Antimicrobial Activity of Roselle (Hibiscus sabdariffa) Aqueous Extracts against Escherichia coli O157:H7 In Vitro and on Fresh Produce

Escherichia coli (E. coli) O157:H7 is one of the most common bacterial pathogens implicated in foodborne outbreaks involving fresh produce in the last decade. In an effort to discover new, natural antimicrobial agents for use on fresh produce, the objective of this study was to evaluate the effectiveness of antimicrobial agent plant extract-concentrate formulations in vitro and on organic fresh produce inoculated with E. coli O157:H7. Roselle calyx paste was prepared to determine the antimicrobial activity against E. coli O157:H7 for in vitro studies. Approximately, 150mu of E. coli O157:H7 "cocktail" (109CFU/ml) was inoculated into 15 g of the prepared roselle calyx paste and into 15 ml of tryptic soy broth (TSB) as preparation for treated and control samples, respectively, both were sampled immediately at 0 h incubation time. Storage for the treated and control samples was at either 4, 8, or 25 °C. Roselle calyx concentrate (RCC) and roselle tea (RT) was prepared at 100% concentrations to determine the bactericidal effects against E. coli O157:H7 on organic romaine lettuce. Additionally, roselle calyx concentrations were prepared at 10, 20, and 30% concentrations to determine the antimicrobial activity against E. coli O157:H7 on organic baby spinach. Each leaf sample was washed, dip inoculated with E. coli O157:H7 "cocktail" (106CFU/ml), and dried. Inoculated leaves were immersed in the treatment solution for 2 minutes and individually incubated at 4 °C. One 10 g sample of romaine lettuce was taken at 0 h and the other at 24 h after storage at 4 °C for the enumeration of surviving bacteria. After incubation, baby spinach samples were taken at 0, 24, and 48 h for enumeration of surviving bacteria. The results showed that the antimicrobial agent activity was both concentration and time dependent. The roselle calyx paste exhibited strong antibacterial activity against E. coli O157:H7 as a result of the in vitro study. No E. coli O157:H7 survivors were detected at 24 h and all temperatures. The roselle calyx concentrate and tea inactivated E. coli O157:H7 to undetectable levels on romaine lettuce over a period of 24 h. Roselle calyx concentrations at 20 and 30% concentrations suggest these concentrations can reduce E. coli O157:H7 populations by 1-2 logs on baby spinach. This study demonstrates the potential of a natural plant aqueous extract formulations to inactivate E. coli O157:H7 in vitro and on organic fresh produce.