Aims: The objective of this study was to evaluate the inhibitory activity of several natural organic compounds alone or in combination with nisin against Escherichia coli and Salmonella Typhimurium.

Methods and Results: The minimum inhibitory concentration (MIC) of five natural organic compounds were determined, and the effect of their combinations with nisin was evaluated by the checkerboard assay using the Bioscreen C. As expected, nisin by itself showed no inhibition against either of the Gram‐negative bacteria. Thymol was found to be the most effective with the lowest MIC values of 1·0 and 1·2 mmol 1⁻¹ against Salm. Typhimurium and E. coli, respectively. After thymol, the antimicrobial order of the natural organic compounds was carvacrol > eugenol > cinnamic acid > diacetyl. However, the combination of nisin with the natural organic compounds did not result in the enhancement of their antimicrobial activities. On the contrary, combination of nisin with diacetyl against Salm. Typhimurium resulted in an antagonism of diacetyl activity.

Conclusions: While the individual natural organic compounds showed inhibitory activity against the two Gram‐negatives, their combinations with nisin showed no improvement of antimicrobial activity.

Significance and Impact of the Study: This study shows the potential of the natural organic compounds to control E. coli and Salm. Typhimurium.