Effect of temperature, pH, and NaCl on the inactivation kinetics of murine norovirus

We investigated the resistance of murine norovirus (MNV) and coliphage MS2, a culturable human norovirus surrogate, to temperature, salt, and pH. Virus inactivation was measured by plaque, real-time TaqMan reverse transcription (RT) PCR, and long-template RT-PCR assays. Both MNV and MS2 were rapidly inactivated at temperatures above 60°C. Similarly, MNV tolerated low salt concentrations (0.3% NaCl) to a greater degree than high salt concentrations (3.3 to 6.3% NaCl). MNV was relatively resistant to strong acidic conditions (pH 2) and was more tolerant of slightly acidic (pH 4) or neutral (pH 7) conditions. In contrast, MS2 was resistant to high salinity. Overall, temperature had a greater effect on infectivity than salt or low pH. Additionally, temperature and low pH had a synergistic effect on MNV infectivity. Both real-time and long-template RT-PCR assays significantly underestimated the inactivation by temperature, salt, and pH. The inactivation kinetics of both MNV and MS2 under various environmental conditions gave a good fit by the Weibull model (R² > 0.9). This study suggests both the capacity of infectious human norovirus to persist in the face of various environmental conditions and its sensitivity to high temperatures, which may provide a mechanism of protection against this virus.