Molecular characterization of a murine norovirus isolate from Sweden and detection of noroviruses in artificially contaminated raspberries

Sammanfattning: Frozen, imported raspberries have been linked to several outbreaks of human norovirus (NoV) gastroenteritis in Sweden. Noroviruses are highly infectious and are often presented in low numbers in contaminated foods. Detection with RT-PCR must therefore be preceded by a method that extract and concentrate viruses from the tested food samples, but most available methods are laborious and/or inefficient. Studies on noroviruses are further complicated by the fact that human noroviruses are unable to grow in routinely used cell culture models. The murine norovirus (MNV), a common pathogen of immunocompromised mice, can be cultivated in macrophage-like cells and is often used as a model for studies on human noroviruses. In the present study, a previously unidentified MNV isolate from Sweden was sequenced and molecularly characterized. This isolate also served as a surrogate for testing and further developing a method to concentrate noroviruses from raspberries. Pathatrix (Life Technologies) is an automated magnet separation system that allows concentration of viruses or bacteria from large sample volumes. This system was tested together with cationic paramagnetic beads that attract the negatively charged surface of NoV particles. Results from this study shows that few modifications of the Pathatrix protocol might enhance viral recovery. For further evaluation, the Pathatrix method was compared with polyethylene glycol (PEG) precipitation for concentrating MNV, NoV GI, and GII from artificially contaminated raspberries. PEG precipitation was clearly more efficient but displayed a high degree of inhibition in RT-PCR. Altogether, this study shows that the Pathatrix method is a convenient and quick alternative. However, it needs to be further optimized before it can be used to concentrate noroviruses from raspberries.