Inhibitory effects of grape seed extract on growth, quorum sensing, and virulence factors of CDC “top-six” non-O157 Shiga toxin producing E. coli

• GSE markedly inhibited bacterial growth of non-O157 STEC at low inoculation level.
• At high inoculation level, GSE reduced quorum sensing of all non-O157 STECs tested.
• GSE reduced swimming motility of all the top-six non-O157 STECs.
• GSE reduced flagella and Shiga toxin contents of E. coli O103:H2 and O111:H2.

Non-O157 Shiga toxin producing Escherichia coli (STECs) have become a growing concern to the food industry. Grape seed extract (GSE), a byproduct of wine industry, is abundant in polyphenols that are known to be beneficial to health. The objective of this study was to evaluate the effect of GSE on the growth, quorum sensing, and virulence factors of Centers for Disease Control and Prevention (CDC) “top-six” non-O157 STECs. Minimal inhibitory concentration (MIC) of GSE was 2 mg/ml against E. coli O26:H11, and 4 mg/ml against the other non-O157 STECs tested. Minimal bactericidal concentration (MBC) was the same as MIC for all six non-O157 STECs tested. At 5 × 105 CFU/ml inoculation level, 4 mg/ml GSE effectively inhibited the growth of all tested strains, while 0.25–2 mg/ml GSE delayed bacterial growth. At a higher inoculation level (1 × 107 CFU/ml), GSE had less efficacy against the growth of the selected six non-O157 STECs. Its impact on bacterial virulence was then assessed at this inoculation level. Autoinducer-2 (AI-2) is a universal signal molecule mediating quorum sensing (QS). GSE at concentration as low as 0.5 mg/ml dramatically reduced AI-2 production of all non-O157 STECs tested, with the inhibitory effect proportional to GSE levels. Consistent with diminished QS, GSE at concentration of 0.125 mg/ml caused marked reduction of swimming motility of all motile non-O157 STECs tested. In agreement, GSE treatment reduced the production of flagella protein FliC and its regulator FliA in E. coli O103:H2 and E. coli O111:H2. Furthermore, 4 mg/ml GSE inhibited the production of Shiga toxin, a major virulence factor, in E. coli O103:H2 and E. coli O111:H2. In summary, GSE inhibits the growth of “top-six” non-O157 STECs at the population level relevant to food contamination. At higher initial population, GSE suppresses QS with concomitant decrease in motility, flagella protein expression and Shiga toxin production. Thus, GSE has the potential to be used in food industry to control non-O157 STEC.