Attachment and survival of viruses on lettuce (Lactuca sativa L. var. capitata L.): role of physicochemical and biotic factors
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Enteric viruses are responsible for a significant amount of foodborne disease in the United States. Foodborne disease associated with enteric viruses has been increasing within the last few years due to technological advances and raised awareness. Salads and salad crops are the principal vector for transmission of enteric viruses. The objective of this study was to determine if viruses are able to attach non-specifically to the surface of lettuce and to determine the forces responsible for non-specific viral adsorption to lettuce. Additionally, the impact of the microbial flora on viral persistence was studied to determine the effect on viruses. The four viruses studied were echovirus 11, feline calicivirus, MS2 and ????X174. The viruses were chosen based on their varying isoelectric points and similar physicochemical attributes. The isoelectric point was not the main factor determining virus attachment to lettuce. Viruses had varying attachment efficiencies, with echovirus 11 having the highest affinity to lettuce and ????X174 the least. Viral adsorption to lettuce was mediated by electrostatic forces due to the removal of virus adsorption at pH 7 and 8 with the addition of 1 M NaCl to the buffer solutions. Microcosm studies indicated that the microbial flora did not have a negative impact on virus survival. The bacteriophages had the highest survival rate. Virus survival in the microcosm studies was not indicative of virus survival on the surface of the lettuce. The animal viruses exhibited survival rates greater than or equal to the survival of bacteriophages at 4???? C, but at room temperature viable animal viruses rapidly declined compared to the bacteriophages. Additional studies also indicated that the microbial flora was not able to degrade the viruses for aerobic microbial growth. Overall, these results indicate that viruses are able to attach to the surface of lettuce, providing a possible explanation for the high incidence of virus associated disease involving salads and fresh produce. More importantly the use of surrogates for virus studies involving fresh produce must be re-evaluated, because of the lack of correlation between animal viruses and bacteriophages. Appropriate viral surrogates, if used, have to be carefully chosen based on viral physicochemical properties as well as the infectious route of the virus.