Antimicrobial effect of lime juice and java plum juice on foodborne pathogenic bacteria

Abstract

Escherichia coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus, and Listeria monocytogenes are common foodborne pathogens in the United States. With the advent of more consumers seeking chemical-free treatments to control pathogenic organisms in food, there has been an increasing interest in antimicrobials from natural sources. This study aimed to evaluate the antimicrobial effect of lime juice and java plum juice on aforementioned foodborne pathogenic bacteria in microbiological media and food models. First, agar well diffusion assay was conducted to test if the juices were effective against the four bacteria. After confirming its effectiveness, the antimicrobial activity of diluted lime juice was further confirmed in water and nutrient broth inoculated with the pathogenic bacteria. The survival of the bacteria in water and nutrient broth was determined on plate count agar (PCA). S. Typhimurium, S. aureus, and L. monocytogenes in water supplemented with 50% lime juice were inactivated at room temperature within 3, 8, and 48 h, respectively. Inactivation of the pathogens was faster in nutrient broth incubated at 35°C than in water. Nutrient broth supplemented with 50% java plum juice inactivated S. Typhimurium and S. aureus in 12 h, and E. coli O157:H7 and L. monocytogenes in 24 h. The juices were then tested on food models such as shrimp and cut cantaloupe. The pathogenic bacteria on the foods were enumerated on selective agars, with xylose lysine decarboxylase (XLD) agar for S. Typhimurium and modified Oxford (MOX) agar for L. monocytogenes. The overlay agar method was also employed to recover the injured cells on PCA before enumerating on the selective agars. S. Typhimurium was used as a representative pathogen to test its susceptibility to lime juice on shrimp. S. Typhimurium-inoculated shrimps were marinated in diluted lime juice at 7°C for 0, 2, 4, 8, 24, and 48 h. By 24 h, a significant difference (P<0.05) between the 25% treatment and control started to show. Comparing to the control, the 50% treatment reduced the count of S. Typhimurium on shrimp by 2.07, and 3.34 logs at 24 h, and 48 h, respectively. S. Typhimurium and L. monocytogenes, the two pathogens that have caused several multistate foodborne illness outbreaks with cantaloupe, were tested for their susceptibility to java plum juice treatments on the fruit. After dipping the inoculated cantaloupe cubes in diluted java plum juice for an hour, the fruit was stored at 7°C for 8 days. Under refrigeration, both L. monocytogenes and S. Typhimurium were able to grow on cut cantaloupe. When the fruit was treated with 50% java plum juice, the growth of L. monocytogenes was restrained. Comparing to the control, the 50% treatment significantly reduced the count of S. Typhimurium by 0.67, 2.86, and 3.41 logs on day 0, day 4 and day 8, respectively. The results from the selective agar and overlay agar were significantly different for S. Typhimurium counts in both solid food models although counts for L. monocytogenes showed no significant difference between the two agars. This indicates that the use of overlay method is crucial for the enumeration of S. Typhimurium when the cells are injured due to exposure to lime juice and java plum juice.