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

Abstract

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 6days. 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>6.5logCFU/cm2 reduction). Neither PA nor NaCl alone were this effective (PA, 1.6–2.7logCFU/cm2 reduction; NaCl, <0.5logCFU/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.