Synergistic bactericidal effect of nisin and phytic acid against Escherichia coli O157:H7

The food industry must prevent food contamination caused by pathogenic Escherichia coli strains as they pose a severe public health threat worldwide and lead to unimaginable economic losses. In this study, the synergistic antibacterial activity in vitro of nisin and phytic acid (PA), a natural metal chelate, against 5 foodborne isolates of E. coli were evaluated by a checkerboard assay. Checkerboard assay showed that nisin and PA showed a synergistic effect (FICI <0.375) on three O157 serogroup strains and an addictive effect (0.5<0.5625) on the remaining two non-O157 serogroup strains. Then the combined bactericidal effect of these compounds against E. coli O157:H7 was investigated. Results showed the number of planktonic and biofilm cells inactivated in groups treated with a combination of 0.512 mg/mL nisin plus 1 x MIC or 2 x MIC PA was significantly higher than the sum of groups treated alone (P < 0.05). Electron scanning microscopy studies revealed that the nisin plus PA treatment had a severe effect on E. coli O157:H7, which after treatment cell morphology appeared deflated with large invaginations. The effect of nisin plus PA against E. coli O157:H7 was furthermore assessed on cold-stored beef, where a similar synergistic bactericidal effect was observed. The studies demonstrate the great potential of the food-approved nisin to control the growth of undesired E. coli pathogens in foods. We used PA to overcome the permeability barrier of the outer membrane to nisin, but other food-grade metal ion chelators such as polyphosphate most likely could be used as well.

Automated summary

In this study, the synergistic antibacterial activity of nisin and phytic acid against foodborne E. coli strains was evaluated. The results showed a synergistic effect on certain strains and an addictive effect on others. The combined bactericidal effect of nisin and phytic acid was also investigated on E. coli O157:H7, and significant effects on cell numbers and morphology were observed.