The synergistic antimicrobial effects of phytic acid (PA), a natural extract from rice bran, plus sodium chloride against Escherichia coli O157:H7 were examined. Exposure to NaCl alone at concentrations up to 36% (wt/wt) for 5 min did not reduce bacterial populations. The bactericidal effects of PA alone were much greater than those of other organic acids (acetic, citric, lactic, and malic acids) under the same experimental conditions (P < 0.05). Combining PA and NaCl under conditions that yielded negligible effects when each was used alone led to marked synergistic effects. For example, whereas 0.4% PA or 3 or 4% NaCl alone had little or no effect on cell viability, combining the two completely inactivated both nonadapted and acid-adapted cells, reducing their numbers to unrecoverable levels (>7-log CFU/ml reduction). Flow cytometry confirmed that PA disrupted the cell membrane to a greater extent than did other organic acids, although the cells remained viable. The combination of PA and NaCl induced complete disintegration of the cell membrane. By comparison, none of the other organic acids acted synergistically with NaCl, and neither did NaCl-HCl solutions at the same pH values as the test solutions of PA plus NaCl. These results suggest that PA has great potential as an effective bacterial membrane-permeabilizing agent, and we show that the combination is a promising alternative to conventional chemical disinfectants. These findings provide new insight into the utility of natural compounds as novel antimicrobial agents and increase our understanding of the mechanisms underlying the antibacterial activity of PA.
Food safety has become an increasingly important aspect of public health. According to the World Health Organization, foodborne and waterborne diarrheal diseases kill an estimated 2 million people per year, many of whom are children (1). Although various chemicals are used to control the transmission of foodborne illnesses via the food and livestock industries (2, 3), several studies have suggested that synthetic sanitizers can have significant side effects (e.g., bleaching, formation of toxic compounds, and off odors) (4, 5). Because modern consumers tend to prefer the use of "natural" agents rather than "synthetic" ones (6), new antimicrobial agents are required; therefore, studies of natural compounds with antimicrobial actions are warranted.Organic acids are a class of such natural antimicrobial agents. Several organic acids (particularly acetic, citric, and lactic acids) have long been used as preservatives or surface sanitizers for beef hides (7), carcasses (8), dairy products (9, 10), fresh-cut vegetables (11), and meat products (12). However, organic acids alone are not powerful enough to inactivate pathogenic bacteria after only a short exposure time (5, 13). Even if this problem could be overcome, applying adequate amounts of organic acids is impractical, because they generate strong acidic odors at working concentrations (5, 13). Thus, it is of vital importance to identify new alternatives ...