Thermal Inactivation Kinetics of Tulane Virus in Cell-Culture Medium and Spinach

Human noroviruses (HNoVs) cause significant gastrointestinal disease outbreaks worldwide. Tulane virus (TV) is a cultivable HNoV surrogate widely used to determine control measures against HNoVs. The objective of this study was to determine the heat inactivation kinetics (D- and z-values) of TV in cell-culture media and on spiked homogenized spinach using the first-order and Weibull models. TV in cell-culture media at approximately 7 log PFU/mL (PFU-plaque forming unit) in 2-mL glass vials was heated at 52, 54, and 56 degrees C for up to 10 min in a circulating water bath. Survivors were enumerated using confluent host LLC-MK2 cells in six-well plates by plaque assay. Data from three replicate treatments assayed in duplicate were analyzed statistically. D-values by the first-order model for TV in cell-culture media at 52, 54, and 56 degrees C were 4.59 +/- 0.05, 2.91 +/- 0.05, and 1.74 +/- 0.07 min, respectively, with a z-value of 9.09 +/- 0.01 degrees C (R-2 = 0.997). The Weibull model showed t(d = 1) values of 2.53 +/- 0.08, 1.99 +/- 0.10, and 0.57 +/- 0.64 min, respectively, at the same temperatures. The D-values for TV in spinach were 7.94 +/- 0.21, 4.09 +/- 0.04, and 1.43 +/- 0.02 min and the z-value was 10.74 +/- 0.01 degrees C (R-2 = 0.98) by the first-order model and 4.89 +/- 0.02, 3.21 +/- 0.45, and 0.25 +/- 0.38 min for the Weibull model at 50, 54, and 58 degrees C, respectively. In comparison to previously reported results for the cultivable HNoV surrogate, murine norovirus -1, TV in cell-culture media and spiked on spinach homogenates showed lower D- and z-values. TV may not be an ideal HNoV surrogate for heat inactivation studies in cell-culture media or homogenized spinach in vacuum bags.