Combined Effects of Naturally Occurring Antimicrobial Agents in Inhibiting the Growth of Bacillus cereus in Infant Rice Cereal

Introduction : One of the most frequently used methods to prevent the growth of foodborne pathogens in food is the addition of chemical agents as a preservative.  However, consumer acceptance for the addition of synthetic chemicals is low due to their potential toxicity. Therefore, the addition of naturally occurring antimicrobial agents in foods to inhibit the growth of foodborne pathogens has been interested.

Purpose : The objectives of this study were to screen natural antimicrobial agents against Bacillus cereus, to test synergistic lethal activities between natural agents, and to confirm their lethal activity against B. cereus in infant rice cereal.

Methods : Plant extracts (2,117 types) were tested for their antimicrobial activity against B. cereus using well diffusion assay.  For selected 13 plant extracts, their minimal inhibitory concentration (MIC) against B. cereus was determined. Combined effects between different extracts against B. cereus were evaluated using a checkerboard test. Finally, the lethal activity of combined extracts against B. cereus was tested in infant rice cereal.

Results : Siegesbeckia glabrescens and Morus alba extracts showed the lowest MICs (0.03 mg/ml) followed by Dryopteris erythrosora (0.06 mg/ml) and Carex pumila (0.06 mg/ml) in tryptic soy broth. A combination of S. glabrescens and C. pumila and a combination of D. erythrosora and C. pumila showed partial synergistic effects against B. cereus with the fractional inhibitory concentration index (FICI) of 0.63. MICs of S. glabrescens, M. alba, D. erythrosora, and C. pumila extracts were significantly increased to 1, 2, 2, and 8 mg/ml, respectively, in infant rice cereal. The combination of D. erythrosora (1 mg/ml) and C. pumila (1 mg/ml) extracts showed partial synergistic lethal activity (FICI = 0.63) in infant cereal.

Significance : Natural antimicrobial agents to inhibit B. cereus were screened. Their individual or combined antimicrobial activities were characterized using a laboratory medium and infant cereal.