Purpose: This study was undertaken to evaluate the effect of cell-free bacterial supernatants on the cytopathic effect of murine norovirus on RAW cells.
Methods: Enterococcus faecalis was grown in TSB, and a commercial probiotic mixture (Lactobacillus acidophilus, L. rhamnosus, Bifidobacterium bifidum, L. salivarius, and S. thermophilus) was grown in UHT-pasteurized milk. Cell-free supernatants (CFS) were prepared by centrifugation of the bacterial suspensions and filtration of the supernatants through 0.2-µm pore filters. The cytopathic effects of 104 infectious MNV on RAW cells was evaluated in the presence of 10% CFS in DMEM or after RAW cells were exposed to CFS and repeatedly washed with HBSS. Cytopathic and cytotoxic effects were evaluated by microscopic examination 48 h post inoculation. At 10% in DMEM, HBSS, TSB, milk, and milk acidified with lactic acid not inoculated with bacteria were included as controls.
Results: Cytopathic effect of MNV was reduced in the presence of CFS of E. faecalis as compared to controls and was inconsistent in the presence of the CFS of the probiotic mix. No cytopathic effect of MNV on RAW cells was observed in the continuous presence of acidified milk or filtered supernatant of acidified milk or even after washing cells of these solutions prior to infection with MNV. Cytopathicity of MNV was not inhibited in cells washed of CFS. The effects were not pH-dependent. There were no apparent cytotoxic effects of the solutions. Challenges were encountered in quantifying the inhibition of viral infection by quantitative reverse-transcriptase PCR.
Significance: Varying degrees of inhibition of viral cytopathic effects were observed with continuous exposure to bacterial CFS; however, the effect was no greater than that observed with acidified milk controls. The nature of viral inhibition may be due to effects on the host cell and remains to be elucidated.