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.