Culturable bacteria resident on lettuce might contribute to accumulation of human noroviruses

Human noroviruses (HuNoVs) are the primary non-bacterial pathogens causing acute gastroenteritis worldwide. Attachment and invasion of HuNoVs are thought to involve histo-blood group antigens (HBGAs). Romaine lettuce, which is usually consumed raw, is a common food-related vehicle for HuNoVs transmission. This study investigated the possibility that bacteria resident on the surface of lettuce leaves contribute to norovirus adherence to this food. To test this hypothesis, bacteria were isolated from romaine lettuce and screened to evaluate whether they produced any polysaccharides with structures resembling HBGAs. Twenty-seven bacterial isolates were screened and 18, belonging to 13 different genera, were found to produce HBGAs-like polysaccharides that were recognized by monoclonal antibodies specific to type A, B, H and Lewis a, b, x and y. One bacterial isolate, belonging to the genus Pseudomonas was further investigated because it produced polysaccharides with the widest range of HBGA types, including type B, H and Lewis a, b and x. The Pseudomonas HBGAs-like polysaccharides were found to be extracellular and their production was enhanced when the bacteria were cultured in oligotrophic medium. HuNoVs capture assays revealed that GI.1, GI.8, and GII.2, GII.3, GII.4, GII.6, GII.12, GII.17 genotypes can be bind to Pseudomonas HBGAs-like polysaccharides. The direct evidence of bacterial production HBGAs-like polysaccharides demonstrates one possible mechanism driving accumulation of HuNoVs on lettuce.