ABSTRAKNisin has been extensively used as a safe food preservative: therefore, the occurrence of nisin resistance in various bacteria including nisin exposed Listeria monocytogenes has increased in recent years. This problem could be overcome by using nisin in combination with other antimicrobial agents resulting in synergistic effects. Citric acid is a safe food additive granted GRAS status by the Food and Drug Administration, USA. In the present investigation, the antibacterial activity of nisin and citric acid alone or in combinations against L. monocytogenes 10403S was determined, and their potential as food preservative in food model systems was evaluated. The nisin and citric acid showed minimum inhibitory concentration (MIC) at 250 and 4,000 mg/ml, respectively. Checkerboard microdilution method using both compounds showed synergistic effect at concentration of 62.5mg/ml and 1,000 mg/ml, respectively, with the fractional inhibitory concentration index (FICI) value of 0.5. The potent anti listeria effect of nisin in combination with citric acid on the growth of L. monocytogenes in pork ham (food model) was observed during six days of storage at 4oC. This might be exploited to inhibit foodborne bacteria and minimize the nisin resistant problem of L. monocytogenes in the food industry.
