Detection of viable salmonella in lettuce by propidium monoazide real-time PCR and control of food-borne pathogenic bacteria by noni juice

Abstract

Pathogenic bacteria have caused a large number of foodborne illnesses in the United States. Detection and control of these pathogens in food are two important measures to enhance the microbiological safety of food. The first objective of this study was to develop a molecular assay for specific detection of viable Salmonella cells in lettuce. Propidium monoazide (PMA), a DNA-modifying dye, was used to treat dead Salmonella cells. Real-time polymerase chain reaction (PCR) results suggest that PMA treatment effectively cross-linked with DNA from as high as 108 CFU/g dead Salmonella Typhimurium cells in lettuce. The PMA real-time PCR assay could selectively detect viable Salmonella at as low as 102 CFU/ml in pure culture and 103 CFU/g in lettuce. Combining a 12-hour enrichment with the assay allowed for the detection of viable Salmonella at 101 CFU/g in lettuce. The PMA real-time PCR assay provides a simple yet accurate tool for detection of Salmonella in food. Moreover, this study also aimed to evaluate the antimicrobial effect of noni juice on major foodborne pathogenic bacteria, including Escherichia coli O157:H7, Salmonella Typhimurium, Listeria monocytogenes and Staphylococcus aureus, in distilled water, a laboratory medium and a food model. After 24 hours of incubation at 35ºC, 10% noni juice reduced the count of E. coli O157:H7, S. Typhimurium, L. monocytogenes and S. aureus in distilled water by 5.2, 4.8, 5.0, and 4.2 log, respectively, compared with the control. The treatment suppressed E. coli O157:H7, S. Typhimurium, and L. monocytogenes in tryptic soy broth by 5.8, 8.4 and 7.9 log, respectively, after five days of incubation. Finally, the antimicrobial activity of noni juice was evaluated in artificially contaminated mushroom soup. At 7ºC, 10% noni juice inactivated L. monocytogenes of 5.02 log CFU/ml in mushroom soup on day 6. At 35ºC, the treatment reduced the count of E. coli O157:H7, S. Typhimurium, and L. monocytogenes in mushroom soup by 8.7, 9.7, and 8.2 log, respectively, after five days of incubation. In addition to being a functional beverage, noni juice holds great promise as a natural preservative.