Factors influencing synergistic antimicrobial activity of thymol and nisin against Shigella spp. in sugarcane juice
Two food-grade antimicrobial substances, thymol and nisin were tested for their antimicrobial activity against 4 species of Shigella including S. boydii, S. dysenteriae, S. flexneri and S. sonnei inoculated in sugarcane juice at 4°C. When used separately, only thymol, but not nisin, exhibited a dose-dependent and species-dependent inhibitory effect on the bacteria. When thymol and nisin were used together, the concentrations of thymol required for complete inhibition were decreased in all cases of the bacteria as the concentrations of nisin were raised, indicating the synergistic antimicrobial activity between both compounds. The effects of bacterial cell number and temperature on the antimicrobial activity of thymol and nisin against S. sonnei in sugarcane juice were investigated. The doses of both compounds required for complete inhibition were directly proportional to the number of bacterial cells in sugarcane juice. Furthermore, they inhibited the bacterium at 30 °C more efficiently than at 4°C. The sensory evaluation showed that thymol-and nisin-treated sugarcane juice was acceptable to consumers. This study suggests that thymol and nisin can be used as food preservatives with the consideration of their dependence on bacteria species, number of contaminated bacterial cells and temperature.
Key wordsnisin Shigella sugarcane juice thymol
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