The Synergistic Effect of the Photosensitizer Curcumin and Ascorbic Acid in Inactivating Listeria monocytogenes and Escherichia coli O157:H7

Introduction: Listeria monocytogenes and Escherichia coli O157:H7 are foodborne pathogens linked to a great number of illnesses and economic losses, annually. Both pathogens have been isolated from fresh produce, meat and meat product, and poultry. Curcumin is the principle curcuminoid of turmeric and approved for food use in Europe (E100) and the USA. L-ascorbic acid (AA) is a GRAS substance commonly used as an antioxidant.

Purpose: The purpose of this study was to evaluate the synergistic effect of curcumin and AA in inactivating Gram positive and Gram negative foodborne pathogens.

Methods: Curcumin was dissolved in 95% EtOH and diluted to desired concentrations with sterile distill water (SDW). AA was dissolved in SDW and filter sterilized. Overnight cultures of L. monocytogenes L008 or E. coli O157:H7 86-24 were diluted to 6.0 log CFU/ml. Curcumin, AA, and cells were mixed in amber tubes and pre-incubated in the dark for 5 min. The mixture was then transferred to a 96-well plate and illuminated (450-455 nm, 0.13 W/cm²) for 1 min (L. monocytogenes) or 10 min (E. coli O157:H7). Number of viable cells was determined; the detection limit was 2.4 log CFU/ml. Results were compared by ANOVA.

Results: A statistically significant, synergistic effect was observed for curcumin/AA concentrations (p < 0.05) in the inactivation of each pathogen. Based on direct plating, no viable L. monocytogenes were detected when exposed to 10 uM of curcumin and 250 ug/ml of AA; while E. coli O157:H7 were below detection limit following treated with 50 uM of curcumin and 25 ug/ml of AA. Populations of E. coli O157:H7 were 4.3 log CFU/ml and 5.9 log CFU/ml when exposed to 50 uM curcumin or 25 ug/ml AA, respectively.

Significance: The combination of photosensitizing curcumin and ascorbic acid has potential application as an antimicrobial treatment for inactivation of foodborne pathogens on the surface of meat, poultry, and fresh produce.