The incidence of foodborne illness linked to fresh fruits and vegetables has increased significantly during the past three decades15. Escherichia coli O157:H7 and Salmonella, both previously regarded as pathogens linked to foods of animal origin, have emerged as common agents of produce-related outbreaks. Initial research into the safety of fresh produce focused primarily on surveys to determine the prevalence of pathogens, as well as the efficacy of various sanitation methods to remove or inactivate bacteria on produce post-harvest. Results indicated that although the incidence of pathogens was low, none of the treatments proposed was completely effective. The rise in the number of produce-related outbreaks, coupled with the lack of an effective intervention, has given rise to an intense research effort into the ecology of human pathogens in the growing environment. Contrary to earlier theory, pathogens have been found to survive for long periods of time in water, animal manure and a variety of agricultural soils.
Recently, the intimate interactions between human pathogens and plant tissues have begun to be characterized. Bacteria have been found to be capable of attaching to and colonizing the surfaces of growing plants. It is now becoming clear that once attached, human pathogens are capable of forming biofilms on plant tissues. This formation of a biofilm was reported to be one of the main factors in failure of washing treatments to remove or inactivate human pathogens on produce surfaces, 2-4.
A biofilm is generally defined as “an assemblage of microorganisms adherent to each other and/or to a surface and embedded in a matrix of exopolymers”8. In food-processing settings, biofilms are found on food-processing surfaces such as stainless steel and glass. The presence of biofilms in processing environments increases the opportunity for contamination of finished product. Biofilms form in a step-wise process of preconditioning, initial reversible attachment, irreversible attachment, formation of microcolonies and maturation of the biofilm. All of these steps are dependent on a variety of factors including nutrient availability, temperature, substratum topography, agitation, flow rate and inoculum density. Bacterial cells embedded in a biofilm are far more resistant to inactivation using chemical sanitizers than their planktonic (free-floating) counterparts. A large number of reports demonstrate resistance of biofilm-associated cells to all major sanitizers common in the food industry such as chlorine, hydrogen peroxide and quaternary ammonium compounds, 7.
Microscopic studies indicate that plant-associated epiphytic bacteria form biofilms on surfaces of a wide variety of plants9, 11. Between 30 and 80 percent of bacteria on plant surfaces exist within biofilms10. The formation of biofilms by bacterial cells on plant surfaces is likely a survival strategy for these cells to withstand the harsh environment of the plant surface (wide temperature changes, desiccation, UV, oxidative stress). Similar to biofilms on food-processing surfaces, bacteria embedded within biofilms on plant tissue are more difficult to remove and more resistant to inactivation than their planktonic counterparts. Three commodities that have been repeatedly linked to outbreaks are cantaloupes, apples (unpasteurized juice or cider), and parsley, each associated with a different human pathogen. Research into the interactions between Salmonella, E. coli, and Shigella and these three vehicles are discussed below.
Cantaloupe melons have been implicated in at least six multistate outbreaks of salmonellosis since 1990. FDA surveys conducted in response to a 1997 outbreak of Salmonella enterica serovar Saphra indicated that approximately 5 percent of imported cantaloupes tested positive for Salmonella17. Three successive outbreaks (2000-2002) linked to the consumption of melons imported from Mexico prompted the FDA to issue an import alert detaining all cantaloupes from Mexico offered for entry at U.S. ports18.| | | Next → | Single Page