Combined effects of plant extracts in inhibiting the growth of Bacillus cereus in reconstituted infant rice cereal

A study was done to determine the potential use of plant extracts to inhibit the growth of Bacillus cereus in reconstituted infant rice cereal. A total of 2116 extracts were screened for inhibitory activity against B. cereus using an agar well diffusion assay. The minimal inhibitory concentrations (MIC) and minimal lethal concentrations (MLC) of 14 promising extracts in tryptic soy broth (TSB) were determined. Dryopteris erythrosora (autumn fern) root extract showed the lowest MIC (0.0156. mg/ml), followed by Siegesbeckia glabrescens (Siegesbeckia herb) leaf (0.0313. mg/ml), Morus alba (white mulberry) cortex (0.0313. mg/ml), Carex pumila (sand sedge) root (0.0625. mg/ml), and Citrus paradisi (grapefruit) seed (0.0625. mg/ml) extracts. The order of MLCs of extracts was D. erythrosora root (0.0156. mg/ml) < S. glabrescens leaf (0.0313. mg/ml) < M. alba cortex (0.0625. mg/ml) = Siegesbeckia pubescens aerial part (0.0625. mg/ml) < C. paradisi seed (0.1250. mg/ml). The combined inhibitory effects of extracts against B. cereus in TSB were determined using a checkerboard assay. A combination of D. erythrosora and C. pumila extracts showed a partial synergistic inhibition, with a fractional inhibitory concentration index (FICI) of 0.75. Single and combined inhibitory activities of selected plant extracts against B. cereus in reconstituted infant rice cereal were investigated. The MICs of S. glabrescens, M. alba, D. erythrosora, and C. pumila extracts against B. cereus were 1.0, 2.0, 2.0, and 8.0. mg/ml, respectively. A combination of D. erythrosora (1.00. mg/ml) and C. pumila (1.00. mg/ml) extracts showed a partial synergistic effect (FICI 0.63) in inhibiting the growth of B. cereus. Results indicate that by combining extracts, the amounts of D. erythrosora and C. pumila extracts can be reduced by 50% and 87.5%, respectively, compared with individual extracts, and give similar inhibitory activity in reconstituted infant rice cereal. Sensory evaluation showed that supplementing reconstituted infant rice cereal with plant extracts reduces sensorial quality. These observations will be useful when developing and applying interventions using natural plant extracts to inhibit B. cereus in foods.