cantaloupe.
The process involves immersing melons in water at 169°F for three minutes to kill the pathogens, then sealing each melon in a plastic bag before rapid cooling in an ice-water bath.
The plastic bag prevents the fruit from potential recontamination in the cold water. The treatment not only enhances the safety of the fruit, but also extends its shelf life by reducing native microflora that may cause spoilage. And it did not harm melon quality.
The US researchers will collaborate with scientists at the University of Navarra, in Spain, to determine how Salmonella develop fimbriae as they grow and how they use them to cling to surfaces. They will also determine the role of specific bacterial polymers involved in surface attachment and biofilm formation by using Salmonella cells that are defective in producing fimbriae and/or cellulose.
The research would be useful in helping to reduce the growing number of food poisioning outbreaks caused by eating contaminated fresh products. According to an analysis of food-poisoning outbreaks by the Centre for Science in the Public Interest (CSPI) fresh produce is catching up with chicken as a major culprit of Salmonella infections in the US.
The CSPI study indicates produce-related outbreaks tend to be larger than poultry-related outbreaks, and sicken more people, sometimes hundreds at a time, CSPI stated.
Fresh produce triggered 554 outbreaks, sickening 28,315 people. Of those 554 outbreaks, 111 were due to Salmonella. Although poultry has historically been responsible for far more Salmonella infections, produce seems to be catching up, CSPI stated.
