Assessment of Listeria monocytogenes Growth Inhibition by Two GRAS Compounds

Introduction: Generally recognized as safe or GRAS compounds are most commonly used as food additives. Many of these compounds are also used as preservatives to control microbial growth. During previous studies with L. monocytogenes, we identified two GRAS compounds, deoxycholic acid and trans-aconitic acid, that inhibited growth as well as or better than other GRAS compounds typically used to inhibit L. monocytogenes, e.g., nisin.

Purpose: To determine if the extent of growth inhibition by deoxycholic acid and trans-aconitic acid was generalized across serotypes of L. monocytogenes strains.

Methods: A panel of 12 L. monocytogenes (Lm) strains consisting of the major disease causing serotypes was evaluated. Growth was tracked in a Bioscreen C at 37°C and at 5°C in brain heart infusion broth (BHI) or BHI supplemented with 7% NaCl. Nisin was also assessed as a control. The area under the curve was calculated for the various curves and a percent inhibition was determined using parallel controls.

Results: Our results indicate increased average inhibition of Lm growth in BHI across all 12 strains by both trans-aconitic acid (9% at 37°C; 18% at 5°C) and deoxycholic acid (29% at 37°C; 22% at 5°C). Nisin, at 0.1%, however showed no inhibition, regardless of temperature. The addition of 7% NaCl to BHI enhanced the effectiveness of all three compounds. However, the effect was most pronounced for deoxycholic acid which showed nearly 100% inhibition of all 12 strains, regardless of temperature. This is notable as both nisin and trans-aconitic acid showed variability across all the serotypes that was not seen for deoxycholic acid in the presence of salt.

Significance: These data indicate that deoxycholic acid may be a potential candidate for use in controlling Lm growth. Further work is necessary to evaluate the interplay between with NaCl and different foods to achieve optimal growth inhibition.