Short communicationSynergistic bactericidal action of phytic acid and sodium chloride against Escherichia coli O157:H7 cells protected by a biofilm

- Sanitizers are less effective against Escherichia coli O157:H7 cells protected by a biofilm.
- The anti-biofilm effect of PA was significantly enhanced by sodium chloride (NaCl) (p < 0.05).
- Treatment with 0.4% PA + 4% NaCl for 5 min completely inactivated E. coli O157:H7 biofilms, without recovery.
- The synergistic antimicrobial interaction between PA and NaCl was visualized by confocal laser scanning microscopy.
- A combination of PA and NaCl may be an effective natural surface sanitizer for use by the food industry.

The food industry must prevent the build-up of strong Escherichia coli O157:H7 biofilms in food processing environments. The present study examined the bactericidal action of phytic acid (PA), a natural extract from rice bran and the hulls/peels of legumes, against E. coli O157:H7 biofilms. The synergistic bactericidal effects of PA plus sodium chloride (NaCl) were also examined. E. coli O157:H7 biofilms were allowed for form on stainless steel coupons by culture in both rich (tryptic soy broth, TSB) and minimal (M9) medium at 22 °C for 6 days. Bacterial cells within biofilms grown in M9 medium were significantly more resistant to PA than those grown in TSB (p < 0.05); thus M9 medium was selected for further experiments. The anti-biofilm effect of PA was significantly increased by addition of NaCl (2-4%) (p < 0.05); indeed, the combination of 0.4% PA plus 3-4% NaCl completely inactivated E. coli O157:H7 biofilms without recovery (a > 6.5 log CFU/cm2 reduction). Neither PA nor NaCl alone were this effective (PA, 1.6-2.7 log CFU/cm2 reduction; NaCl, < 0.5 log CFU/cm2 reduction). Confocal laser scanning microscopy images of propidium iodide-treated cells showed that PA (0.4%) plus NaCl (2-4%) had marked membrane permeabilizing effects. These results suggest that a sanitizer that combines these two naturally occurring antimicrobial agents may be useful to food safety managers who encounter thick biofilm formation in food processing environments.