Effect of bacterial cell-free supernatants on infectivity of norovirus surrogates

Bacterial metabolic products were evaluated for inhibitory effects on viral propagation in cell culture. Cell-free supernatants (CFS) were prepared from growth of Enterococcus faecalis ATCC 19433, Pseudomonas fluorescens ATCC 13525, Escherichia coli 08, Staphylococcus epidermidis ATCC 12228, Bacillus subtilis 168, Bacillus coagulans 185A, B. coagulans 7050, Clostridium sporogenes PA3679, and a commercial probiotic mixture of Lactobacillus acidophilus, Lactobacillus rhamnosus, Bifidobacterium bifidum, Lactobacillus salivarius, and Streptococcus thermophilus in microbiological medium or milk. The inhibitory effects of CFS on the propagation of murine norovirus 1 and Tulane virus in RAW 264.7 and LLCMK2 cells, respectively, were evaluated in the continuous presence of CFS or after exposure of host cells to CFS. Slight inhibition of viral propagation was observed for murine norovirus and Tulane virus in the continuous presence of CFS of B. subtilis 168 and E. faecalis 19433, respectively. CFS cytotoxicity was also determined by microscopic examination. Virus persisted in the CFS that demonstrated cytotoxic effects, suggesting a lack of direct effect of CFS on virions. The viral propagation indicates a general lack of competitive inhibition by bacterial extracellular products and bears significance in understanding the persistence of virus in food and human systems shared by bacteria that are recognized for their colonization and competitive capabilities.