P-068. Characterization of Salmonella Isolates from Retail Foods for Biofilm Formation, Inducible Acid-Tolerance and Caco-2 Cell Infectivity

X. Xia1,2, A. Smith2, S. Zhao3, J. McEvoy2, J. Meng1, A. A. Bhagwat2;
1Univ. of Maryland, College Park, MD, 2USDA-ARS, Beltsville, MD, 3US FDA, Laurel, MD.

Background: Foods contaminated with Salmonella spp. are a major health concern in the United States as well as many other countries and a variety of foods have been associated with Salmonella spp. outbreaks. Understanding Salmonella serotype distribution in different food matrices and its virulence properties are important for disease prevention. Methods: Sixteen Salmonella strains isolated from a variety of foods in 2000-2003 by the Florida State Department of Agriculture were characterized by serotyping, antimicrobial susceptibility testing, biofilm formation, inducible acid-tolerance, Caco-2 cell infectivity and Pulse-Field Gel Electrophoresis (PFGE). Results: Among 16 isolates, 15 different serotypes were identified. Most of the isolates were susceptible to the 15 antimicrobials tested except two. PFGE fingerprinting profiles showed that all 16 Salmonella isolates, were genetically diverse with distinguished 16 unique PFGE patterns. The induced acid tolerance level of the isolate obtained from orange juice was not significantly different from the laboratory reference strain SL1344. One strain was defective in constitutive and inducible acid-tolerance while five strains lacked only the constitutive acid-resistance pathway. Upon short term exposure (~40 min) to sliced tomatoes (pH 4.5), some strains were enhanced in their ability to infect Caco-2 cells. Because a Spanish patent (#9700408) suggested a correlation between biofilm formation and virulence, we compared biofilm formation ability of these strains with their ability to infect differentiated Caco-2 cells. Two isolates infected Caco-2 cells with reduced efficiency but formed biofilms in greater quantities. Conclusion: The study indicates that different serotypes of Salmonella were present in a variety of retail foods.Caco-2 cell infectivity was better correlated with acid-tolerance than biofilm formation.