Natural microbiota of raspberries (Rubus idaeus) and strawberries (Fragaria × ananassa): microbial survey, bacterial isolation and identification, and biofilm characterization

Human norovirus (HuNoV) outbreaks have been associated with the consumption of contaminated fresh raspberries (Rubus idaeus) and strawberries (Fragaria × ananassa). The dynamics of HuNoV binding to berries are unknown: viruses may attach to berry-specific ligands; bind non-specifically (electrostatic and hydrophobic interactions); or bind in a facilitated manner such as in association with native berry microflora. In fact, HuNoV have been shown to bind to specific structures found on the extracellular polymeric substances (EPS) produced by biofilm-producing bacteria. A microbial survey was done on (Rubus idaeus var. strigosus 'Heritage') raspberries and 'Chandler' strawberries obtained from local U-Pick farms (Angier, NC and Raleigh, NC, respectively) by plating them on selective and differential media. A total of 172 (n=98 from raspberries; n=74 from strawberries) unique bacterial isolates were identified, with the microbial profiles differing by berry type. Selected isolates were tested for the capacity to form biofilms using a crystal violet-based assay. Biofilm production varied greatly among isolates, but strains of Pantoea agglomerans, Pseudomonas moraviensis, Klebsiella oxytoca, and Enterobacteriaceae spp. were the strongest biofilm producers (A₄₀₅>2.2). These strong biofilm-producing strains will be used in future studies to characterize the attachment of HuNoV to the natural berry microflora and determine if adherence to specific microflora components (e.g., EPS) significantly contributes to the attachment and persistence of HuNoV on berries.