Abstract
In this study, thymoquinone (TQ), a natural active substance, was investigated for its antibacterial activity against Bacillus cereus, and its inhibitory effect on B. cereus in reconstituted infant formula (RIF) was evaluated. In addition, the inhibitory effect of TQ on B. cereus spore germination was explored. The minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of TQ against eight B. cereus strains ranged from 4.0 to 8.0 μg/mL, whereas B. cereus treated with TQ displayed a longer lag phase than the untreated control. TQ exerted a good bactericidal effect on B. cereus in Luria–Bertani broth. In addition, TQ obviously reduced the intracellular ATP concentration of B. cereus, which caused depolarization of the cell membrane, increased the intracellular reactive oxygen species level, impaired the cell morphology, and destroyed proteins or inhibited proteins synthesis. This provides a mechanism for its bacteriostatic effect. TQ also inactivated B. cereus growth in RIF. Moreover, reverse transcription–quantitative polymerase chain reaction illustrated that TQ downregulated the transcription of genes related to hemolysin, non-hemolytic enterotoxin, enterotoxin, and cytotoxin K. Meanwhile, TQ displayed the ability to inhibit the germination of B. cereus spores. These findings indicate that TQ, as an effective natural antimicrobial preservative, has potential applications in controlling food contamination and foodborne diseases caused by B. cereus.
Highlights
Bacillus cereus is a Gram-positive, flagellated, aerobic endospore-forming foodborne pathogenic bacterium that is non-susceptible to extreme environmental conditions, including heat, freezing, drying, and radiation [1]
The results indicated that TQ could effectively reduce the transcription of genes related to toxin production in B. cereus, thereby hindering the synthesis of hemolysin, non-hemolytic enterotoxin, enterotoxin, and cytotoxin K, and reducing the risk of food poisoning caused by B. cereus
TQ exerted its antimicrobial effects on B. cereus by causing depolarization of the cell membrane, decreasing intracellular ATP concentrations, increasing intracellular ROS levels, affecting intracellular protein synthesis, and altering the cellular morphology
Summary
Bacillus cereus is a Gram-positive, flagellated, aerobic endospore-forming foodborne pathogenic bacterium that is non-susceptible to extreme environmental conditions, including heat, freezing, drying, and radiation [1]. B. cereus strains that can cause poisoning usually pollute cereals and recombinant infant formula (RIF), and they are present in vegetables and meat [2,3,4]. Contaminated food is not obviously rotten, which makes B. cereus contamination difficult to observe. Foodborne diseases caused by B. cereus are a matter of great importance for the food industry and consumers. B. cereus usually causes vomiting, diarrhea, and abdominal pain [5], these gastrointestinal symptoms are mild and self-limiting. B. cereus can cause diseases such as severe eye infections, osteomyelitis, hepatitis, and even death in severe cases [6]. In Belgium, B. cereus was the most frequently reported foodborne outbreak pathogen in 2018, and B. cereus was the second most common pathogen in foodborne outbreaks between 2007 and 2014 in France [7]
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